# On Pentafluoroorthotellurates and Related Compounds

Julia Bader, Lukas Fischer, Kurt F Hoffmann, Niklas Limberg, Alexandre Millanvois, Friederike Oesten, Alberto Pérez-Bitrián, Johanna Schlögl, Ahmet N Toraman, Daniel Wegener, et al.

► To cite this version:

Julia Bader, Lukas Fischer, Kurt F Hoffmann, Niklas Limberg, Alexandre Millanvois, et al. On Pentafluoroorthotellurates and Related Compounds. Chemical Reviews, 2025, 10.1021/acs.chemrev.5c00075. [https://doi.org/10.1021/acs.chemrev.5c00075](https://doi.org/10.1021/acs.chemrev.5c00075). [arXiv:2009.09387](https://arxiv.org/abs/2009.09387)

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# On Pentafluoroorthotellurates and Related Compounds

*Published as part of Chemical Reviews special issue "Fluorine-Specific Interactions".*

Julia Bader, Lukas Fischer, Kurt F. Hoffmann, Niklas Limberg, Alexandre Millanvois, Friederike Oesten, Alberto Pérez-Bitrián, Johanna Schlögl, Ahmet N. Toraman, Daniel Wegener, Anja Wiesner, and Sebastian Riedel*

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Cite This: *Chem. Rev.* 2025, 125, 9140–9186

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**ABSTRACT:** This Review surveys the properties and applications of the pentafluoroorthotellurate ("teflate", OTeF₅) ligand and highlights the syntheses of the known teflate-based compounds across the periodic table. Due to the accessibility to several useful teflate transfer reagents and its unique properties, including strong electron-withdrawing character, considerable steric bulk, and stability against oxidation, a variety of intriguing *p*-block and *d*-block species have been reported. These encompass highly reactive Lewis acids, versatile weakly coordinating anions, neutral and cationic noble gas compounds, and a wide number of transition metal complexes. The lower analogues of the pentafluoroorthochalcogenate group, OSeF₅ and OSF₅, are described as well, although fewer examples are known. Recent progress in the derivatization of the OTeF₅ group to *cis*- and *trans*-PhTeF₄O or *trans*-(C₆F₅)₂TeF₃O moieties is also discussed, opening pathways to exciting new research directions.

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## CONTENTS

<table><tbody><tr><td>1. Introduction</td><td>9141</td></tr><tr><td>1.1. Scope of the Review</td><td>9141</td></tr><tr><td>2. Fundamental Compounds and Their Applications</td><td>9141</td></tr><tr><td>2.1. Pentafluoroorthotelluric Acid</td><td>9141</td></tr><tr><td>2.2. Reactive Synthons</td><td>9142</td></tr><tr><td>2.3. Properties</td><td>9142</td></tr><tr><td>2.3.1. Lewis Acidity</td><td>9144</td></tr><tr><td>2.3.2. Weakly Coordinating Anions with Te-flate Groups</td><td>9144</td></tr><tr><td>2.3.3. Ligand Properties</td><td>9145</td></tr><tr><td>2.4. Selenium-Based Derivatives</td><td>9145</td></tr><tr><td>2.5. Sulfur-Based Derivatives</td><td>9145</td></tr><tr><td>2.6. Derivatives of the Teflate Group</td><td>9147</td></tr><tr><td>3. Group 1 Teflates</td><td>9148</td></tr><tr><td>4. Group 2 Teflates</td><td>9148</td></tr><tr><td>5. Group 13 Teflates</td><td>9149</td></tr><tr><td>5.1. Boron Teflates</td><td>9149</td></tr><tr><td>5.2. Aluminum Teflates</td><td>9149</td></tr><tr><td>5.3. Gallium Teflates</td><td>9151</td></tr><tr><td>5.4. Thallium Teflates</td><td>9152</td></tr><tr><td>6. Group 14 Teflates</td><td>9152</td></tr><tr><td>6.1. Carbon Teflates</td><td>9152</td></tr><tr><td>6.1.1. Hydrocarbons (C<sub>n</sub>H<sub>2n+2</sub>)</td><td>9152</td></tr><tr><td>6.1.2. Fluorocarbons</td><td>9153</td></tr><tr><td>6.2. Silicon Teflates</td><td>9154</td></tr><tr><td>6.3. Germanium, Tin, and Lead Teflates</td><td>9155</td></tr><tr><td>7. Group 15 Teflates</td><td>9155</td></tr><tr><td>7.1. Nitrogen Teflates</td><td>9155</td></tr><tr><td>7.2. Phosphorus Teflates</td><td>9155</td></tr></tbody></table>

<table><tbody><tr><td>7.3. Arsenic Teflates</td><td>9156</td></tr><tr><td>7.4. Antimony Teflates</td><td>9156</td></tr><tr><td>7.5. Bismuth Teflates</td><td>9157</td></tr><tr><td>8. Group 16 Teflates</td><td>9158</td></tr><tr><td>8.1. Oxygen Teflates</td><td>9158</td></tr><tr><td>8.2. Sulfur Teflates</td><td>9158</td></tr><tr><td>8.3. Selenium Teflates</td><td>9159</td></tr><tr><td>8.4. Tellurium Teflates</td><td>9159</td></tr><tr><td>9. Group 17 Teflates</td><td>9159</td></tr><tr><td>9.1. Hypofluorite</td><td>9160</td></tr><tr><td>9.2. Hypochlorite</td><td>9161</td></tr><tr><td>9.3. Hypobromite and Bromine Teflates</td><td>9161</td></tr><tr><td>9.4. Iodine Teflates</td><td>9161</td></tr><tr><td>10. Group 18 Teflates</td><td>9162</td></tr><tr><td>10.1. Krypton Teflates</td><td>9162</td></tr><tr><td>10.2. Xenon(II) Teflates</td><td>9163</td></tr><tr><td>10.3. Xenon(IV) Teflates</td><td>9164</td></tr><tr><td>10.4. Xenon(VI) Teflates</td><td>9165</td></tr><tr><td>11. d-Block and f-Block Metal Teflates</td><td>9166</td></tr><tr><td>11.1. Homoleptic d-Block Metal Teflates</td><td>9166</td></tr><tr><td>11.1.1. Neutral Complexes</td><td>9166</td></tr><tr><td>11.1.2. Anionic Complexes</td><td>9168</td></tr><tr><td>11.2. Heteroleptic d-Block Metal Teflates</td><td>9171</td></tr></tbody></table>

Received: January 25, 2025

Revised: May 19, 2025

Accepted: June 18, 2025

Published: September 16, 2025

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<figure><img src="image_10.png" /><figcaption>Figure 1. Periodic table of teflate compound elements and their oxidation states.</figcaption></figure>

<table><tbody><tr><td>11.2.1.</td><td>Metal Teflate Complexes Containing Oxo Ligands</td><td>9171</td></tr><tr><td>11.2.2.</td><td>Metal Teflate Complexes Containing Halide Ligands</td><td>9172</td></tr><tr><td>11.2.3.</td><td>Metal Teflate Complexes Containing O-, N- and P-Donor Ligands</td><td>9173</td></tr><tr><td>11.2.4.</td><td>Organometallic Teflate Complexes</td><td>9175</td></tr><tr><td>11.3.</td><td>f-Block Metal Teflates</td><td>9176</td></tr><tr><td>12.</td><td>Conclusion</td><td>9177</td></tr><tr><td colspan="2">Author Information</td><td>9177</td></tr><tr><td colspan="2">Corresponding Author</td><td>9177</td></tr><tr><td colspan="2">Authors</td><td>9177</td></tr><tr><td colspan="2">Author Contributions</td><td>9178</td></tr><tr><td colspan="2">Notes</td><td>9178</td></tr><tr><td colspan="2">Biographies</td><td>9178</td></tr><tr><td colspan="2">Acknowledgments</td><td>9179</td></tr><tr><td colspan="2">Abbreviations</td><td>9179</td></tr><tr><td colspan="2">References</td><td>9179</td></tr></tbody></table>

# 1. INTRODUCTION

The element fluorine possesses the highest electronegativity among the elements, giving rise to unique properties of its compounds. In fact, almost every element in the periodic table reacts with fluorine, with the exception of the noble gases He and Ne, to form a huge variety of different species. By substituting fluorides in element-fluoride compounds with sterically demanding groups with high electronegativities, some of these properties like melting point, vapor pressure, coordination behavior etc. can be manipulated. Famous examples of these endeavors are perfluorinated aryl, aryloxy, alkyl, and alkyloxy groups or perfluorinated carborates. A highly promising candidate in this regard is the pentafluoroorthotellurate group (OTeF<sub>5</sub>), also known as the teflate group, which displays similarly strong electron-withdrawing properties when compared to fluorine while having a greater volume and a reduced lone pair back bonding. The rich chemistry of this "big brother" of fluorine across the periodic table is shown in Figure 1.

## 1.1. Scope of the Review

Following up on an initial review by Seppelt from 1982,¹ this Review aims to provide a comprehensive overview of all hitherto reported teflate compounds across the periodic table of elements. Furthermore, the selenates (OSeF<sub>5</sub>) and sulfonates (OSF<sub>5</sub>) will also be addressed. Starting with a general description of the peculiarities of the OTeF<sub>5</sub> group, the following sections focus on different element—OTeF<sub>5</sub> compounds, thereby discussing their syntheses, aspects of their characterization and properties, and their possible applications in preparative chemistry.

# 2. FUNDAMENTAL COMPOUNDS AND THEIR APPLICATIONS

## 2.1. Pentafluoroorthotelluric Acid

The pentafluoroorthotelluric acid HOTEF<sub>5</sub> (teflic acid) was accidentally discovered in 1964 by Engelbrecht and Sladky when they reacted BaTeO<sub>4</sub> and fluorosulfonic acid in a molar ratio of 1:10.² The reaction was accompanied by several byproducts, such as F<sub>5</sub>TeOSO<sub>3</sub>H, F<sub>5</sub>TeOSO<sub>2</sub>F, (F<sub>5</sub>TeO)<sub>2</sub>SO<sub>2</sub>, TeF<sub>6</sub>, SiF<sub>4</sub> (reaction with the glassware) and SO<sub>3</sub>, and therefore the yield of isolated HOTEF<sub>5</sub> was relatively low.³ However, they optimized the reaction conditions and found that the use of barium tellurate with the exact stoichiometric composition BaH<sub>4</sub>TeO<sub>6</sub> (also written as BaTeO<sub>4</sub>·2H<sub>2</sub>O) increases the yield to 95% (see eq 1).⁴

$$ \mathrm{BaTeO_4 \cdot 2H_2O} + 7\mathrm{HSO_3F} \xrightarrow{-20 \text{ to } 160^\circ\mathrm{C}, 3 \text{ h}} \mathrm{HOTEF_5} + \mathrm{Ba(SO_3F)_2} + 5\mathrm{H_2SO_4} \quad (1) $$

At ambient conditions HOTEF<sub>5</sub> is a glass-like solid (mp = 40 ± 1 °C, bp = 60 ± 1 °C, ρ = 2.626 g/cm³) with a vapor pressure of 53 mbar (40 Torr) at 0 °C and 147 mbar (110 Torr) at 17 °C, making it a convenient reagent that can be easily condensed at room temperature.²,³ Still, caution must be taken when exposing HOTEF<sub>5</sub> and some of its derivatives to moisture, since contact with water leads to hydrolysis under formation of telluric acid Te(OH)<sub>6</sub> and hydrogen fluoride (HF).
<figure><img src="image_11.png" /><figcaption>Figure 2. ¹⁹F NMR spectra (377 MHz, r.t.) of B(OTeF₅)₃, [Sb(OTeF₅)₆]⁻, HOTeF₅ and AgOTeF₅ illustrating different appearances of the A₄B or AB₄ spin systems. The ¹²⁵Te satellites are marked with *.</figcaption></figure>

Several years after its initial discovery, a more convenient route to obtain HOTeF₅ in good yields (70–85%) was developed by reacting Te(OH)₆ and HSO₃F as shown by eq 2.⁵⁻⁷

$$Te(OH)_6 + 5HSO_3F \xrightarrow{170^{\circ}\text{C, 5 to 6 h}} \text{HOTeF}_5 + 5H_2SO_4 \quad (2)$$

The compound HOTeF₅ is easily characterized by ¹⁹F NMR spectroscopy due to its resulting AB₄ spin system (see Figure 2). As expected, the compound reacts as a strong Brønsted acid, and its acidity has been determined to range between those of HCl and HNO₃.⁸ For a more specific application, the synthesis of the deuterated acid DOTeF₅ starting from [NBu₄][OTeF₅] and excess of sulfuric acid-d₂ in glassware that is treated with D₂O and flame-dried beforehand is reported.⁷

## 2.2. Reactive Synthons

Besides the acid HOTeF₅, typical reactive synthons to introduce the teflate ligand into main group and transition metal compounds are B(OTeF₅)₃, AgOTeF₅ and Xe(OTeF₅)₂.⁹⁻¹⁴ For B(OTeF₅)₃ and AgOTeF₅ the reactions can be described as metathesis reactions with the formation of BF₃ and AgCl, respectively, as driving force, enabling the synthesis of e.g. As(OTeF₅)₃, Sb(OTeF₅)₃, U(OTeF₅)₆, Xe(OTeF₅)₄, and Xe(OTeF₅)₆ (with B(OTeF₅)₃) and Me₃GeOTeF₅ (with AgOTeF₅).¹⁵⁻²⁰ In contrast to this, Xe(OTeF₅)₂ introduces OTeF₅ groups by an oxidative addition leading to compounds in high oxidation states such as As(OTeF₅)₅, I(OTeF₅)₅, or Te(OTeF₅)₆.¹⁵,²¹,²² Another effective oxidative transfer reagent is the hypochlorite ClOTeF₅, which has been applied for the synthesis of [Au(OTeF₅)₃]₂, Fe(OTeF₅)₃, Ga(OTeF₅)₃, and Tl(OTeF₅)₃.⁵,²³⁻²⁷ The commonly used synthesis of B-(OTeF₅)₃, AgOTeF₅, Xe(OTeF₅)₂ and ClOTeF₅ as well as

their general introduction pathways are summarized in Scheme 1 (see also Sections 5.1, 9.2, 10.2, and 11.1.1).

## 2.3. Properties

The access to unusually high oxidation states in teflate-based compounds,¹⁶,²²,²⁸ that are typically only achieved with fluoride

### Scheme 1. Synthesis and Utilization of the Most Commonly Used OTeF₅ Transfer Reagents

#### synthesis of reactive synthons of the OTeF₅ ligand

$$3 \text{HOTeF}_5 + \text{BCl}_3 \rightarrow \text{B(OTeF}_5)_3 + 3 \text{HCl}$$

$$\text{HOTeF}_5 + \text{AgF} \rightarrow \text{AgOTeF}_5 + \text{HF}$$

$$^{2/3}B(\text{OTeF}_5)_3 + \text{XeF}_2 \rightarrow \text{Xe(OTeF}_5)_2 + ^{2/3}\text{BF}_3$$

$$\text{B(OTeF}_5)_3 + 3 \text{ClF} \rightarrow 3 \text{ClOTeF}_5 + \text{BF}_3$$

#### introduction of the OTeF₅ ligand

$$n \text{B(OTeF}_5)_3 + 3 \text{EF}_n \rightarrow 3 \text{E(OTeF}_5)_n + n \text{BF}_3$$

$$n \text{AgOTeF}_5 + \text{ECI}_n \rightarrow \text{E(OTeF}_5)_n + n \text{AgCl}$$

$$\text{Xe(OTeF}_5)_2 + \text{EX}_n \rightarrow \text{EX}_n(\text{OTeF}_5)_2 + \text{Xe}$$

$$n \text{ClOTeF}_5 + \text{ECI}_n \rightarrow \text{E(OTeF}_5)_n + n \text{Cl}_2$$

$$E = \text{element}, X = F, \text{OTeF}_5$$
<figure><img src="image_12.png" /><figcaption>**Figure 3.** Comparison of FIA values⁵² of monomeric boron, aluminum, arsenic and antimony compounds. Substances in quotation marks are not synthetically available. The gray dashed line indicates the FIA value of SbF₅ and therefore the threshold for Lewis superacidity.</figcaption></figure>

ligands, led to the investigation of the electron-withdrawing properties of the OTeF₅ group and the comparison to those of the most electronegative element, fluorine. Different methods have been applied such as comparison of the chemical shift differences of the compounds Me-X and Et-X (X = F, Cl, Br, I, OTeF₅) in ¹H NMR spectra,²⁹,³⁰ the preference of equatorial OTeF₅ substitution on square pyramidal IFₙ(OTeF₅)₅-ₙ (n = 1–4) and the infrared and multinuclear NMR experiments of OPF₂X (X = Br, Cl, F or OTeF₅).²¹,³¹ Additionally, ¹²⁷I and ¹²⁹Xe Mössbauer together with ¹²⁵Te and ¹²⁹Xe NMR spectroscopy on different teflate and fluoride complexes were carried out for this purpose.³² These experimental investigations came to opposite conclusions. A very recent work reinvestigated this topic with a combination of different quantum-chemical methods and concluded that the teflate group exhibits extraordinary electron-withdrawing properties, but less pronounced than in the case of fluorine.³³ When having a closer look at the QTAIM charges (QTAIM = quantum theory of atoms in molecules) of the different atoms within the OTeF₅ fragment, it is clear that the bonding within the teflate moiety is significantly ionic. In addition, it was also shown that the OSF₅, OSeF₅ and OTeF₅ group exhibit nearly identical electron-withdrawing properties.

A comparison of the negatively charged OTeF₅ with the fluoride ligand in terms of electronic properties have been investigated on the homoleptic transition metal teflate complexes [Ni(OTeF₅)₄]²⁻, [Co(OTeF₅)₄]²⁻, [Mn(OTeF₅)₄]²⁻ and [Mn(OTeF₅)₅]²⁻.³⁴⁻³⁶ From SQUID measurements and UV/visible spectra accompanied with quantum-chemical calculations it was deduced that the teflate group is a weak/medium-field ligand and can be included in the spectrochemical series at a similar position as the fluorido ligand.

A striking difference between the teflate and the fluoride ligand is their size. The higher steric demand of the OTeF₅ group results in a reduced tendency to act as a bridging ligand. This is demonstrated in the case of [Ni(OTeF₅)₄]²⁻, [Mn(OTeF₅)₄]²⁻ or [Mn(OTeF₅)₅]²⁻, which exist in a monomeric

form while the [NiF₄]²⁻, [MnF₄]²⁻ and [MnF₅]²⁻ ions built extended structures.³⁷⁻³⁹ Therefore, the typical oligomerization is prevented. In the case of the gold-based teflate compound, [Au(OTeF₅)₃]₂, dimers are formed in the solid state, whereas AuF₃ shows a helical polymeric structure.⁴⁰,⁴¹

Remarkably, the negative charge of the teflate group is distributed over the whole periphery of the ligand leading to very weak intermolecular interactions. Therefore, neutral teflate compounds such as B(OTeF₅)₃, Ti(OTeF₅)₄, U(OTeF₅)₆ and Xe(OTeF₅)₂ possess a high vapor pressure and are sublimable at moderate temperatures despite their size and high molecular weight.⁹,¹²,¹⁷,⁴² Combined with its high oxidation stability, these properties render the OTeF₅ group ideal for the application in weakly coordinating anions (WCAs), as described in Section 2.3.2).⁴³,⁴⁴

Reports discussing the stability of the teflate group toward bases or electrophiles are scarce in the literature. Teflic acid is prone to hydrolysis when dissolved in water.² Tötsch and Sladky conducted a detailed study on the hydrolysis of TeF₆ and found that the hydrolysis of HOTEF₅ to cis-TeF₄(OH)₂ proceeds rapidly.⁴⁵ In contrast, the hydrolysis of cis-tetrafluorotelluric acid to *fac*- and *mer*-TeF₃(OH)₃ requires several days. For the subsequent hydrolysis step, only the configuration is discussed in the literature without specifying reaction times. Ultimately, TeF(OH)₅ readily hydrolyzes or disproportionates to Te(OH)₆ and cis-TeF₂(OH)₄. The reaction mechanism likely involves tellurium in a coordination number of 7.⁴⁶⁻⁴⁹

Based on these observations, we generally avoid alcohols as solvents in our work with teflate compounds. From our experience in this field, we have concluded that hydrolysis occurs primarily when excess water is added to the reaction mixture. For instance, the anion [Al(OTeF₅)₄]⁻ forms aqua complexes before decomposition. We have crystallized two unpublished examples: Na[Al(H₂O)₂(OTeF₅)₄] and K₂[Al(H₂O)(OTeF₅)₅].

Other Lewis bases are not known to induce decomposition of the teflate ligand. Only for the related systems TeF₆, MeOTeF₅,
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Computed electrostatic potentials plotted on the electron density (BP86/def2-SVP level, isovalue: 0.025 e bohr⁻³) using ORCA 5.0.4<sup>80</sup> visualized with Chemcraft.<sup>81</sup></figcaption></figure>

and (MeO)₂TeF₄, the addition of one fluoride (from reactions with [NMe₄]F) has been reported, resulting in hepta-coordinated tellurium.<sup>48,49</sup>

The aluminate [Al(OTeF₅)₄]⁻ is unfortunately not resistant to the strongest class of electrophiles<sup>50</sup>—silylum cations. Attempts in our group toward the synthesis of alkylated silylum cations [SiEt₃]<sup>+</sup> suggest that they may form at low temperatures but readily react to produce Et₃SiOTeF₅.<sup>51</sup> Other strong electrophiles, such as protons, carbocations, or halonium ions, have been reported as salts of teflate-based WCAs and are discussed in Section 2.3.2.

Another significant observation from our research over recent years is that reducing agents, such as RₓSiHᵧ (x = 3, 2, 1, 0; y = 4-x) compounds or hydrides, tend to reduce the OTeF₅ group to elemental tellurium. This reductive decomposition pathway represents an important limitation when designing synthetic approaches involving teflate-containing compounds.

The teflate group is readily characterized by ¹⁹F NMR spectroscopy. The fluorine atom in the *trans* position to the oxygen atom is magnetically inequivalent to the four fluorine atoms in the *cis* position, giving rise to an AB₄ or A₄B spin system (see Figure 2). The AB₄ pattern can appear as a pseudo quintet and pseudo doublet as in the case of AgOTeF₅, but often the close values of δ(¹⁹Fₐ) and δ(¹⁹Fₑ) result in a highly convoluted pattern of higher order as in HOTeF₅ and B(OTeF₅)₃. It is also possible that the A and B part almost fully coincide as in the spectrum of the [Sb(OTeF₅)₆]⁻ anion. In those cases, the satellites of the ¹²³Te and ¹²⁵Te nuclei (0.91 and 7.14% natural abundance) with characteristic ¹J(F–Te) coupling constants in the range of 2500 to 3700 Hz are more informative. The ²J(Fₐ⁻Fₑ) coupling constant is usually in the range of 175–190 Hz.

**2.3.1. Lewis Acidity.** The high electronegativity of fluorine allows the synthesis of very strong Lewis acids. A well-known example is SbF₅, which is regarded as one of the strongest conventional Lewis acids. One of the most commonly used methods to measure the Lewis acidity of a given compound is the so-called fluoride ion affinity (FIA). This affinity is defined as the negative reaction enthalpy of the adduct formation of a given Lewis acid and a fluoride ion and can be easily obtained by quantum-chemical calculation.<sup>52,53</sup> Other typical experimental methods for estimating Lewis acidity are the Gutmann–Beckett method or the formation of e.g. nitrile adducts. For more details on these techniques along with an overview of known Lewis superacids we refer to a recently published review article about this topic.<sup>54</sup>

The substitution of fluoride ligands with bulkier electron-withdrawing groups is an effective strategy to obtain even stronger Lewis acids. This becomes most evident when comparing the corresponding FIA values (see Figure 3). The Lewis acidity of element halides is usually outperformed by oxygen-bound Lewis acids such as triflate, perfluoro-tert-butyl alkoxide or teflate compounds. Triflate-based Lewis acids can suffer from bidentate binding of the SO₃CF₃ group toward the central atom which explains comparably low FIA values in the case of aluminum and antimony compounds.<sup>52</sup>

Another significant factor for these high Lewis acidities, besides the electron-withdrawing properties of the teflate group, is the reduced lone pair back bonding of the O atoms of the teflate group toward the electron deficient central element, in contrast to direct element-fluorine bonds. Still, the OTeF₅ ligand, like all oxygen-bound ligands, can delocalize electron density from the free electron pairs at the oxygen atom to the central element.

Several Lewis acids based on teflates have been already reported, such as B(OTeF₅)₃, As(OTeF₅)₅ and more recently, the compounds Al(OTeF₅)₃ and Ga(OTeF₅)₃, which possess exceptionally high FIA values.<sup>26,55</sup> In the solid state, the Al and Ga teflate have a dimeric structure, which can be easily converted into soluble solvent adducts by the addition of appropriate donor solvents.<sup>26,56</sup> Depending on the donor solvent, its Lewis acidity will, of course, be lowered.

The compound Sb(OTeF₅)₅ is assumed to be one of the strongest possible neutral Lewis acids, but its synthetic preparation as a neat species or as a solvent adduct remains still elusive.<sup>15,18</sup> For the lower homologue, the 5-fold coordinated As(OTeF₅)₅, several procedures were reported in literature and it is expected to show a remarkable acidity.<sup>15,52,57</sup> The potential of this compound is still underexplored, since only one report shows the usage of As(OTeF₅)₅ as a reactant for the synthesis of the arsenium cations [AsCl₄]⁺ and [AsBr₄]⁺.<sup>58</sup> Analogous strong Lewis acids can also be prepared by combining teflates with transition metals, as it was shown by the synthesis of [Au(OTeF₅)₃]₂. While its first synthesis was already reported in 1985,<sup>40</sup> a recent publication demonstrates an alternative synthetic approach and is more focused on the Lewis acidic properties of this species.<sup>24</sup>

**2.3.2. Weakly Coordinating Anions with Teflate Groups.** Due to the low polarizability of fluorine and based on its small size combined with the high electronegativity, anions with fluorine atoms at the periphery of the molecule are excellent candidates for the formation of weakly coordinating
anions. Several review articles on that topic have been published in the past,⁵⁹⁻⁶² but the teflates only played a minor role, possibly due to their comparably complex synthesis and their expensive starting materials. Still, teflate-based WCAs are of scientific interest due to their high stability toward strong oxidizers. In the 1980s and 1990s, several tetra- and hexacoordinate anions of group 5, 13, and 15 elements have been investigated, namely [B(OTeF₅)₄]⁻ and [M(OTeF₅)₆]⁻ (with M = As, Nb, Sb, Ta, and Bi).⁴⁴,⁵⁷,⁶³⁻⁶⁵ More recently the heavier analogues of the borate ion, [Al(OTeF₅)₄]⁻ and [Ga(OTeF₅)₄]⁻ have been reported.⁵⁵,⁶⁶ Especially the [Al(OTeF₅)₄]⁻ and [Sb(OTeF₅)₆]⁻ anions have been used to stabilize strong oxidizing cations such as the xenonium ions [Xe(OTeF₅)(SO₂ClF)]⁺, [Xe(OTeF₅)(C₅F₅N)]⁺, and [Xe(OTeF₅)(C₅H₃F₂N)]⁺ or carbocations like [CCl₃]⁺, [CBr₃]⁺, [CFCl₂]⁺, [CFBr₂]⁺, [C(OTeF₅)₃]⁺, [CBr(OTeF₅)₂]⁺, and [C(C₆F₇)₃]⁺.⁴³,⁶⁷⁻⁷¹ Furthermore, the aluminate has been utilized for the formation of different reactive cations such as [C₆H₇]⁺, [C₉H₁₃]⁺, [P₄H]⁺, [NO]⁺, [NOP₄]⁺, [ClMe₂]⁺, [(CF₃CH₂)₂Cl]⁺, [C₅F₆NH]⁺, [C₅F₄ClNH]⁺, [(C₅F₅N)₂H]⁺, and [(C₅Cl₅N)₂H]⁺.⁸¹,⁵⁵,⁷²⁻⁷⁵ The examples of protonated arenes and pyridines demonstrate the robustness of the anion against the smallest electrophile, the proton. Labile cationic complexes, such as the acetylene, ethene or P₄ complexes of silver(I) ions⁷⁶⁻⁷⁸ or the recently reported transition metal carbonyl cations,⁷⁹ have not been realized so far with teflate-based WCAs. Still, teflates possess a well delocalized negative charge over the 20 or 30 fluorine atoms and could be useful in this regard. A common method to visualize the coordination ability of WCAs is the depiction of the calculated electrostatic potential (ESP) plotted on the electron density. In Figure 4, the more negatively polarized regions in the fluorides ([BF₄]⁻, [AlF₄]⁻ and [SbF₆]⁻) are compared with the teflate-based anions, where the charge delocalization is clearly distinguishable. Within the group of teflates the negatively polarized areas decrease in the row [B(OTeF₅)₄]⁻ > [Al(OTeF₅)₄]⁻ > [Sb(OTeF₅)₆]⁻, indicated by the change of orange and yellow areas to more yellow and green areas at the fluorine atoms.

### 2.3.3. Ligand Properties.
The recent synthesis of homoleptic 3d metal complexes based on Ni and Co has made it possible to further understand the properties of the teflate ligand in coordination chemistry, proving that it behaves like the fluoride also in ligand-field terms.³⁴,³⁶ In fact, [NEt₄]₂[Ni(OTeF₅)₄] and [NEt₄]₂[Co(OTeF₅)₄] represent analogues of the polymeric [MF₄]²⁻ species (M = Ni, Co) but with monomeric structures. Through a combination of magnetic and electronic spectroscopy measurements on [NEt₄]₂[Ni(OTeF₅)₄], the ligand-field properties of the teflate ligand indicate a weak/medium-field character. Additionally, the inferred value for the ligand-field parameter allowed it to be included in the spectrochemical series at a similar position as the fluoride ion and next to the fluorinated O-donor ligands [OC₄F₉]⁻ and [OC₆F₅]⁻.³⁴,⁸² In the nephelauxetic series of ligands a Racah parameter of *B* = 896 cm⁻¹ of the OTeF₅ group positions it before the heavier halides and again next to [OC₄F₉]⁻ and [OC₆F₅]⁻ ligands. In the case of the [Co(OTeF₅)₄]²⁻ anion, a comparative bonding study with the hypothetical monomeric [CoF₄]²⁻ anion through the interacting quantum atoms (IQA) energy decomposition scheme⁸³ demonstrated a similar strength in the Co-X (X = OTeF₅, F) bonds.³⁶ This reinforces the similar electron-withdrawing properties of both groups.

## 2.4. Selenium-Based Derivatives

Already in 1959 the syntheses of the pentafluoroselenium hypofluorite FOSeF₅ and the peroxide F₅SeOOSeF₅ starting from the fluorination of SeO₂ or SeOCl₂ were reported.⁸⁴ The reaction is accompanied by the formation of SeOF₂ and SeF₆. A more selective reaction uses Hg(OTeF₅)₂ and fluorine in different molar ratios.⁸⁵ In case of a molar ratio of Hg-(OSeF₅)₂:F₂ = 1:1, the corresponding peroxide is formed. If the amount of fluorine is further increased to a ratio of 1:2.6, the hypofluorite can be formed. The mercury(II) pentafluoroothoselenate is also a useful starting material for the introduction of OSeF₅ ligands.¹⁸,²³,⁸⁵⁻⁸⁷ It is slowly formed by the reaction of HOSeF₅ with HgF₂⁸⁸ and can be used for the synthesis of As(OTeF₅)₃¹⁸ CH₉(OTeF₅)₄ₙ (n = 0, 1, 3),⁸⁶ CIOSeF₅,⁸⁵ F₅SeOCN,⁸⁹ F₂Se(OTeF₅)₂,⁷⁷ F₅SeO-COCF₃,⁹⁰ F₅SeOOSeF₅,⁸⁵ NO₂(OTeF₅),²³ POF₂(OTeF₅),¹⁸ OSe-(OSeF₅)₂,²³ CrO₂(OTeF₅)₂⁴² and VO(OTeF₅)₃.⁴² The corresponding acid HOSeF₅ is another synthetically very useful synthon. Three different preparation routes have been reported.⁹¹⁻⁹³ The most favorable is the reaction of SeO₂F₂ with HF. The pentafluoroorthoselenic acid was used to synthesize e.g. the alkaline metal salts MOSeF₅ (M = Li, Na, K, Rb, Cs), the silane Me₃SiOSeF₅, the xenon compound Xe(OTeF₅)₂ as well as the above-mentioned Hg(OTeF₅)₂ compound.⁸⁸,⁹⁴,⁹⁵

Unfortunately, the selenium compounds in oxidation state +VI are not as stable as the tellurium-based compounds, due to the *d*-block contraction caused by the additional 3d¹⁰ electrons.⁹⁶ Therefore, the selenium of the OSeF₅ group is easily reduced to the oxidation state +IV and acts as a better oxidizer.¹ For instance, HOSeF₅ readily oxidizes alkali chlorides¹,⁸⁸,⁹³ and HCl⁹⁴ to chlorine, is observed to attack glass and metal apparatuses⁹³ and thermally decomposes⁹¹ under the formation of selenium tetrafluoride. Chlorine formation is also reported when Hg(OTeF₅)₂ is reacted with TiCl₄ and WCl₆.⁴² The desired substitution products Ti-(OSeF₅)₄ and W(OSeF₅)₆ are not stable and undergo a formal elimination of SeOF₄. The pentavalent compound dimerizes above –100 °C to diselenium dioxide octafluoride Se₂O₂F₈.⁹⁴,⁹⁷ The colorless liquid Se₂O₂F₈ is also formed in the synthetic attempts toward Se(OSeF₅)₄⁸⁷ from SeCl₄ and Hg(OSeF₅)₂ or during storage of the instable As(OSeF₅)₃.¹⁸ In contrast, the corresponding teflate species As(OTeF₅)₃, Ti(OTeF₅)₄ and W(OTeF₅)₆,⁹⁵ except for Se(OTeF₅)₄, are synthetically available. Since fundamental starting materials for the introduction of the OSeF₅ group are either too reactive (HOSeF₅) or not accessible (B(OSeF₅)₃ or AgOSeF₅), only a limited number of OSeF₅-based compounds have been realized and are summarized in Table 1.

## 2.5. Sulfur-Based Derivatives

In contrast to the OTeF₅ and OSeF₅ groups, most of the reported OSF₅ compounds can be found bearing an organic backbone. This moiety is found as alkyl and aryl derivatives and these are mainly obtained from highly reactive reagents prepared from scarcely available thionyl tetrafluoride (SOF₄).¹⁰⁹⁻¹¹¹ To prepare these alkyl derivatives, hypochlorite (ClOSF₅) and hypofluorite (FOSF₅) are used and are respectively obtained from direct treatment of chlorine monofluoride (CIF) and fluorine with SOF₄ (see Scheme 2A).¹¹²,¹¹³ With these reagents in hand, an addition on halogenated alkenes can be performed.¹¹⁴⁻¹¹⁷ In the case of aryl-OSF₅ species, the first reported procedure required the pentafluorooxosulfate peroxide
<table><caption>Table 1. Overview of OSeF<sub>5</sub> Compounds</caption><thead><tr><th>Group</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="2">1</td><td>HOSeF<sub>5</sub></td><td>91–93</td></tr><tr><td>MOSeF<sub>5</sub> (M = Li, Na, K, Rb, Cs)</td><td>87,88</td></tr><tr><td rowspan="8">14</td><td>CH<sub>3</sub>OSeF<sub>5</sub></td><td>21,86</td></tr><tr><td>CH<sub>2</sub>(OSeF<sub>5</sub>)<sub>2</sub></td><td>86</td></tr><tr><td>CH(OSeF<sub>5</sub>)<sub>3</sub></td><td>86</td></tr><tr><td>C(OSeF<sub>5</sub>)<sub>4</sub></td><td>86,87</td></tr><tr><td>F<sub>5</sub>SeO-COCl</td><td>86</td></tr><tr><td>F<sub>5</sub>SeOCN</td><td>87,89</td></tr><tr><td>F<sub>5</sub>SeO-COCF<sub>3</sub></td><td>90</td></tr><tr><td>C<sub>3</sub>F<sub>9</sub>OSeF<sub>5</sub></td><td>98</td></tr><tr><td rowspan="3">15</td><td>F<sub>5</sub>SeO-COF</td><td>98</td></tr><tr><td>Me<sub>3</sub>SiOSeF<sub>5</sub></td><td>94</td></tr><tr><td>NH<sub>4</sub>[OSeF<sub>5</sub>]</td><td>88</td></tr><tr><td rowspan="3">16</td><td>[NO<sub>2</sub>][OSeF<sub>5</sub>]/NO<sub>2</sub>(OSeF<sub>5</sub>)</td><td>23</td></tr><tr><td>POF<sub>3</sub>(OSeF<sub>5</sub>)</td><td>18</td></tr><tr><td>As(OSeF<sub>5</sub>)<sub>3</sub></td><td>18</td></tr><tr><td rowspan="7">17</td><td>F<sub>5</sub>SeOSeF<sub>5</sub></td><td>85,87,98–101</td></tr><tr><td>F<sub>5</sub>SeOOSeF<sub>5</sub></td><td>84,85,87,99</td></tr><tr><td>F<sub>5</sub>SeSO<sub>2</sub>-OSO<sub>2</sub>F</td><td>90</td></tr><tr><td>F<sub>5</sub>SeOSF<sub>5</sub></td><td>98</td></tr><tr><td>F<sub>5</sub>SeSO<sub>2</sub>F</td><td>99</td></tr><tr><td>cis-BrCH<sub>2</sub>-SF<sub>4</sub>-OSeF<sub>5</sub></td><td>102</td></tr><tr><td>F<sub>2</sub>Se(OSeF<sub>5</sub>)<sub>2</sub></td><td>87</td></tr><tr><td>OSe(OSeF<sub>5</sub>)<sub>2</sub></td><td>23,87</td></tr><tr><td rowspan="8">18</td><td>FOSeF<sub>5</sub></td><td>84,85,87,98</td></tr><tr><td>CIOSeF<sub>5</sub></td><td>85</td></tr><tr><td>BrOSeF<sub>5</sub></td><td>85</td></tr><tr><td>Br(OSeF<sub>5</sub>)<sub>3</sub></td><td>85</td></tr><tr><td>Rb[Br(OSeF<sub>5</sub>)<sub>4</sub>]</td><td>23</td></tr><tr><td>I(OSeF<sub>5</sub>)<sub>3</sub></td><td>85</td></tr><tr><td>F<sub>n</sub>I(OSeF<sub>5</sub>)<sub>5-n</sub></td><td>21</td></tr><tr><td>Xe(OSeF<sub>5</sub>)<sub>2</sub></td><td>87,103–107</td></tr><tr><td>XeF(OSeF<sub>5</sub>)</td><td>103,106,107</td></tr><tr><td>[Xe(OSeF<sub>5</sub>)][AsF<sub>6</sub>]</td><td>106</td></tr><tr><td>Xe(OSeF<sub>5</sub>)(OTeF<sub>5</sub>)</td><td>5,107</td></tr><tr><td rowspan="4">transition metal</td><td>VO(OSeF<sub>5</sub>)<sub>3</sub></td><td>42</td></tr><tr><td>CrO<sub>2</sub>(OSeF<sub>5</sub>)<sub>2</sub></td><td>42</td></tr><tr><td>[Mn(CO)<sub>5</sub>(OSeF<sub>5</sub>)]</td><td>108</td></tr><tr><td>[Re(CO)<sub>5</sub>(OSeF<sub>5</sub>)]</td><td>108</td></tr><tr><td></td><td>Hg(OSeF<sub>5</sub>)<sub>2</sub></td><td>87,88</td></tr></tbody></table>

F<sub>5</sub>SOOSF<sub>5</sub> (see Scheme 2B),<sup>118</sup> which can be prepared via several methods like the reaction of FOSF<sub>5</sub> with SOF<sub>4</sub> or SOF<sub>2</sub> or by photolysis of FOSF<sub>5</sub>.<sup>119,120</sup> In 2021, Haupt and co-workers explored the reactivity of the pentafluorooxosulfate silver salt (AgOSF<sub>5</sub>) that was synthesized from AgF and SOF<sub>4</sub>.<sup>110</sup> The silver salt is only stable in MeCN solution and was utilized for the formation of three first-row transition metal complexes bearing the OSF<sub>5</sub> ligand, namely, [(bpy)<sub>2</sub>Ni(OSF<sub>5</sub>)<sub>2</sub>] and [(bpy)<sub>2</sub>Cu(OSF<sub>5</sub>)<sub>n</sub>] (n = 1, 2; bpy = 2,2'-bipyridine) through a metathesis reaction. Furthermore, AgOSF<sub>5</sub> was reported to react with halides to CuOSF<sub>5</sub>, [NEt<sub>3</sub>Me][OSF<sub>5</sub>], [PPH<sub>4</sub>][OSF<sub>5</sub>] and [PNP][OSF<sub>5</sub>] (PNP = bis(triphenylphosphine)iminium) again via metathesis reactions.<sup>109</sup> The poor nucleophilicity of the

<caption>Scheme 2. Overview of Synthetic Routes toward Organic OSF<sub>5</sub> Species (OTf = triflate, DBDHM = 1,3-dibromo-5,5-dimethylhydantoin)</caption>

### synthetic routes toward organic OSF<sub>5</sub> derivatives

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

[OSF<sub>5</sub>]<sup>-</sup> anion was demonstrated with an inefficient addition (see Scheme 2C) at an *in situ* generated aryne (12% yield).<sup>110</sup> Recently, a solution of AgOSF<sub>5</sub> in MeCN was synthesized from commercially available SOCl<sub>2</sub>, AgF<sub>2</sub> and [NEt<sub>4</sub>]Cl (ratio SOCl<sub>2</sub>:AgF<sub>2</sub>:[NEt<sub>4</sub>]Cl = 1:16:2) and reacted with primary amines to sulfurimidoyl fluorides.<sup>121</sup> The excess of AgF<sub>2</sub> was reduced when AgF instead of tetraethylammonium chloride was added to the reaction mixture (ratio SOCl<sub>2</sub>:AgF<sub>2</sub>:AgF = 1:2.5:1.2).<sup>122</sup> Then, the pentafluorooxosulfate anion was reacted with dibenzobromolium and dibenzochlorolium salts to yield a set of new aryl-OSF<sub>5</sub> species (see Scheme 2D). The lipophilicity of those compounds was investigated through reverse phase HPLC (HPLC = high-performance liquid chromatography) and gave the following order of lipophilicity when compared to related fluorinated derivatives: SCF<sub>3</sub> > OSF<sub>5</sub> > OCF<sub>3</sub> ≈ SF<sub>5</sub> > CF<sub>3</sub>. Finally, Pitts et al. used the acetonitrile solution of AgOSF<sub>5</sub> to functionalize cyclobutanes with an OSF<sub>5</sub> group (see Scheme 2E) and discussed the products' potential as bioisosteres of para-substituted benzene rings.<sup>123</sup>

Until now, the limited availability of starting materials has prevented chemists from the synthesis of OSF<sub>5</sub>-based compounds. Besides the comparably new synthon AgOSF<sub>5</sub> (in acetonitrile solutions), only the peroxide (OSF<sub>5</sub>)<sub>2</sub>,<sup>119</sup> the fluorosulfonyl FSO<sub>3</sub>SF<sub>5</sub>,<sup>124</sup> and the hypofluorite FOSF<sub>5</sub><sup>125</sup> are found to be handleable reactants. The Brønsted acid HOSF<sub>5</sub> already decomposes at -78 °C to HF and SOF<sub>4</sub>.<sup>126</sup> Metal salts of the [OSF<sub>5</sub>]<sup>-</sup> anion are prone to metal fluoride formation and SOF<sub>4</sub> elimination.<sup>127</sup> The hypochlorite ClOSF<sub>5</sub> is only stable for
Scheme 3. Overview of the Synthetic Routes toward Organic Derivatives of the OTeF₅ Group and Reactivity of B(OTeF₃(C₆F₅)₂)₃

<figure><img src="image_19.png" /><figcaption>B) synthesis of compounds containing the trans-PhTeF₄O moiety</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>C) synthesis of compounds containing the trans-(C₆F₅)₂TeF₃O moiety</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>D) synthesis of B[OTeF₃(C₆F₅)₂]₃ and its use as a OTeF₃(C₆F₅)₂ transfer reagent</figcaption></figure>

<figure><img src="image_22.png" /></figure>

a limited amount of time even stored at lower temperature (−40 °C) and decomposes to its starting materials ClF and SOF₄.

Due to the few examples of OSF₅-bearing compounds, little is known about their toxicity. Only in the case of the oxide F₅SOSF₅ a provisional operational limit of 0.5 ppm was discussed.¹²⁸

2.6. Derivatives of the Teflate Group

Despite the advantageous properties of the pentafluoro-
orthotellurate group that allowed the synthesis of a variety of
different species presented in this review, the teflate ligand
unfortunately shows some drawbacks. These include the
occasionally observed sensitivity to hydrolysis leading to the
formation of decomposition products (e.g., HF), as well as the
necessity of special starting materials (Te(OH)₆ and HSO₃F)
for the synthesis of the most commonly used teflate source
HOTeF₅.¹,⁷

Consequently, a modification of the teflate ligand and in
particular the substitution of some of the fluorine atoms of the
OTeF₅ unit by (perfluoro)aryl groups was sought. The reactivity
of the phenyltellurium(VI) compound PhTeF₅ and its
derivatives trans-Ph₂TeF₄ and mer-Ph₃TeF₃ toward alcohols,

amines and silicon reagents had been already studied in 1989.¹²⁹
However, the obtained teflate derivatives have never been
employed, most likely due to the inconvenient synthesis of the
starting material PhTeF₅ and its derivatives at that time.¹³⁰⁻¹³³
In 2019 an easy-to-handle system for the oxidative fluorination
of diaryltellurides consisting of trichloroisocyanuric acid
(TCICA), KF and trifluoroacetic acid was reported by Togni,
Pitts et al.¹³⁴,¹³⁵ This system made the synthesis of a broad scope
of TeF₅-substituted arenes, including PhTeF₅, attainable. The
latter was found to be air-stable, but hydrolyzes quantitatively to
the cis-PhTeF₄OH compound in an acetonitrile/water mix-
ture,¹³⁴ and therefore entails the first organotellurium derivative
of HOTeF₅, which was easily accessible.

This outcome was used as starting point for the synthesis of
two different sets of air-stable aryl derivatives of the teflate
group. The preparation of cis-PhTeF₄OH was improved and
further transformed into the corresponding silver salt, which
could be used as a cis-PhTeF₄O transfer reagent in its reaction
with [PPh₄]Cl (see Scheme 3A).¹³⁶

An approach for the synthesis of the trans-PhTeF₄O moiety
was made by Dielmann et al. with the use of an oxidative
fluorination of diphenylditelluride by a $[SF_5]^{-}$ salt to yield the elusive $[PhTeF_4]^{-}$ anion. Further oxidation of this species led to the formation of the *trans-*-[PhTeF$_4$O]⁻ anion, which could be subsequently alkylated to *trans*-PhTeF$_4$OR (R = alkyl, allyl, benzyl) (see Scheme 3B).¹³⁷

Furthermore, the synthesis of a perfluorinated derivative *trans*-(C$_6$F$_5$)$_2$TeF$_3$OH was achieved by a selective hydrolysis of *trans*-(C$_6$F$_5$)$_2$TeF$_4$, which can be prepared through fluorination of (C$_6$F$_5$)$_2$Te using the TCICA/KF oxidation system, as shown in Scheme 3C. Quantum-chemical calculations revealed a higher Brønsted acidity and robustness against fluoride abstraction of the *trans*-(C$_6$F$_5$)$_2$TeF$_3$OH compared to the phenyl analogue.

The teflic acid derivative *trans*-(C$_6$F$_5$)$_2$TeF$_3$OH (*trans*-(C$_6$F$_5$)$_2$TeF$_3$O moiety simplified as OTeF$_3$-(C$_6$F$_5$)$_2$) was then applied for the syntheses of B-(OTeF$_3$-(C$_6$F$_5$)$_2$)$_3$ and Al-(OTeF$_3$-(C$_6$F$_5$)$_2$)$_3$.¹³⁸ The boron Lewis acid was realized by reacting HOTeF$_3$-(C$_6$F$_5$)$_2$ with BCl$_3$ or BCl$_3$.SMe$_2$ (see Scheme 3D).¹³⁹ This Lewis acid has a high thermal stability up to 300 °C and one of the most sterically encumbered boron centers known in the literature (see Figure 5). The Lewis acidity of this

<figure><img src="image_23.png" /><figcaption>**Figure 5.** Molecular structure (left, perfluorophenyl substituents are shown as wireframes) and space-filling model (right) of B-(OTeF$_3$-(C$_6$F$_5$)$_2$)$_3$.¹³⁹</figcaption></figure>

compound, evaluated by theoretical (fluoride ion affinity and global electrophilicity index) and experimental (Gutmann-Beckett, $\nu$(CN)) methods, was found to be comparable to that of B-(C$_6$F$_5$)$_3$ and slightly lower than that of the related B-(OTeF$_5$)$_3$. The affinity of B-(OTeF$_3$-(C$_6$F$_5$)$_2$)$_3$ toward pyridine was evaluated using isothermal titration calorimetry (ITC), which confirmed the above-mentioned trend in Lewis acidity. Inspired by the reactivity of B-(OTeF$_5$)$_3$, the use of B-(OTeF$_3$-(C$_6$F$_5$)$_2$)$_3$ as a ligand-transfer reagent was first demonstrated by the reaction with [NMe$_4$][OTeF$_3$-(C$_6$F$_5$)$_2$]. Starting from the corresponding fluoro-compounds and following a similar procedure, the boron Lewis acid was then used in the synthesis of the Au(III) species [PPh$_4$] [(CF$_3$)$_3$Au(OTeF$_3$-(C$_6$F$_5$)$_2$)], and furthermore in the preparation of a Togni-type hypervalent iodine compound (see Scheme 3D). Teflate-based derivatives and their compounds are gathered in Table 2.

### 3. GROUP 1 TEFLATES

Alkali metal teflates MOTeF$_5$ (M = Li, Na, K, Rb, Cs) can all be prepared in a similar manner following a method introduced by Sladky et al.¹¹,¹⁴⁰,¹⁴¹ This reaction relies on the condensation of a 10-fold excess of HOTeF$_5$ onto the respective alkali metal chloride MCl and warming the reaction mixture to room temperature, whereby evolution of HCl gas takes place (see eq

<table><caption>**Table 2.** Summary of Teflate-Based Derivatives</caption><thead><tr><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td>PhTeF<sub>4</sub>X, Ph<sub>2</sub>TeF<sub>3</sub>X, Ph<sub>3</sub>TeF<sub>2</sub>X (X = OMe, NMe<sub>2</sub>, NEt<sub>2</sub>)</td><td>129</td></tr><tr><td>[cat][trans-PhTeF<sub>4</sub>O] (cat = Ag, PPh<sub>4</sub>)</td><td>137</td></tr><tr><td>trans-PhTeF<sub>4</sub>OR (R = alkyl, allyl, benzyl)</td><td>137</td></tr><tr><td>PhTeF<sub>5</sub>, cis-PhTeF<sub>4</sub>OH</td><td>134–136</td></tr><tr><td>trans-(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>TeF<sub>3</sub>OH</td><td>136</td></tr><tr><td>[PPh<sub>4</sub>][cis-PhTeF<sub>4</sub>O]</td><td>136</td></tr><tr><td>K[OTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]</td><td>136</td></tr><tr><td>[NMe<sub>4</sub>][OTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]</td><td>139</td></tr><tr><td>[PPh<sub>4</sub>][OTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]</td><td>139</td></tr><tr><td>B(OTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)<sub>3</sub></td><td>139</td></tr><tr><td>Al(OTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)<sub>3</sub></td><td>138</td></tr><tr><td>[PPh<sub>4</sub>][(F<sub>3</sub>O)<sub>3</sub>AuOTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]</td><td>139</td></tr><tr><td>C<sub>9</sub>H<sub>10</sub>OIOTeF<sub>3</sub>(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub></td><td>139</td></tr></tbody></table>

3). After removal of the excess of acid under reduced pressure white, crystalline solids are obtained.

$$ \mathrm{MCl} + 10\mathrm{HOTeF_5} \xrightarrow[-9\mathrm{HOTeF_5}]{\mathrm{M} = \mathrm{Li, Na, K, Rb, Cs}} \mathrm{MOTeF_5} + \mathrm{HCl} \quad (3) $$

For KOTeF$_5$, RbOTeF$_5$, and CsOTeF$_5$ this reaction is quantitative due to the good solubility of the respective chloride salt in HOTeF$_5$. However, in the case of LiOTeF$_5$ and NaOTeF$_5$ a full conversion can only be achieved by grinding the reaction mixture after each addition of HOTeF$_5$ or by performing the synthesis in acetonitrile.¹¹ In 1974 Seppelt et al. proposed another route for the synthesis of LiOTeF$_5$, as shown in eq 4.⁹⁴ The lithium salt can be obtained by reacting LiOMe with an equimolar amount of HOTeF$_5$. Nevertheless, the yield is only 38%, as the formed methanol solvates the product and can only be removed in high vacuum at higher temperatures, leading to the decomposition of the product. The sodium salt, on the other hand, can be obtained in almost quantitative yield by the reaction of Me$_3$SiOTeF$_5$ with NaOSiMe$_3$ (see eq 5). All alkali metal salts are white crystalline solids that do not melt and are thermally stable up to at least 250 °C.¹¹,¹⁴⁰ They are nonhygroscopic but slowly hydrolyze under moist air or in aqueous solution. All compounds have been characterized by NMR, IR, and Raman spectroscopy, as well as by powder-XRD.¹¹,¹⁴¹ In fact, it has been shown that all MOTeF$_5$ compounds exhibit the structure of KOsF$_6$, and due to identical unit cell volumes in their $Cs^+$ salts, it could be concluded that $[OTeF_5]^{-}$ has a volume identical to that of the isoelectronic $[SbF_6]^{-}$.¹¹

$$ \mathrm{LiOMe} \xrightarrow[-\mathrm{MeOH}]{\mathrm{HOTeF_5}} \mathrm{LiOTeF_5} \quad (4) $$

$$ \mathrm{NaOSiMe_3} \xrightarrow[-(\mathrm{Me_3Si})_2\mathrm{O}]{\mathrm{Me_3SiOTeF_5}} \mathrm{NaOTeF_5} \quad (5) $$

The respective alkali metal pentafluoroorthoselenates MOSeF$_5$ (M = Li, Na, K, Rb, Cs) are also known. Except for the sodium compound, all can be prepared by the conversion of the alkali metal fluoride with HOSeF$_5$. Pure NaOSeF$_5$ can be obtained by reacting Me$_3$SiOSeF$_5$ with NaOSiMe$_3$.⁸⁸,⁹⁴ Group 1 teflates and selenates are gathered in Table 3.

### 4. GROUP 2 TEFLATES

So far, no earth alkali metal teflate compounds have been described in the literature.
<table><caption>**Table 3.** Overview of Group 1 (Alkali Metal) Teflate and Selenate Compounds</caption><thead><tr><th>Element</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="2">Li</td><td>LiOTeF<sub>5</sub></td><td>11,94</td></tr><tr><td>LiOSeF<sub>5</sub></td><td>88</td></tr><tr><td rowspan="2">Na</td><td>NaOTeF<sub>5</sub></td><td>11,94</td></tr><tr><td>NaOSeF<sub>5</sub></td><td>88,94</td></tr><tr><td rowspan="2">K</td><td>KOTeF<sub>5</sub></td><td>11,140</td></tr><tr><td>KOSeF<sub>5</sub></td><td>88,94</td></tr><tr><td rowspan="2">Rb</td><td>RbOTeF<sub>5</sub></td><td>11,141</td></tr><tr><td>RbOSeF<sub>5</sub></td><td>88</td></tr><tr><td rowspan="2">Cs</td><td>CsOTeF<sub>5</sub></td><td>11,140,141</td></tr><tr><td>CsOSeF<sub>5</sub></td><td>88</td></tr></tbody></table>

# 5. GROUP 13 TEFLATES

## 5.1. Boron Teflates

Boron tris(pentafluoroorthotellurate), B(OTeF<sub>5</sub>)<sub>3</sub>, was first prepared by Sladky et al. through the neat reaction of BCl<sub>3</sub> with 3 equiv of HOTeF<sub>5</sub> at -80 °C.<sup>9</sup> The compound forms hexagonal colorless crystals and can be sublimed at room temperature *in vacuo*.<sup>142</sup> B(OTeF<sub>5</sub>)<sub>3</sub> has a remarkably high thermal stability up to 140 °C, which is unexpected, as comparable compounds containing fluorine atoms in the β-position to the boron center, such as B(OCF<sub>3</sub>)<sub>3</sub> or B(SCF<sub>3</sub>)<sub>3</sub>, decompose above -20 °C by β-fluoride elimination.<sup>54,63,143</sup> As mentioned earlier, B(OTeF<sub>5</sub>)<sub>3</sub> possesses an especially high FIA value of 506 kJ mol<sup>-1</sup> and can therefore be considered as a Lewis superacid.<sup>52,54</sup> In fact, its Lewis acidity was determined calorimetrically to be comparable to that of BCl<sub>3</sub>, and it forms stable adducts with pyridine, tetrahydropyran and acetonitrile.<sup>9</sup> Anyhow, this compound is mainly used as OTeF<sub>5</sub> transfer reagent in metathetical reactions with fluorides, in which the driving force is the formation of the volatile BF<sub>3</sub> compound. B(OTeF<sub>5</sub>)<sub>3</sub> enabled the formation of a variety of different teflate compounds containing several different central atoms such as As,<sup>18</sup> Au,<sup>40</sup> Bi,<sup>44</sup> Cl,<sup>27</sup> F,<sup>144</sup> Nb,<sup>145</sup> Sb,<sup>15</sup> Se,<sup>87</sup> Ta,<sup>145</sup> Te<sup>22,28,57</sup> or Xe<sup>1</sup>. For more reaction details on each formed compound class see the corresponding section.

The corresponding anion [B(OTeF<sub>5</sub>)<sub>4</sub>]<sup>-</sup> was in the past considered as a candidate for the least coordinating anion.<sup>146</sup> Therefore, the possibility to generate weakly coordinated metal and metalloid cations with the aforementioned borate as counterion has been thoroughly studied. The Lewis acid-base reaction of B(OTeF<sub>5</sub>)<sub>3</sub> with MOTeF<sub>5</sub> (M = Ag, Tl) in the weakly coordinating solvents mesitylene (Mes), dichloromethane (DCM), 1,2-dichloroethane (DCE), and 1,1,2-trichloro-1,2,2-trifluoroethane (F-113) generates solutions of [M(solv)<sub>n</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>] (n = 0-2) (see eq 6).<sup>147</sup> The nonsolvated compound Ag[B(OTeF<sub>5</sub>)<sub>4</sub>] is isolated from a F-113 solution, whereas the nonsolvated salt Tl[B(OTeF<sub>5</sub>)<sub>4</sub>] is isolated from DCM or F-113.<sup>147</sup> Both salts, Ag[B(OTeF<sub>5</sub>)<sub>4</sub>] and Tl[B(OTeF<sub>5</sub>)<sub>4</sub>] are thermally unstable, slowly decomposing to the corresponding metal teflate salt and the volatile B(OTeF<sub>5</sub>)<sub>3</sub>. The molecular structure of Ag[B(OTeF<sub>5</sub>)<sub>4</sub>] reveals weak interactions between the Ag<sup>+</sup> cations and the borate anion through three Ag-O contacts.<sup>147</sup> In the case of Tl<sup>+</sup> as counterion, molecular structures in the solid state are described

with either 1,2-dichloroethane or mesitylene. In the first example, the solvent acts as a chelating ligand and, consequently, the salt [Tl(1,2-C<sub>2</sub>H<sub>2</sub>Cl<sub>2</sub>)][B(OTeF<sub>5</sub>)<sub>4</sub>] could be isolated by Strauss et al.<sup>148</sup> The compound [Tl(η<sup>6</sup>-Mes)<sub>2</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>] comprises a chain of [Tl(Mes)<sub>2</sub>]<sup>+</sup> cations and [B(OTeF<sub>5</sub>)<sub>4</sub>]<sup>-</sup> anions connected by weak Tl-F interactions (see Figure 6).<sup>146</sup>

<figure><img src="image_24.png" /><figcaption>**Figure 6.** Molecular structure of [Tl(Mes)<sub>2</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>] in the solid state.<sup>146</sup></figcaption></figure>

$$ \underset{-3\text{HCl}}{\text{BCl}_3} \xrightarrow{3\text{HOTeF}_5} \underset{\text{solv}}{\text{B(OTeF}_5)_3} \xrightarrow{\text{MOTeF}_5} \begin{array}{c} [\text{M(solv)}_n][\text{B(OTeF}_5)_4] \\ \text{M} = \text{Ag, Tl}; n = 0-2 \\ \text{solv} = \text{DCE, DCM, F-113, Mes} \end{array} \qquad (6) $$

However, attempts to generate the cations [Fe(OEP)]<sup>+</sup>, [Fe(TPP)]<sup>+</sup> or [SiPh<sub>3</sub>]<sup>+</sup> by the reaction of Ag[B(OTeF<sub>5</sub>)<sub>4</sub>] or Tl[B(OTeF<sub>5</sub>)<sub>4</sub>] with Fe(OEP)Cl (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate dianion), Fe(TPP)Cl (TPP = 5,10,15,20-tetraphenylporphyrinate dianion) or Ph<sub>3</sub>SiCl in dichloromethane or mesitylene failed. Instead, an OTeF<sub>5</sub> abstraction occurs and B(OTeF<sub>5</sub>)<sub>3</sub> together with [Fe(OEP)(OTeF<sub>5</sub>)], [Fe(TPP)(OTeF<sub>5</sub>)] or Ph<sub>3</sub>SiOTeF<sub>5</sub> are obtained, respectively. The reaction of the tritylium salt [CPh<sub>3</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>] with Ph<sub>3</sub>SiH in dichloromethane was also not successful and produced Ph<sub>3</sub>SiOTeF<sub>5</sub>.<sup>147</sup>

Nevertheless, due to the weakly coordinating character of the [B(OTeF<sub>5</sub>)<sub>4</sub>]<sup>-</sup> anion the first isolation of the silver carbonyl complexes [AgCO]<sup>+</sup> and [Ag(CO)<sub>2</sub>]<sup>+</sup> was possible.<sup>149-151</sup> The nature of these compounds have been comprehensively studied by X-ray crystallography as well as solid-state IR- and multinuclear NMR spectroscopy. Other reactive cations, such as [XeC<sub>6</sub>F<sub>5</sub>]<sup>+</sup>, could also be stabilized through the use of this weakly coordinating anion. This cation has been prepared in a metathesis reaction of [XeC<sub>6</sub>F<sub>5</sub>][BF<sub>4</sub>] with the corresponding M[B(OTeF<sub>5</sub>)<sub>4</sub>] salt (M = K or Cs),<sup>152</sup> which itself has been synthesized by reacting MOTeF<sub>5</sub> with B(OTeF<sub>5</sub>)<sub>3</sub>.<sup>63</sup>

Quaternary ammonium salts also exist with this counterion. The [NMe<sub>4</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>] salt has been synthesized by the reaction of [NMe<sub>4</sub>][OTeF<sub>5</sub>] with B(OTeF<sub>5</sub>)<sub>3</sub>, and was compared to the isoelectronic C(OTeF<sub>5</sub>)<sub>4</sub> species.<sup>153</sup>

## 5.2. Aluminum Teflates

The Lewis superacid Al(OTeF<sub>5</sub>)<sub>3</sub> is synthesized by reacting either triethyl aluminum or trimethyl aluminum with teflic acid in n-pentane, with the use of AlMe<sub>3</sub> leading to a purer product. If the reaction temperature is kept at -40 °C, only two of the three methyl groups of AlMe<sub>3</sub> are substituted by the OTeF<sub>5</sub> ligand
resulting in AlMe(OTeF$_5$)$_2$, which is found to be an oxygen-bridged dimer [AlMe(OTeF$_5$)$_2$]$_2$ in the solid state (see eq 7). When the reaction mixture is allowed to warm up to room temperature, the homoleptic compound Al(OTeF$_5$)$_3$ is formed.⁵⁶ The vibrational analysis in combination with quantum-chemical calculations of this Lewis acid suggests that it exists as oxygen-bridged dimer [Al(OTeF$_5$)$_3$]$_2$.⁵⁵ The dimeric structure can readily be broken up by addition of weakly coordinating solvents such as fluorobenzene or sulfuryl chloride fluoride, forming [Al(OTeF$_5$)$_3$(C$_6$H$_5$F)$_2$] and [Al(OTeF$_5$)$_3$(SO$_2$ClF)$_2$], respectively.⁵⁶ Additionally, several adducts of the Lewis acid containing up to three ligands per aluminum center have been described. These are either synthesized by starting from [Al(OTeF$_5$)$_3$(SO$_2$ClF)$_2$] and exchanging the SO$_2$ClF with diethyl ether (Et$_2$O) or triethylphosphine oxide, or by using the desired ligand as solvent as seen with MeCN and PhCN. Interestingly, the 6-fold coordinated nitrile adducts [Al(OTeF$_5$)$_3$(RCN)$_3$] (R = Me, Ph) autoionize (see eq 8) in solution and coexist with the ion-pair [Al(OTeF$_5$)$_2$(RCN)$_4$][Al(OTeF$_5$)$_4$(RCN)$_2$]. However, in the solid state, [Al(OTeF$_5$)$_3$(PhCN)$_3$] crystallizes as the neutral compound. The autoionization is induced by addition of 2,2'-bipyridine to a [Al(OTeF$_5$)$_3$(PhCN)$_3$] solution, leading to the salt [Al(OTeF$_5$)$_2$(bpy)$_2$][Al(OTeF$_5$)$_4$(bpy)] (see eq 8).

$$2\text{AlMe}_3 + 6\text{HOTeF}_5 \xrightarrow{-4\text{CH}_4} [\text{AlMe}(\text{OTeF}_5)_2]_2 + 2\text{HOTeF}_5 \xrightarrow{-2\text{CH}_4} [\text{Al}(\text{OTeF}_5)_3]_2 \quad (7)$$

$$\begin{align*} \text{AlEt}_3 + 3\text{HOTeF}_5 &\xrightarrow{\text{RCN}} [\text{AlMe}(\text{OTeF}_5)_3(\text{RCN})_3] \\ &\rightleftharpoons \frac{1}{2}[\text{Al}(\text{OTeF}_5)_2(\text{RCN})_4][\text{Al}(\text{OTeF}_5)_4(\text{RCN})_2] \\ &\qquad \text{R = Me, Ph} \tag{8} \end{align*}$$

The toluene adduct of the Lewis acid is accessible by a synthetic detour. First, the protonated methyl benzenium cation and the weakly coordinating [Al(OTeF$_5$)$_4$]$^-$ anion are formed by treating triethyl aluminum with 4 equiv of teflic acid in toluene, (for further information on those Brønsted acids see below). In a second step, addition of Et$_3$SiH leads to the formation of the toluene adduct [Al(OTeF$_5$)$_3$(η$^1$-C$_7$H$_8$)], presumably via a cationic silylum species [SiEt$_3$]$^+$ (see eq 9).

$$\begin{align*} \text{AlEt}_3 + 4\text{HOTeF}_5 &\xrightarrow{\text{toluene}} [\text{H-C}_7\text{H}_8][\text{Al}(\text{OTeF}_5)_4] \xrightarrow{\text{Et}_3\text{SiH}} \\ &[\text{SiEt}_3][\text{Al}(\text{OTeF}_5)_4] \xrightarrow{\text{(not isolable)}} [\text{Al}(\text{OTeF}_5)_3(\text{η}^1-\text{C}_7\text{H}_8)] \tag{9} \end{align*}$$

The Lewis acidity at the Al center of the experimentally observed compounds decreases in the order [Al(OTeF$_5$)$_2$]$_2$ > [Al(OTeF$_5$)$_3$(η$^1$-C$_7$H$_8$)] > [Al(OTeF$_5$)$_3$(SO$_2$ClF)$_2$] > [Al(OTeF$_5$)$_3$(C$_6$H$_5$F)$_2$] > [Al(OTeF$_5$)$_3$(MeCN)$_3$] > [Al(OTeF$_5$)$_3$(PhCN)$_3$] > [Al(OTeF$_5$)$_3$(Et$_2$O)$_2$]. Among these the FIA values of [Al(OTeF$_5$)$_2$], [Al(OTeF$_5$)$_3$(C$_7$H$_8$)], [Al(OTeF$_5$)$_3$(SO$_2$ClF)$_2$], and [Al(OTeF$_5$)$_3$(C$_6$H$_5$F)$_2$] surpass the FIA value of SbF$_5$, thus classifying them as Lewis superacids.⁵³

Interestingly, [Al(OTeF$_5$)$_3$]$_2$ can also be utilized to access an anion-doped aluminum chlorofluoride of the type AlCl$_0.1$F$_2.8$(OTeF$_5$)$_0.1$. This was achieved by treating a mixture of AlCl$_3$ and [Al(OTeF$_5$)$_3$]$_2$ with CCl$_3$F. Further analysis

revealed the highly Lewis acidic nature of this material and its potential for catalyzing dehydrofluorination reactions.¹⁵⁴

The addition of 1 equiv of HOTEF$_5$ to the Lewis acid results in the formation of a conjugated Brønsted/Lewis acid HOTEF$_5$/ Al(OTeF$_5$)$_3$ in analogy to the well-known binary systems HF/ SbF$_5$ and HSO$_3$F/SbF$_5$.¹⁵⁵ Based on the basicity of the used solvent, the Brønsted acidity can be tuned, whereas the use of different arenes is reported.⁵⁵ Experiments showed ortho-difluorobenzene to be a promising candidate for this purpose by combining a low proton affinity (leading consequently to a high Brønsted acidity of its protonated derivative) and a good solubility even at low temperatures. The Brønsted superacid [H-C$_6$H$_4$F$_2$][Al(OTeF$_5$)$_4$]$_{(solv)}$ is synthesized according to Figure 7.

### reactive cations stabilized by [Al(OTeF$_5$)$_4$]$^-$

<figure><img src="image_25.png" /></figure>

### molecular structure and space-filling model of [Al(OTeF$_5$)$_4$]$^-$

<figure><img src="image_26.png" /><figcaption>Figure 7. Synthetic route to the acid [H-C$_6$H$_4$F$_2$][Al(OTeF$_5$)$_4$]$_{(solv)}$ and follow-up chemistry (upper chart). Molecular structure (lower chart) in the solid state of [PPh$_4$][Al(OTeF$_5$)$_4$] (left) and the space-filling representation (right).⁵⁵ Cations are omitted for clarity. Thermal ellipsoids are set at 50% probability.</figcaption></figure>

and can be further reacted with either chloride salts to form corresponding salts of the WCA [Al(OTeF$_5$)$_4$]$^{−}$⁵⁵,¹⁵⁶,¹⁵⁷ or with more basic substrates than ortho-difluorobenzene to protonate these. It was shown that the protonation of arenes such as benzene and mesitylene,⁵⁵ or several pyridines⁵⁵,⁷⁴ can be achieved. Remarkably, even white phosphorus⁷² is protonated, forming a [P$_4$H]$^+$ cation with the proton on a P-P edge of the P$_4$ tetrahedron. Additionally, when C$_5$F$_5$N and C$_5$Cl$_5$N are used in excess, the corresponding pyridine adducts of the pyridinium cations [(C$_5$F$_5$N)$_2$H]$^+$ and [(C$_5$F$_5$N)$_2$H]$^+$ are available.

The in situ generation of the Brønsted superacid in chloromethane gives rise to the strong methylation agent [Cl(Me)$_2$][Al(OTeF$_5$)$_4$]. Hereby, chloromethane is partially protonated and reacts with another equivalent of MeCl to the dimethyl chloronium cation. Several weak bases have been methylated, including MeBr, MeI, PF$_3$, P(CF$_3$)$_3$, C$_5$H$_5$P, 2-
(Me₃Si)C₅H₄P, and 2,3,5,6-(Me₃Si)₄C₅HP. The methylated products are depicted in Figure 8.⁷³,¹⁵⁸

<figure><img src="image_27.png" /><figcaption>**Figure 8.** Methylation products of the reaction of [Cl(CH₃)₂][Al(OTeF₅)₄] with MeBr (a), MeI (b), PF₃ (c), P(CF₃)₃ (d), C₅H₅P (e), 2-(Me₃Si)C₅H₄P (f), and 2,3,5,6-(Me₃Si)₄C₅HP (g). The [Al(OTeF₅)₄]⁻ is omitted for clarity.</figcaption></figure>

In order to form the synthetically useful silver salt Ag[Al(OTeF₅)₄], which has the potential to be used as a halide abstraction or oxidizing agent, the acid [H-C₆H₄F₂][Al(OTeF₅)₄](solv) is reacted with AgF.⁵¹ Other silver salts, containing weakly coordinating anions, have successfully been used in the synthesis of reactive cations.¹⁵⁹⁻¹⁶⁵ Furthermore, the strongly oxidizing [NO]⁺, [NOP₄]⁺, [Xe(OTeF₅)(C₅H₃F₂N)]⁺ and [Xe(OTeF₅)(C₅F₅N)]⁺ salts of the WCA have been prepared.⁵¹,⁷⁰ In the case of the nitrosonium salt, the metathesis reaction of Li[Al(OTeF₅)₄] and [NO][SbF₆] is performed. Upon addition of P₄, [NOP₄][Al(OTeF₅)₄] is formed quantitatively. The xenonium ions are synthesized starting from the Lewis superacid [Al(OTeF₅)₃]₂ and Xe(OTeF₅)₂, followed by the addition of the respective pyridine (see eq 10). This reaction demonstrates on the one hand the high Lewis acidity of [Al(OTeF₅)₃]₂ and on the other hand the robustness of the WCA [Al(OTeF₅)₄]⁻ toward strong oxidizers. Upon addition of an excess of CH₂ClCF₃ to a mixture of [Al(OTeF₅)₃]₂ and Xe(OTeF₅)₂ in SO₂ClF, the chlorine atom of the chlorofluoroalkane is oxidized, leading to the fluorinated dialkyl chloronium salt [Cl(CH₂CF₃)₂][Al(OTeF₅)₄].⁷⁵

$$ \frac{1}{2}[\text{Al(OTeF}_5)_3]_2 + \text{Xe(OTeF}_5)_2 \xrightarrow{\substack{1. \text{SO}_2\text{ClF} \\ 2. \text{py}^F}} [\text{Xe(OTeF}_5)(\text{py}^F)][\text{Al(OTeF}_5)_4] \\ \text{py}^F = \text{C}_5\text{H}_3\text{F}_2\text{N}, \text{C}_5\text{F}_5\text{N} \qquad (10) $$

Interestingly, the tetrahedral coordination sphere of the anion [Al(OTeF₅)₄]⁻ can be further extended. For instance, treatment of [Al(OTeF₅)₃](SO₂ClF)₂ with an excess of [NEt₄][OTeF₅] gives rise to the trigonal bipyramidal dianion [NEt₄]₂[Al(OTeF₅)₅].⁵⁶

### 5.3. Gallium Teflates

The Lewis superacid Ga(OTeF₅)₃ is obtained by the neat reaction of GaCl₃ and ClOTeF₅ with the release of gaseous Cl₂, as shown in eq 11.²⁶ The thermally stable product is a rare

example of an insoluble gallium-based hard and soft Lewis superacid as indicated by its high fluoride and hydride ion affinity. The shift difference in the ³¹P NMR spectra upon coordination of OPPh₃ to the Lewis acid (modified Gutmann–Beckett method) shows that its Lewis acidity is comparable to that of the aluminum-based Lewis superacid Al(OTeF₅)₃. In analogy to this Lewis acid, vibrational analysis in combination with quantum-chemical calculations suggests that Ga(OTeF₅)₃ forms the oxygen-bridged dimer [Ga(OTeF₅)₃]₂ in the solid state. The high Lewis acidity of this compound allows the formation of adducts with weak nucleophiles like pentafluoropyridine C₅F₅N (see Figure 9, left).

<figure><img src="image_28.png" /><figcaption>**Figure 9.** Structures of [Ga(OTeF₅)₃(C₅F₅N)] (left) and Ga₂Et₃(OTeF₅)₃ (right) in the solid state.²⁶,⁶⁶ Thermal ellipsoids are set at 50% probability.</figcaption></figure>

$$ \mathrm{GaCl_3} \xrightarrow[\mathrm{-3Cl_2}]{\mathrm{3ClOTeF_5}} \mathrm{Ga(OTeF_5)_3} \qquad (11) $$

The addition of 1 equiv of HOTeF₅ leads to the formation of a conjugated Brønsted/Lewis acid HOTeF₅/Ga(OTeF₅)₃ as already reported for the aluminum analogue. With diethyl ether this acidic system forms the oxonium salt [H(Et₂O)₂][Ga(Et₂O)₂(OTeF₅)₄] (see eq 12). The high Lewis acidity of Ga(OTeF₅)₃ can further be utilized to access a strong oxidizing system with Xe(OTeF₅)₂ in SO₂ClF, which is able to oxidize chlorine atoms of chloromethane MeCl to give the dimethyl chloronium salt [ClMe₂][Ga(OTeF₅)₄] (see eq 13).²⁶

$$ \mathrm{Ga(OTeF_5)_3} \xrightarrow{\mathrm{HOTeF_5, 4Et_2O}} [\mathrm{H(Et_2O)_2}][\mathrm{Ga(Et_2O)_2(OTeF_5)_4}] \qquad (12) $$

$$ \mathrm{Ga(OTeF_5)_3} \xrightarrow{\mathrm{Xe(OTeF_5)_2, MeCl}} [\mathrm{ClMe_2}][\mathrm{Ga(OTeF_5)_4}] \qquad (13) $$

The corresponding weakly coordinating anion [Ga(OTeF₅)₄]⁻ is prepared as the Ag⁺ salt by the reaction of GaCl₃ with AgOTeF₅, as shown in eq 14. Subsequent reaction of the silver salt with [PPh₄]Cl, and CClPh₃, led to the formation of the [PPh₄]⁺ and the [CPh₃]⁺ salt of [Ga(OTeF₅)₄]⁻.⁶⁶

$$ \mathrm{GaCl_3} \xrightarrow[\mathrm{-3AgCl}]{\mathrm{4AgOTeF_5}} \mathrm{Ag[GA(OTeF_5)_4]} \xrightarrow{\substack{[cat]Cl \\ -AgCl}} [\mathrm{cat}][\mathrm{Ga(OTeF_5)_4}] \\ \mathrm{cat} = \mathrm{PPh_3, CPh_3} \qquad (14) $$

In addition, the partially teflate-substituted anion [GaEt(OTeF₅)₃]⁻ was isolated as several different salts, resulting from the incomplete reaction of GaEt₃ and HOTeF₅. Within the latter process, it was possible to isolate and characterize the reactive species Ga₂Et₃(OTeF₅)₃, which can be described as a contact ion pair between a cationic [GaEt₂]⁺ and an anionic [GaEt(OTeF₅)₃]⁻ fragment (see Figure 9, right). It does not react
<table><caption>Table 4. Overview of Group 13 Teflate Compounds</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="10">B</td><td rowspan="10">III</td><td>B(OTeF<sub>5</sub>)<sub>3</sub></td><td>9,63,142</td><td>[Tl(DCE)][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>148</td></tr><tr><td>B[(OTeF<sub>5</sub>)<sub>3</sub>(py)]</td><td>9</td><td>[Tl(η<sup>6</sup>-Mes)<sub>2</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>146</td></tr><tr><td>B[(OTeF<sub>5</sub>)<sub>3</sub>(THP)]</td><td>9</td><td>[N(nBu)<sub>4</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>146</td></tr><tr><td>B[(OTeF<sub>5</sub>)<sub>3</sub>(MeCN)]</td><td>9,63</td><td>[AgCO][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>149,150</td></tr><tr><td>B[(OTeF<sub>5</sub>)<sub>3</sub>(OPEt<sub>3</sub>)]</td><td>139</td><td>[Ag(CO)<sub>2</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>150,151</td></tr><tr><td>B[(OTeF<sub>5</sub>)<sub>3</sub>(OPPh<sub>3</sub>)]</td><td>26</td><td>[NMe<sub>4</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>153</td></tr><tr><td>Cs[B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>63</td><td>[XeC<sub>6</sub>F<sub>5</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>152</td></tr><tr><td>Ag[B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>147</td><td>[SeF<sub>3</sub>][B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>153</td></tr><tr><td>Tl[B(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>147</td><td></td><td></td></tr><tr><td rowspan="16">Al</td><td rowspan="16">III</td><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>]<sub>2</sub></td><td>56</td><td>[{(C<sub>6</sub>H<sub>4</sub>F<sub>2</sub>)<sub>3</sub>P<sub>2</sub>N}][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>157</td></tr><tr><td>[AlMe(OTeF<sub>5</sub>)<sub>3</sub>]<sub>2</sub></td><td>56</td><td>[CPh<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>55</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(η<sup>1</sup>-C<sub>7</sub>H<sub>8</sub>)]</td><td>56</td><td>[C(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>71</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(OPEt<sub>3</sub>)]</td><td>56</td><td>[C<sub>6</sub>H<sub>7</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>55</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(OPPh<sub>3</sub>)]</td><td>26</td><td>[C<sub>9</sub>H<sub>13</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>55</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(C<sub>6</sub>H<sub>5</sub>F<sub>2</sub>)<sub>2</sub>]</td><td>56</td><td>[P<sub>4</sub>H][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>72</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(Et<sub>2</sub>O)<sub>2</sub>]</td><td>56</td><td>[C<sub>5</sub>F<sub>5</sub>NH][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>74</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(SO<sub>2</sub>CIF)<sub>2</sub>]</td><td>56</td><td>[C<sub>5</sub>F<sub>4</sub>ClNH][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>74</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(MeCN)<sub>3</sub>]</td><td>56</td><td>[[(C<sub>5</sub>F<sub>4</sub>N)<sub>2</sub>H][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>74</td></tr><tr><td>[Al(OTeF<sub>5</sub>)<sub>3</sub>(PhCN)<sub>3</sub>]</td><td>56</td><td>[[(C<sub>5</sub>Cl<sub>5</sub>N)<sub>2</sub>H][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>74</td></tr><tr><td>[H-C<sub>6</sub>H<sub>4</sub>F<sub>2</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]<sub>(solv)</sub></td><td>55</td><td>[Me<sub>2</sub>X][Al(OTeF<sub>5</sub>)<sub>4</sub>] (X = Cl, Br, I)</td><td>73</td></tr><tr><td>[Ag(C<sub>6</sub>H<sub>4</sub>F<sub>2</sub>)<sub>2</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>51</td><td>[X(CH<sub>2</sub>CF<sub>3</sub>)][Al(OTeF<sub>5</sub>)<sub>4</sub>] (X = Cl, I)</td><td>75</td></tr><tr><td>M[Al(OTeF<sub>5</sub>)<sub>4</sub>] (M = Li, Na, K, Rb, Cs)</td><td>51,55</td><td>[MePF<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>73</td></tr><tr><td>[NO][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>51</td><td>[MeP(CF<sub>3</sub>)<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>73</td></tr><tr><td>[P<sub>4</sub>NO][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>51</td><td>[C<sub>5</sub>H<sub>5</sub>PMe][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>158</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>5</sub>F<sub>5</sub>N)][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>70</td><td>[2-(Me<sub>3</sub>Si)C<sub>5</sub>H<sub>4</sub>PMe][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>158</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>5</sub>H<sub>3</sub>F<sub>2</sub>N)][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>70</td><td>[2,3,5,6-(Me<sub>3</sub>Si)<sub>4</sub>C<sub>5</sub>HPMe][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>158</td></tr><tr><td>[NR<sub>4</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>] (R = Me, Et, nPr, nBu)</td><td>156</td><td>[NEt<sub>4</sub>]<sub>2</sub>[Al(OTeF<sub>5</sub>)<sub>3</sub>]</td><td>56</td></tr><tr><td>[NMeEt<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>156</td><td>[Al(OTeF<sub>5</sub>)<sub>2</sub>(bpy)<sub>2</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>(bpy)]</td><td>56</td></tr><tr><td>[PPh<sub>4</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>55</td><td></td><td></td></tr><tr><td rowspan="4">Ga</td><td rowspan="4">III</td><td>Ga(OTeF<sub>5</sub>)<sub>3</sub></td><td>26</td><td>[H(Et<sub>2</sub>O)<sub>2</sub>][Ga(Et<sub>2</sub>O)<sub>2</sub>(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>26</td></tr><tr><td>Ga[(OTeF<sub>5</sub>)<sub>3</sub>(C<sub>3</sub>F<sub>5</sub>N)]</td><td>26</td><td>[cat][Ga(Et)(OTeF<sub>5</sub>)<sub>3</sub>] (cat = PPh<sub>4</sub>, CPh<sub>3</sub>)</td><td>66</td></tr><tr><td>Ga[(OTeF<sub>5</sub>)<sub>3</sub>(OPPh<sub>3</sub>)]</td><td>26</td><td>Ga<sub>2</sub>(Et)<sub>3</sub>(OTeF<sub>5</sub>)<sub>3</sub></td><td>66</td></tr><tr><td>[cat][Ga(OTeF<sub>5</sub>)<sub>4</sub>] (cat = Ag, PPh<sub>4</sub>, CPh<sub>3</sub>, ClMe<sub>2</sub>)</td><td>26,66</td><td></td><td></td></tr><tr><td>Tl</td><td>I</td><td>[Tl(η<sup>6</sup>-Mes)<sub>2</sub>OTeF<sub>5</sub>]<sub>2</sub></td><td>146,167</td><td></td><td></td></tr><tr><td></td><td>III</td><td>Tl(OTeF<sub>5</sub>)<sub>3</sub></td><td>27</td><td></td><td></td></tr></tbody></table>

further with HOTeF<sub>5</sub> in contrast to the Al analogue Al<sub>2</sub>Et<sub>3</sub>(OR<sup>F</sup>)<sub>3</sub> (R<sup>F</sup> = C(CF<sub>3</sub>)<sub>3</sub>), in which the ethyl groups can be further substituted by OC(CF<sub>3</sub>)<sub>3</sub> substituents.<sup>166</sup>

## 5.4. Thallium Teflates

TlOTeF<sub>5</sub> has been synthesized by the reaction of TlF with HOTeF<sub>5</sub>.<sup>146,167</sup> The salt is soluble in toluene and mesitylene and can be crystallized as [TlOTeF<sub>5</sub>(Mes)<sub>2</sub>]<sub>2</sub>.Mes. The molecular structure reveals a η<sup>6</sup>-coordination of mesitylene to the thallium atoms and two bridging OTeF<sub>5</sub> groups that is comparable with reported structures of AgOTeF<sub>5</sub>.<sup>168</sup> The neutral Tl(III) teflate can be synthesized from the reaction of TlCl<sub>3</sub> and 3 equiv of ClOTeF<sub>5</sub>, or from TlCl<sub>3</sub> and Xe(OTeF<sub>5</sub>)<sub>2</sub> upon formation of Cl<sub>2</sub> and Xe, respectively. The molecular structure reveals that this compound forms a dimer [Tl(OTeF<sub>5</sub>)<sub>3</sub>]<sub>2</sub> in the solid state.<sup>27</sup> Group 13 teflates are gathered in Table 4.

# 6. GROUP 14 TEFLATES

## 6.1. Carbon Teflates

In contrast to the well-explored inorganic compounds bearing the teflate ligand, only a few organic derivatives have been

prepared so far. Two classes of compounds were reported, hydrocarbons (C-H) and fluorocarbons (C-F). Furthermore, the two homoleptic carbon teflate compounds [C(OTeF<sub>5</sub>)<sub>3</sub>]<sup>+</sup> and C(OTeF<sub>5</sub>)<sub>4</sub> will be highlighted.

### 6.1.1. Hydrocarbons (C<sub>n</sub>H<sub>2n+2</sub>).
Most synthetic efforts proceed via a unique pathway developed by Fraser et al., which involves the direct reaction of tellurium hexafluoride (TeF<sub>6</sub>) with alcohols (R-OH, R = linear and branched alkyl) (see eq 15 and Figure 10).<sup>169-171</sup> These derivatives as well as the bis-alkoxy derivatives TeF<sub>4</sub>(OR)<sub>2</sub> found their place as alkylation reagents in 1979.<sup>172</sup>

$$ TeF_6 + R-OH \rightarrow R-OTeF_5 + HF \quad (15) $$

This synthetic route enables the formation of alkoxotellurium derivatives in moderate to good yields (up to 75%), with a scope including linear and branched alcohols (C<sub>1</sub>-C<sub>4</sub>). The formed HF is deactivated with NaF, Na<sub>2</sub>CO<sub>3</sub> or pyridine. These compounds have been observed analytically in similar vibrational or chemical shift ranges in spectroscopic techniques (IR and NMR), indicating minimal influence of the alkyl substituents on the tellurium center. The typical AB<sub>4</sub> spin
<figure><img src="image_29.png" /><figcaption>**branched alkanes**</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure 10.** Selected teflate-functionalized hydrocarbons.</figcaption></figure>

system of the teflate group is observed in the ¹⁹F NMR spectra of these compounds, consistent with other covalently bonded OTeF₅ groups. Further NMR studies and structural analysis of the MeOTeF₅ compound were conducted by Seppelt et al.⁴⁹ A recent report on a modern teflate-based reagent (dimethylchloronium salt [ClMe₂][Al(OTeF₅)₄]⁷³ also mentions the generation of carbon-based species CH₂ClOTeF₅ and CH₂(OTeF₅)₂ by its decomposition. The compounds were not isolated.

Finally, this strategy was extended by using diols, enabling the synthesis of bridged teflates F₅TeO-(CH₂)ₙ-OTeF₅ (n = 3, 4, 10).¹⁷³ Furthermore, only two branched polyols (trimethylol-ethane and pentaerythritol) have been successfully converted into the desired organic teflate derivatives (see Figure 10). More recently, both CH₂ClOTeF₅ and CH₂(OTeF₅)₂ were observed from the decomposition of [Al(OTeF₅)₄]⁻.¹⁷⁴

**6.1.2. Fluorocarbons.** In contrast to the hydrocarbon compounds discussed above, the synthesis of fluorocarbon derivatives utilizes various established OTeF₅ transfer reagents like Xe(OTeF₅)₂ and XOTeF₅ (X = Cl, F). First, hypohalites were engaged with fluorinated olefins, enabling the synthesis of linear derivatives (C₂, C₃ and C₅) in good yields (60–86%).¹⁷⁵ The use of fluoroolefins with Xe(OTeF₅)₂ allows the synthesis of fluorinated bis-OTeF₅ species via a formal 1,2-addition reaction over the double bond and liberation of Xe (see Figure 11).¹⁷⁶ Christe and Schack successfully prepared cyclic derivatives from the hypofluorite FOTeF₅ and synthesized an example of an unsaturated bis-teflate compound, as shown in Table 5, which provides an overview of hydro- and fluorocarbon-based teflate compounds.¹⁷⁷ Finally, fluorinated alkyl chains bearing the OTeF₅ group were prepared through the direct reaction of the hypochlorite ClOTeF₅ with fluorocarbon iodides, allowing excess to CF₃OTeF₅, C₂F₅OTeF₅ and n-C₃F₇OTeF₅.¹⁷⁸ Several patents from Christe's work were subsequently filed.¹⁷⁹–¹⁸¹

In 2004, Schrobilgen et al. presented the first carbon-based teflate compounds with nonorganic substituents, synthesizing [CBrₙ(OTeF₅)₃-n]⁺ (n = 0–2)⁶⁷ by oxidizing CBr₄ with stoichiometric amounts of the strong oxidizer, [XeOTeF₅] [Sb(OTeF₅)₆], followed by cascade reactions in which the formed [CBr₃]⁺ reacts further with BrOTeF₅ that formed during the process (see Scheme 4).

All compounds are stable at 0 °C for several hours. The OTeF₅ derivatives were confirmed by ¹⁹F NMR and ¹³C NMR spectroscopy except for [CBr₂(OTeF₅)]⁺ since that ¹³C chemical shift could not be detected. The cation [C(OTeF₅)₃]⁺

<figure><img src="image_31.png" /><figcaption>**disubstituted halocarbons**</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 11.** Exemplary synthesis and selected examples of halocarbon-based teflate compounds from hypohalites (upper chart) and from Xe(OTeF₅)₂ (lower chart).</figcaption></figure>

is isoelectronic and isostructural to B(OTeF₅)₃ and was also characterized in the solid state (see Figure 12, left).⁶⁷

The reaction of CBr₄ with stoichiometric amounts of BrOTeF₅ yielded the homoleptic C(OTeF₅)₄⁶⁷ which decomposes at 10 °C to O(TeF₅)₂ and CO₂ in MeCN.¹⁵³ The formation of C(OTeF₅)₄ and the ¹³C-enriched ¹³C(OTeF₅)₄ was validated by ¹³C NMR, ¹⁹F NMR, and ¹²⁵Te NMR spectroscopy⁶⁷ as well as by single crystal X-ray diffraction of C(OTeF₅)₄ (see Figure 12, right).¹⁵³ Due to the more covalent C–O bond character in [C(OTeF₅)₃]⁺ the C–O bond lengths in C(OTeF₅)₄ are longer than in the corresponding cation (135(2)–142(2) pm vs 125.8(15)–131.3(16) pm).¹⁵³

Carbon-based OSeF₅ representatives are CH₃OSeF₅, CH₂(OSeF₅)₂, CH(OSeF₅)₃ and C(OSeF₅)₄. Except for CH₂(OSeF₅)₂, all four other compounds can be obtained by the OSeF₅ transfer reagent Hg(OSeF₅)₂. Surprisingly, this procedure was not successful for CH₂Cl₂ and yielded CH₂(OSeF₅)₂ only in traces. However, if CH₂Br₂ and ClOSeF₅ are reacted the desired product is formed.⁸⁶

In a similar way the compounds F₅SeO-COCl⁸⁶ and F₅SeO-COCF₃⁹⁰ are synthesized, starting from Hg(OSeF₅)₂ and phosgene or trifluoroacetyl chloride.

Similarly to the pentafluorooxosulfate chemistry described in Section 2.5, Cady relied on the hypofluorite (FOSeF₅) for the direct [2+2] reaction with a fluorinated alkene (i.e., perfluorocyclopentene).⁹⁸ This allowed the synthesis of the desired fluorinated cyclic five-member ring C₅F₉OSeF₅ in high yield. Furthermore, the oxidative ability of this reagent was highlighted by its reaction with carbon monoxide affording the corresponding acyl fluoride bearing the pentafluoroorthoselenate moiety (F₅SeO-COF).⁹⁸
Scheme 4. Oxidation of CBr₄ to [CBr₇(OTeF)₃]⁺ Cations (n = 0, 1, 3)

synthesis of the [C(OTeF₅)₃]⁺ cation

$$3 [\text{XeOTeF}_5][\text{Sb(OTeF}_5)_6] + 3 \text{CBr}_4 \rightarrow 3 [\text{CBr}_3][\text{Sb(OTeF}_5)_6] + 3 \text{BrOTeF}_5 + 3 \text{Xe}$$

reaction cascades

$$\begin{align*} [\text{CBr}_3][\text{Sb(OTeF}_5)_6] & \xrightarrow[\text{Br}_2]{\text{BrOTeF}_5} [\text{CBr}_2(\text{OTeF}_5)][\text{Sb(OTeF}_5)_6] \\ [\text{CBr}_2(\text{OTeF}_5)][\text{Sb(OTeF}_5)_6] & \xrightarrow[\text{Br}_2]{\text{BrOTeF}_5} [\text{CBr(OTeF}_5)_2][\text{Sb(OTeF}_5)_6] \\ [\text{CBr(OTeF}_5)][\text{Sb(OTeF}_5)_6] & \xrightarrow[\text{Br}_2]{\text{BrOTeF}_5} [\text{C(OTeF}_5)_3][\text{Sb(OTeF}_5)_6] \end{align*}$$

<figure><img src="image_33.png" /><figcaption>Figure 12. Structures of [C(OTeF₅)₃][Sb(OTeF₅)₆]·3SO₂ClF (left) and C(OTeF₅)₄ (right) in the solid state.⁶⁷,¹⁵³ Thermal ellipsoids are set at 50% and 30% probability, respectively. The counterion [Sb(OTeF₅)₆]⁻ is omitted for clarity.</figcaption></figure>

The homoleptic silicon teflate Si(OTeF₅)₄ was briefly mentioned by Sladky et al. in 1973 and its characterization is limited to the melting point of 37 °C.¹⁹ All other known silicon teflates contain only one teflate group.

The synthesis of Me₃SiOTeF₅ was first described in 1973 using Me₃SiCl and HOTeF₅ and the compound was characterized by the boiling point. Sladky et al. tested Me₃SiOTeF₅ as a teflate transfer reagent in metathesis reactions with PF₅ and SF₄.¹⁹ In those two reactions the tellurium atom was fluorinated, resulting in TeF₆, Me₃SiF, POF₃, and SOF₂. In the case of TeF₆ no reaction took place. For the covalent halide AsF₅ it was observed that F₂AsOTeF₅ is obtained, which further dismutates to AsF₃ and As(OTeF₅)₃.¹⁹ In 2018 the compound Me₃SiOTeF₅ was fully characterized by Riedel et al. and its application as teflate transfer reagent could be expanded to the heteroleptic organo gold(III) complex [AuF₃(SIMes)] (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene). Due to the introduced teflate group the Lewis acidity of the Au moiety could be increased in [AuF₂(OTeF₅) (SIMes)].¹⁸²

The compound Et₃SiOTeF₅ was observed only as a side product in the reaction of [H-C₇H₈][Al(OTeF₅)₄] with Et₃SiH and was not isolated.⁵⁶ Another example of a monoteflate substituted silane is Ph₃SiOTeF₅. It was formed by using Ph₃SiCl and TlOTeF₅.¹⁴⁷ Upon reaction of Ph₃SiH and the hydride abstractor [CPh₃][B(OTeF₅)₄] instead of the expected [SiPh₃]⁺, Ph₃SiOTeF₅ is formed.¹⁴⁷

For the selenate group only one species was published so far. Contrary to the teflate analogue, Me₃SiOSeF₅ was not formed by reacting the corresponding chlorosilane, but rather the silylamine (Me₃Si)₃N with the selenate transfer reagent HOSeF₅.⁹⁴

### 6.3. Germanium, Tin, and Lead Teflates

Me₃GeOTeF₅ is the only literature-known germanium teflate to date.¹⁹ The tin compound Me₃SnOTeF₅ is one of only two known tin teflates. According to Sladky et al., contrary to its lighter homologues, its synthesis is possible not only by reacting Me₃ECI (E = Si, Ge, Sn) with a teflate transfer reagent but also through the reaction of Me₄Sn with HOTeF₅.¹⁹ However, a later study showed that the first approach mainly produces Me₂SnClOTeF₅ and Me₃SnOTeF₅ is the minor product.¹⁸³ Moreover, the two solvent adducts [Me₃SnOTeF₅-((CD₃)₂CO)] and [Me₃SnOTeF₅(CD₃CN)] are reported.¹⁸³

No synthesis of teflate-based lead compounds has so far been reported. Group 14 teflates and selenates are gathered in Table 5.

## 7. GROUP 15 TEFLATES

### 7.1. Nitrogen Teflates

The only reported compound containing the N-OTeF₅ fragment is the nitrosyl teflate, which is obtained from NOCl and Hg(OTeF₅)₂.¹⁸⁷ The covalent bond between the nitrogen and the teflate group can only be observed in the gas phase using IR spectroscopy. In the solid state or in solution, the molecule ionizes to form [NO][OTeF₅] (see eq 17). This structural property has also been described for the related selenium compound NO₂(OSeF₅).¹⁸,²³

$$\begin{align*} \text{Hg(OTeF}_5)_2 + 2\text{NOCl} &\xrightarrow{-\text{HgCl}_2} 2[\text{NO}][\text{OTeF}_5]_{(\text{s})} &\rightleftharpoons 2\text{NO(OTeF}_5)_{(\text{g})} \end{align*}$$

### 7.2. Phosphorus Teflates

The phosphorus(III) teflate P(OTeF₅)₃ is prepared from PCl₃ and Hg(OTeF₅)₂ in decent yields but slowly decomposes at room temperature (see Scheme 5).¹⁵ The low stability of this compound is attributed to its inner redox potential, which can be observed through the formation of an elemental tellurium mirror over time.

Scheme 5. Synthetic Routes for P(OTeF₅)₃ and OP(OTeF₅)₃ synthesis of P(OTeF₅)₃

$$3 \text{Hg(OTeF}_5)_2 + 2 \text{PCl}_3 \xrightarrow{-3 \text{HgCl}_2} 2 \text{P(OTeF}_5)_3$$

synthetic attempts toward P(OTeF₅)₅

<figure><img src="image_34.png" /></figure>

All efforts to obtain the phosphorus(V) teflate using different synthetic pathways such as the oxidation of the P(III) compound with Xe(OTeF₅)₂ or the ligand exchange of PCl₅ with ClOTeF₅ failed. In both cases ligand elimination from P(OTeF₅)₅ could not be suppressed, giving exclusively the phosphorus(V) oxo-teflate OP(OTeF₅)₃ and O(TeF₅)₂ as the products (see Scheme 5). Mixed fluoro-oxo-teflate P(V) compounds are selectively synthesized by reacting POF₂Cl
with Hg(OTeF<sub>5</sub>)<sub>2</sub> yielding OPF<sub>2</sub>(OTeF<sub>5</sub>).<sup>15</sup> In an analogous way using Hg(OSeF<sub>5</sub>)<sub>2</sub> the lighter OPF<sub>2</sub>(OSeF<sub>5</sub>) can be obtained.<sup>18</sup>

Cationic phosphonium(V) compounds containing the OTeF<sub>5</sub> group have been shown to form via the decomposition of [MePF<sub>3</sub>][Al(OTeF<sub>5</sub>)<sub>4</sub>] in SO<sub>2</sub>.<sup>73</sup> The cationic species [MePF<sub>2</sub>(OTeF<sub>5</sub>)]<sup>+</sup> and [MePF(OTeF<sub>5</sub>)<sub>2</sub>]<sup>+</sup> have been observed by NMR spectroscopy. However, they have not been isolated.

### 7.3. Arsenic Teflates

The first arsenic teflate was prepared in 1973 by the reaction of AsF<sub>3</sub> with Me<sub>3</sub>SiOTeF<sub>5</sub> stating that initially, the monosubstituted AsF<sub>2</sub>(OTeF<sub>5</sub>) is formed, which then dismutates upon distillation to AsF<sub>3</sub> and As(OTeF<sub>5</sub>).<sup>19</sup> The characterization of As(OTeF<sub>5</sub>)<sub>3</sub> was then limited to the boiling point only. Further characterization and an improved synthesis of As(OTeF<sub>5</sub>)<sub>3</sub> were published a few years later by Seppelt et al. in 1977. The procedure utilizes AsF<sub>3</sub> and B(OTeF<sub>5</sub>)<sub>3</sub> as the teflate transfer reagent to obtain analytically pure As(OTeF<sub>5</sub>)<sub>3</sub> (see Scheme 6).<sup>18</sup> In contrast to its lighter homologue As(OSeF<sub>5</sub>)<sub>3</sub>, which

<figure><img src="image_35.png" /><figcaption>formation of salts of the WCA [As(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup></figcaption></figure>

<figure><img src="image_36.png" /></figure>

decomposes over time to [SeF<sub>3</sub>][AsF<sub>6</sub>], the dimeric (OSeF<sub>4</sub>)<sub>2</sub> and further unidentified products, the As(III) compound As(OTeF<sub>5</sub>)<sub>3</sub> is stable over a long period of time. This difference in stability can be attributed to the higher oxidation power of Se(VI) compared to Te(VI) leading to intramolecular redox reactions in As(OSeF<sub>5</sub>)<sub>3</sub>, which do not take place in As(OTeF<sub>5</sub>)<sub>3</sub>.<sup>18</sup>

Despite the free electron pair on As(OTeF<sub>5</sub>)<sub>3</sub>, the strong electron-withdrawing property of the teflate group leads to an increased Lewis acidity comparable with that of AsF<sub>3</sub>. As(OTeF<sub>5</sub>)<sub>3</sub> forms adducts with simple Lewis bases like MeCN, Cl<sup>-</sup> and [OTeF<sub>5</sub>]<sup>-</sup> exhibiting σ acidity.<sup>188</sup> Furthermore, As(OTeF<sub>5</sub>)<sub>3</sub> binds to low valent transition metals as shown in W(CO)<sub>5</sub>(As(OTeF<sub>5</sub>)<sub>3</sub>) exhibiting π acidity.<sup>188</sup> The Lewis acidity drastically increases by oxidizing the As(III) center to As(V). With a FIA value of 580 kJ mol<sup>-1</sup> for As(OTeF<sub>5</sub>)<sub>5</sub>, it is considered the only neutral arsenic-based Lewis superacid,<sup>189</sup> surpassing other strong arsenic-based Lewis acids like AsF<sub>5</sub> (439 kJ mol<sup>-1</sup>) and the threshold for Lewis superacidity SbF<sub>5</sub> (496 kJ mol<sup>-1</sup>).<sup>52,53</sup>

The synthesis of As(OTeF<sub>5</sub>)<sub>5</sub> is achieved by oxidation of As(OTeF<sub>5</sub>)<sub>3</sub> with Xe(OTeF<sub>5</sub>)<sub>2</sub> (see Scheme 6).<sup>15</sup> This reaction however yields, besides the desired As(V) compound, unidentified side products, which could not be separated. An improved synthesis starts from AsF<sub>5</sub> as an As(V) precursor and stoichiometric amounts of B(OTeF<sub>5</sub>)<sub>3</sub> in SO<sub>2</sub> and results in pure As(OTeF<sub>5</sub>)<sub>5</sub> (see Scheme 6).<sup>57</sup>

The understoichiometric use of B(OTeF<sub>5</sub>)<sub>3</sub> yields the mixed fluoro-teflate compound AsF(OTeF<sub>5</sub>)<sub>4</sub>. Both Lewis acids, As(OTeF<sub>5</sub>)<sub>5</sub> and AsF(OTeF<sub>5</sub>)<sub>4</sub>, have been used in the synthesis of the tetrahalopnictogen(V) salts [AsX<sub>4</sub>][As(OTeF<sub>5</sub>)<sub>6</sub>] and [AsX<sub>4</sub>][AsF(OTeF<sub>5</sub>)<sub>5</sub>] (X = Cl, Br) (see Scheme 6).<sup>58</sup> In the oxidation reaction of AsX<sub>3</sub> with the corresponding hypohalite XOTeF<sub>5</sub>, the Lewis acids As(OTeF<sub>5</sub>)<sub>5</sub> or AsF(OTeF<sub>5</sub>)<sub>4</sub> act as [OTeF<sub>5</sub>]<sup>-</sup> anion acceptors forming the respective weakly coordinating anions [As(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> or [AsF(OTeF<sub>5</sub>)<sub>5</sub>]<sup>-</sup>. Further studies on the [OTeF<sub>5</sub>]<sup>-</sup> anion acceptor properties of As(OTeF<sub>5</sub>)<sub>5</sub> in the reaction with Te(OTeF<sub>5</sub>)<sub>4</sub> lead to the observation of the tellurium(IV) salt [Te(OTeF<sub>5</sub>)<sub>3</sub>][As(OTeF<sub>5</sub>)<sub>6</sub>] in solution (see Scheme 6).<sup>57</sup> However, no full conversion and isolation of the product could be achieved.

Regarding the stability of the weakly coordinating arsenate anions, the homoleptic [As(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion shows the highest resistance against nucleophilic attacks compared to [AsF(OTeF<sub>5</sub>)<sub>5</sub>]<sup>-</sup> and [AsF<sub>6</sub>]<sup>-</sup>.<sup>58</sup>

The [As(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion has been subject of detailed vibrational and multinuclear NMR spectroscopic studies with the noteworthy clear observation of the quadrupolar <sup>75</sup>As core.<sup>44,190</sup> Compared to the neutral As(OTeF<sub>5</sub>)<sub>5</sub>, which does not produce an observable signal in the <sup>75</sup>As NMR spectrum, the highly symmetric octahedral environment around the arsenic center in the [As(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion, leads to well-resolved <sup>75</sup>As NMR spectra of the cesium and ammonium salts even depicting the spin coupling to the <sup>125</sup>Te isotope present in the teflate group as <sup>125</sup>Te satellites. The studied salts have been synthesized by reaction of the Lewis acid As(OTeF<sub>5</sub>)<sub>5</sub> and the corresponding teflate salt [cat][OTeF<sub>5</sub>] (cat = Cs<sup>+</sup>, [NMe<sub>4</sub>]<sup>+</sup>).<sup>44,57</sup>

### 7.4. Antimony Teflates

The first antimony teflates were mentioned in 1977 by Seppelt et al.<sup>18</sup> In his report he describes the reaction of SbF<sub>5</sub> and B(OTeF<sub>5</sub>)<sub>3</sub> with a stoichiometry of 1:2, which does not lead to the desired Sb(V) compound Sb(OTeF<sub>5</sub>)<sub>5</sub> but rather yields the Sb(III) compound Sb(OTeF<sub>5</sub>)<sub>3</sub> via the elimination of the peroxide F<sub>5</sub>TeO<sub>2</sub>TeF<sub>5</sub>. He later revised this claim after synthesizing analytically pure Sb(OTeF<sub>5</sub>)<sub>3</sub> from SbF<sub>3</sub> and B(OTeF<sub>5</sub>)<sub>3</sub>.<sup>15</sup> Similar to the lighter As(III) homologue, Sb(OTeF<sub>5</sub>)<sub>3</sub> has a Lewis acidic character and acts as a [OTeF<sub>5</sub>]<sup>-</sup> anion acceptor. Upon reaction with ammonium teflates [NR<sub>4</sub>][OTeF<sub>5</sub>] the corresponding Sb(III) salts [NR<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>4</sub>] (R = Me, Et) are obtained.<sup>44</sup>

Despite great efforts, the successful synthesis of Sb(OTeF<sub>5</sub>)<sub>5</sub>, often being considered one of the strongest neutral Lewis acids achievable (FIA value of 623 kJ mol<sup>-1</sup>),<sup>52</sup> has not been realized. Only mixed fluoro-teflate Sb(V) compounds SbF<sub>5-n</sub>(OTeF<sub>5</sub>)<sub>n</sub> (n = 1, 2) have been reported in stoichiometric reactions of SbF<sub>5</sub> with B(OTeF<sub>5</sub>)<sub>3</sub> (see Scheme 7).<sup>191</sup>

It comes as no surprise that the corresponding Sb(V) teflate anion [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> has proven itself to be an excellent weakly coordinating anion, resisting and stabilizing highly oxidizing and strongly electrophilic cations. The [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion has been subject to detailed vibrational and multinuclear NMR spectroscopy.<sup>44</sup>
**Scheme 7. Syntheses of Sb(OTeF<sub>5</sub>)<sub>3</sub>, the Anion [Sb(OTeF<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, and the Mixed SbF<sub>5-n</sub>(OTeF<sub>5</sub>)<sub>n</sub> with n = 1, 2**
synthetic routes toward antimony-based teflates

<figure><img src="image_37.png" /></figure>

Synthesis of the [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion can be achieved by a ligand exchange reaction starting from a Sb(V) precursor. Antimony pentachloride SbCl<sub>5</sub> reacts with AgOTeF<sub>5</sub> under the elimination of AgCl to give the synthetically useful silver salt Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>] (see Scheme 8).<sup>192</sup> Via simple metathesis

**Scheme 8. Synthesis and Reactivity of Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>]**
synthetic usage of Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>]**

<figure><img src="image_38.png" /></figure>

reactions of the silver antimonate salt is converted to the cesium salt Cs[Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>193</sup> and the tritium salt [CPh<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>].<sup>192</sup> Reactions of Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>] with the elemental chalcogens sulfur and selenium in SO<sub>2</sub> gave the respective salts [Ag(S<sub>8</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>194</sup> or [Ag<sub>2</sub>Se<sub>6</sub>(SO<sub>2</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>195</sup>. The undistorted structure of the [Ag(S<sub>8</sub>)<sub>2</sub>]<sup>+</sup> cation in the solid state showcases the weakly coordinating nature of the [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion, as the cation displays significant distortion in the chemically related salt [Ag(S<sub>8</sub>)<sub>2</sub>][SbF<sub>6</sub>], originating from cation-anion interactions. Reactions of the silver antimonate salt with elemental tellurium in the presence of TeBr<sub>4</sub> led to the dicationic [Te<sub>4</sub>]<sup>2+</sup> species, while the reaction of Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>] with TeBr<sub>4</sub> yields the tellurium-(IV) cation [TeBr<sub>3</sub>]<sup>+</sup> (see Scheme 8).<sup>196</sup>

Simple ammonium salts [NR<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (R = Me, Et, nBu) are obtained by the metathesis reaction of Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>] with ammonium chlorides [NR<sub>4</sub>]Cl (R = nBu)<sup>192</sup> or by the oxidation of the Sb(III) salts [NR<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>4</sub>] (R = Me, Et) with Xe(OTeF<sub>5</sub>)<sub>2</sub>.<sup>44</sup>

Oxidation of the Sb(III) teflate Sb(OTeF<sub>5</sub>)<sub>3</sub> with elemental chlorine or bromine leads to ligand scrambling and the formation of the respective tetrahalostibonium(V) salts [SbX<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (X = Cl, Br).<sup>197</sup>

The [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion shows a high stability against these strongly oxidizing cations with its salts being indefinitely stable at room temperature. Furthermore, the weakly coordinating nature of the [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion leads to an almost perfectly tetrahedral structure of the [SbX<sub>4</sub>]<sup>+</sup> cation (X = Cl, Br) in the

solid state with the anion and the cation being well separated with only weak interactions (see Figure 13). Compared to the

reactive cations stabilized by [Sb(OTeF<sub>5</sub>)<sub>6</sub>]**

<figure><img src="image_39.png" /></figure>

molecular structure and space-filling model
of [Sb(OTeF<sub>5</sub>)<sub>6</sub>]**

<figure><img src="image_40.png" /><figcaption>**Figure 13.** Examples of reactive cations stabilized by the simultaneous formation of [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> (upper chart). Molecular structure (lower chart) in the solid state of [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> (left) and the space-filling representation (right).<sup>70</sup> Cations are omitted for clarity.</figcaption></figure>

three other anions [Sb<sub>2</sub>F<sub>11</sub>]<sup>-</sup>, [Sb<sub>2</sub>Cl<sub>0.5</sub>F<sub>10.5</sub>]<sup>-</sup> and [Sb<sub>2</sub>Cl<sub>2</sub>F<sub>9</sub>]<sup>-</sup>, which have been reported to stabilize the [SbCl<sub>4</sub>]<sup>+</sup> cation,<sup>198-200</sup> the [Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> anion is the weakest coordinating one in this group.

Another strongly oxidizing salt, the noble gas compound [Xe(OTeF<sub>5</sub>)(SO<sub>2</sub>ClF)][Sb(OTeF<sub>5</sub>)<sub>6</sub>], can be obtained by the oxidation of the Sb(III) teflate Sb(OTeF<sub>5</sub>)<sub>3</sub> with 2 equiv of Xe(OTeF<sub>5</sub>)<sub>2</sub>.<sup>43,69</sup>

The structure of the salt in the solid state reveals the high Lewis acidity of the xenonium ion as it forms an adduct with the weak Lewis base SO<sub>2</sub>ClF. The SO<sub>2</sub>ClF can be displaced by other oxidation-resistant Lewis bases as shown in the synthesis of [Xe(OTeF<sub>5</sub>)(C<sub>5</sub>F<sub>5</sub>N)][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (see Figure 13).<sup>70</sup> An excess of Xe(OTeF<sub>5</sub>)<sub>2</sub> in the synthesis of the xenonium salt and subsequent removal of all SO<sub>2</sub>ClF revealed the formation of [Xe<sub>2</sub>(OTeF<sub>5</sub>)<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] as identified by Raman spectroscopy.<sup>69</sup>

Furthermore, the strong oxidizing salt [Xe(OTeF<sub>5</sub>)(SO<sub>2</sub>ClF)][Sb(OTeF<sub>5</sub>)<sub>6</sub>] finds application in the synthesis of halogenated carbocations [CX<sub>3</sub>]<sup>+</sup>, [CFX<sub>2</sub>]<sup>+</sup> (X = Cl, Br), the teflate substituted carbocations [CBr<sub>n</sub>(OTeF<sub>5</sub>)<sub>3-n</sub>]<sup>+</sup> (n = 0, 1, 2) and the chloronium cations [Cl(R<sub>2</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (R = Me, CH<sub>2</sub>CF<sub>3</sub>).<sup>67,68,75</sup> Oxidation of the hypobromite BrOTeF<sub>5</sub> with [Xe(OTeF<sub>5</sub>)(SO<sub>2</sub>ClF)][Sb(OTeF<sub>5</sub>)<sub>6</sub>] leads to the Br(III) salt [Br(OTeF<sub>5</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>].<sup>67</sup>

7.5. Bismuth Teflates

Low oxidation state bismuth teflates are unknown; however, the neutral Bi(V) teflate Bi(OTeF<sub>5</sub>)<sub>5</sub> and the bismuthate anion [Bi(OTeF<sub>5</sub>)<sub>6</sub>]<sup>-</sup> have been reported.<sup>44</sup> The neutral Lewis acid Bi(OTeF<sub>5</sub>)<sub>5</sub> is obtained from the reaction of BiF<sub>5</sub> and
$B(OTeF_5)_3$ (see eq 18). Given that $Bi(OTeF_5)_5$ acts as a $[OTeF_5]^- $ acceptor, the synthesis of the anionic species is analogous to that of its arsenic counterpart $[As(OTeF_5)_6]^-$. The Bi(V) teflate $Bi(OTeF_5)_5$ reacts with ammonium teflate salts $[NR_4][OTeF_5]$ yielding $[NR_4][Bi(OTeF_5)_6]$ ($R = Me, Et$).⁴⁴

$$ \mathrm{BiF_5} \xrightarrow[{-\frac{5}{3}\mathrm{BF_3}}]{\frac{5}{3}\mathrm{B(OTeF_5)_3}} \mathrm{Bi(OTeF_5)_5} \xrightarrow[\mathrm{R} = \mathrm{Me, Et}]{[NR_4][OTeF_5]} [NR_4][\mathrm{Bi(OTeF_5)_6}] \quad (18) $$

Comparing the color of the $[E(OTeF_5)_6]^- $ salts ($E = As, Sb, Bi$) the unique property of the bismuth compound is displayed, as $[Bi(OTeF_5)_6]^- $ salts are yellow, differing from the colorless $[As(OTeF_5)_6]^- $ and $[Sb(OTeF_5)_6]^- $ salts. The yellow color, which is also observed for the neutral $Bi(OTeF_5)_5$, is believed to be a consequence of relativistic effects on the heavy Bi atom. Apart from the color, the $[Bi(OTeF_5)_6]^- $ anion also differs in its stability from the lighter homologues. Salts of the $[Bi(OTeF_5)_6]^- $ anion are stable solids, that slowly decompose in $SO_2ClF$ and rapidly decompose in MeCN at room temperature. $^{19}$F NMR spectroscopy revealed the formation of the oxo-teflate anion $[OBI(OTeF_5)_4]^- $ and $O(TeF_5)_2$ and therefore a similar ligand elimination as reported for $P(OTeF_5)_5$. Further multi-nuclear NMR spectroscopic investigation of the $[Bi(OTeF_5)_6]^- $ anion led to the observation of the extremely rare spin coupling of the $^{209}$Bi nuclei to another atom, in this case, the $^2J$ coupling to the $^{125}$Te nuclei present in the teflate ligand.⁴⁴ This spin coupling is observable due to the highly symmetric octahedral environment of the quadrupolar $^{209}$Bi center, which is also found in the $[BiF_6]^- $ anion.²⁰¹ Group 15 teflates and selenates are gathered in Table 6.

# 8. GROUP 16 TEFLATES

## 8.1. Oxygen Teflates

The oxo compound $F_5TeOTeF_5$ can be regarded as the anhydride of teflic acid, $HOTeF_5$, and is obtained either by fluorination of $TeO_2$¹⁰⁰,¹⁰¹,²⁰² or the reaction of $(F_5TeO)_2SO_2$ with $KOTeF_5$ or $CsCl$ (see Scheme 9).⁴ Structural information of $F_5TeOTeF_5$ obtained by electron diffraction measurements show a relatively large Te–O–Te bond angle (145.5°) and remarkably short O–Te bond lengths (183.2 pm).¹⁰⁰,¹⁰¹ Interestingly, the conformation of the TeF$_5$ groups is eclipsed, even though it is sterically the least favored. These structural properties of $F_5TeOTeF_5$ are attributed to a (pd)π bonding between the tellurium and the oxygen atoms.¹⁰⁰,¹⁰¹ Furthermore, this (pd)π bonding is observed in the peroxide $F_5TeOOteF_5$ leading to the remarkable stability of this compound. The O–O bond is reported to be unusually strong as judged by the high O–O valence frequencies in the Raman spectrum.⁵,²⁰³ The peroxide shows no homolytic decomposition under ambient conditions; however, it decomposes to the oxo compound $F_5TeOTeF_5$ if heated above 80 °C.⁵ The selective synthesis of the peroxide is achieved either by thermal or photolytically decomposition of the noble gas compounds $XeF(OTeF_5)$, $Xe(OTeF_5)_2$, $Xe(OTeF_5)_4$, $Xe(OTeF_5)_6$ or $Kr(OTeF_5)_2$ or the photolytical decomposition of $ClOTeF_5$ (see Scheme 9).¹,⁵,¹⁶,²⁰⁴,²⁰⁵ On the other hand, the thermal decomposition of the lighter derivative $Xe(SeF_5)_2$ does not yield the peroxide but the anhydride $O(SeF_5)_2$.⁸⁵ The peroxide $F_5SeOOSeF_5$, however, can be synthesized by fluorination of $SeO_2$.⁸⁴,⁹⁹

<table><caption>**Table 6. Overview of Group 15 Teflate and Selenate Compounds**</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="2">N</td><td>III</td><td>[NO][OTeF<sub>5</sub>]/NO(OTeF<sub>5</sub>)</td><td>187</td></tr><tr><td>V</td><td>[NO<sub>2</sub>][OSeF<sub>5</sub>]/NO<sub>2</sub>(OSeF<sub>5</sub>)</td><td>23</td></tr><tr><td rowspan="4">P</td><td>III</td><td>P(OTeF<sub>5</sub>)<sub>3</sub></td><td>15</td></tr><tr><td rowspan="3">V</td><td>OP(OTeF<sub>5</sub>)<sub>3</sub></td><td>15</td></tr><tr><td>OPF<sub>2</sub>(OTeF<sub>5</sub>)</td><td>15</td></tr><tr><td>OPF<sub>2</sub>(OSeF<sub>5</sub>)</td><td>15</td></tr><tr><td rowspan="10">As</td><td>III</td><td>As(OTeF<sub>5</sub>)<sub>3</sub></td><td>18</td></tr><tr><td rowspan="5">V</td><td>W(CO)<sub>5</sub>(As(OTeF<sub>5</sub>)<sub>3</sub>)</td><td>188</td></tr><tr><td>As(OTeF<sub>5</sub>)<sub>5</sub></td><td>15,57</td></tr><tr><td>AsF(OTeF<sub>5</sub>)<sub>4</sub></td><td>58</td></tr><tr><td>[AsX<sub>4</sub>][As(OTeF<sub>5</sub>)<sub>6</sub>] (X = Cl, Br)</td><td>58</td></tr><tr><td>[AsX<sub>4</sub>][AsF(OTeF<sub>5</sub>)<sub>5</sub>] (X = Cl, Br)</td><td>58</td></tr><tr><td rowspan="5">Sb</td><td>Cs[As(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>57</td></tr><tr><td>[NMe<sub>4</sub>][As(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>44</td></tr><tr><td>III</td><td>Sb(OTeF<sub>5</sub>)<sub>3</sub></td><td>15</td></tr><tr><td>[NMe<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>44</td></tr><tr><td>[NEt<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>44</td></tr><tr><td rowspan="14">V</td><td>SbF<sub>4</sub>(OTeF<sub>5</sub>)</td><td>191</td></tr><tr><td>SbF<sub>3</sub>(OTeF<sub>5</sub>)<sub>2</sub></td><td>191</td></tr><tr><td>Ag[Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>192</td></tr><tr><td>Cs[Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>193</td></tr><tr><td>[CPh<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>192</td></tr><tr><td>[Ag(S<sub>8</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>194</td></tr><tr><td>[Ag<sub>2</sub>Se<sub>6</sub>(SO<sub>2</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sub>2</sub></td><td>195</td></tr><tr><td>[TeBr<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>196</td></tr><tr><td>[Cl<sub>3</sub>Te-F-TeCl<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>196</td></tr><tr><td>[Te<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]<sub>2</sub></td><td>196</td></tr><tr><td>[NMe<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>44</td></tr><tr><td>[NEt<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>44</td></tr><tr><td>[N(nBu)<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>192</td></tr><tr><td>[SbX<sub>4</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (X = Cl, Br)</td><td>197</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(SO<sub>2</sub>ClF)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>43,69</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>2</sub>F<sub>5</sub>N)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>70</td></tr><tr><td>[Xe<sub>2</sub>(OTeF<sub>5</sub>)<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>69</td></tr><tr><td>[CX<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (X = Cl, Br)</td><td>67,68</td></tr><tr><td>[CFX<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>] (X = Cl, Br)</td><td>68</td></tr><tr><td>[C(OTeF<sub>5</sub>)<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>67,68</td></tr><tr><td>[CMe<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[CetMe<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[C<sub>6</sub>H<sub>11</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[Br(OTeF<sub>5</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>67</td></tr><tr><td>[ClMe<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[Cl(CH<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[I(CH<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td>[F<sub>5</sub>C<sub>5</sub>N(CH<sub>2</sub>CF<sub>3</sub>)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>75</td></tr><tr><td rowspan="3">Bi</td><td>V</td><td>Bi(OTeF<sub>5</sub>)<sub>5</sub></td><td>44</td></tr><tr><td></td><td>[NMe<sub>4</sub>][Bi(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>44</td></tr><tr><td></td><td>[NEt<sub>4</sub>][Bi(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>44</td></tr></tbody></table>

## 8.2. Sulfur Teflates

The earliest mention of sulfur teflate compounds coincides with the first reported synthesis of teflic acid $HOTeF_5$. In 1964 Sladky et al. reported on the reaction of barium tellurate, $BaTeO_4$, with fluorosulfonic acid $FSO_3H$ yielding, depending on the water content of the barium tellurate, $HOTeF_5$ and the three teflate-
**Scheme 9. Synthesis of the Anhydride F<sub>5</sub>TeO<sub>2</sub>F<sub>5</sub> and the Peroxide F<sub>5</sub>TeO<sub>2</sub>TeF<sub>5</sub>**

**synthetic routes toward F<sub>5</sub>TeO<sub>2</sub>F<sub>5</sub>**

<figure><img src="image_41.png" /></figure>

**synthetic routes toward F<sub>5</sub>TeO<sub>2</sub>TeF<sub>5</sub>**

<figure><img src="image_42.png" /></figure>

containing sulfur compounds F<sub>5</sub>TeOSO<sub>2</sub>F, (F<sub>5</sub>TeO)<sub>2</sub>SO<sub>2</sub>, and F<sub>5</sub>TeOSO<sub>3</sub>H in differing ratios.<sup>3</sup> After their accidental discovery the three sulfur-oxo-teflates were selectively synthesized (see eqs 19–21).<sup>4</sup>

$$ \mathrm{HOTeF_5} \xrightarrow{\mathrm{SO_3}} \mathrm{F_5TeOSO_3H} \qquad (19) $$

$$ \mathrm{F_5TeOSO_3H} \xrightarrow[\mathrm{-H_2SO_4}]{\mathrm{HSO_3X}} \mathrm{F_5TeOSO_2X} \qquad (20) $$

$$ \mathrm{F_5TeOSO_3H} \xrightarrow{\mathrm{CsCl}} (\mathrm{F_5TeO})_2\mathrm{SO_2} \qquad (21) $$

Additionally, F<sub>5</sub>TeOSO<sub>2</sub>Cl was synthesized from F<sub>5</sub>TeOSO<sub>3</sub>H and chlorosulfonic acid ClSO<sub>3</sub>H.<sup>4</sup> Their physical properties and their vibrational and <sup>19</sup>F NMR spectra have been described in detail in the literature.<sup>3,4,206,207</sup>

The only other reported compound containing a S-OTeF<sub>5</sub> fragment is the thiazyl-pentafluoroothotellurate NS-OTeF<sub>5</sub>, which is obtained by the reaction of the thiazyl salt [NS][SbF<sub>6</sub>] and CsOTeF<sub>5</sub>.<sup>208</sup> Other pathways, like the reaction of NSF with B(OTeF<sub>5</sub>)<sub>3</sub>, also yield NS-OTeF<sub>5</sub>; however, significant by-products like the isomerized OSNTeF<sub>5</sub> and the sulfur diimide S(NTeF<sub>5</sub>)<sub>2</sub> are observed.

Sulfur compounds containing the OSeF<sub>5</sub> group are rare. Examples are F<sub>5</sub>SeO-SO<sub>2</sub>-OSO<sub>2</sub>F, F<sub>5</sub>SeOSF<sub>5</sub> or cis-BrCH<sub>2</sub>SF<sub>4</sub>-OSeF<sub>5</sub>, which can be prepared starting from SO<sub>3</sub>, SF<sub>4</sub> or CH<sub>2</sub>SF<sub>4</sub> and the corresponding hypohalite XOSeF<sub>5</sub> (X = F, Br).<sup>90,98,102</sup> Furthermore, the reaction of the two peroxides F<sub>5</sub>SeOOSeF<sub>5</sub> and S<sub>2</sub>O<sub>6</sub>F<sub>2</sub> yields the fluorosulfate F<sub>5</sub>SeO-SO<sub>2</sub>F.<sup>99</sup>

**8.3. Selenium Teflates**

Attempts to obtain the homoleptic Se(IV) teflate Se(OTeF<sub>5</sub>)<sub>4</sub> from the stoichiometric reaction of SeF<sub>4</sub> and B(OTeF<sub>5</sub>)<sub>3</sub> lead to the isolation of the oxo-teflate compound OSe(OTeF<sub>5</sub>)<sub>2</sub>, while

the use of substoichiometric amounts of B(OTeF<sub>5</sub>)<sub>3</sub> leads to a product mixture of OSeF(OTeF<sub>5</sub>), F<sub>2</sub>Se(OTeF<sub>5</sub>)<sub>2</sub> and [SeF<sub>3</sub>] [B(OTeF<sub>5</sub>)<sub>4</sub>].<sup>87</sup> So far, these are the only reported selenium teflate species. The lighter homologues containing the OSeF<sub>5</sub> group are only known for the oxo and difluoro selenium compounds OSe(OSeF<sub>5</sub>)<sub>2</sub> and F<sub>2</sub>Se(OSeF<sub>5</sub>)<sub>2</sub>.<sup>23,87</sup>

**8.4. Tellurium Teflates**

The accidental discovery<sup>18</sup> of trans-TeF<sub>2</sub>(OTeF<sub>5</sub>)<sub>4</sub> gave rise to the systematic synthesis of a great variety of homoleptic tellurium teflate and heteroleptic tellurium fluorido teflate compounds.

The homoleptic Te(IV) teflate Te(OTeF<sub>5</sub>)<sub>4</sub> is obtained by the stoichiometric reaction of TeF<sub>4</sub> and B(OTeF<sub>5</sub>)<sub>3</sub>.<sup>22,28,57</sup> Its structure was assigned via low-temperature <sup>125</sup>Te NMR spectroscopy to have a pseudo trigonal bipyramidal configuration at the Te center.<sup>32</sup> Oxidation of Te(OTeF<sub>5</sub>)<sub>4</sub> with the noble gas compound Xe(OTeF<sub>5</sub>)<sub>2</sub> yields the homoleptic Te(VI) teflate Te(OTeF<sub>5</sub>)<sub>6</sub> (see eqs 22 and 23).<sup>22</sup>

$$ 3\mathrm{TeF_4} \xrightarrow[\mathrm{-4BF_3}]{\mathrm{4B(OTeF_5)_3}} 3\mathrm{Te(OTeF_5)_4} \qquad (22) $$

$$ \mathrm{Te(OTeF_5)_4} \xrightarrow[\mathrm{-Xe}]{\mathrm{Xe(OTeF_5)_2}} \mathrm{Te(OTeF_5)_6} \qquad (23) $$

The reaction of TeF<sub>4</sub> with substoichiometric amounts of B(OTeF<sub>5</sub>)<sub>3</sub> leads to the formation of the heteroleptic Te(IV) fluorido teflate F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>2</sub> which can be oxidized with the hypofluoride FOTeF<sub>5</sub> to obtain mer-F<sub>3</sub>Te(OTeF<sub>5</sub>)<sub>3</sub>.<sup>87</sup>

The cis- and trans-isomers of F<sub>4</sub>Te(OTeF<sub>5</sub>)<sub>2</sub> are obtained as a product mixture by the oxidation of TeF<sub>4</sub> with Xe(OTeF<sub>5</sub>)<sub>2</sub>. Separation of the two isomers can be achieved either by fractional crystallization or by crystallization of the trans-isomer followed by the distillation of the cis-isomer.<sup>22,28</sup>

If Te(OTeF<sub>5</sub>)<sub>4</sub> is reacted with XeF<sub>2</sub> the main product of the reaction is the cis-isomer of F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>4</sub>, while the reaction with diluted fluorine leads to a cis- and trans-isomer mixture of F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>4</sub>. In both cases, FTe(OTeF<sub>5</sub>)<sub>5</sub> is formed as a side product, which can be isolated by fractional crystallization.<sup>22,28</sup>

The Te(IV) compound Te(OTeF<sub>5</sub>)<sub>4</sub> can act as a OTeF<sub>5</sub> group or fluoride ion acceptor, forming the anionic [Te(OTeF<sub>5</sub>)<sub>5</sub>]<sup>−</sup> or [TeF(OTeF<sub>5</sub>)<sub>4</sub>]<sup>−</sup>.<sup>209</sup> On the other hand Te(OTeF<sub>5</sub>)<sub>4</sub> is able to act as an OTeF<sub>5</sub> group donor toward strong Lewis acids like AsF<sub>5</sub>, forming the mixed teflate fluoride cations [TeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>n−3</sub>]<sup>+</sup> with n = 0–3.<sup>57</sup> Group 16 teflates and selenates are gathered in Table 7.

**9. GROUP 17 TEFLATES**

The whole set of hypohalites of the TeF<sub>5</sub> moiety, XOteF<sub>5</sub> (X = F, Cl, Br, I), was recently investigated from a computational point of view using state-of-the-art methods of quantum chemical topology.<sup>33</sup> When analyzing the QTAIM charges, it is interesting to point out that the total charge of the TeF<sub>5</sub> moiety is independent of the atom X. It is in fact the O atom the most affected by the nature of X, therefore acting as a charge buffer between the X atom and the TeF<sub>5</sub> fragment. This way, the charge at the O atom shifts from −0.37 au in the case of X = F to −1.01 au for X = I. As expected, the charge of the oxygen bound F atom of FOTeF<sub>5</sub> is negative (−0.14 au). The other halogen atoms of the hypohalite group become increasingly positively charged (Cl = 0.29, Br = 0.40, and I = 0.61 au). The IQA (interacting quantum atoms) analysis also points to both F and OTeF<sub>5</sub> having similar electron-withdrawing properties in this
<table><caption>**Table 7.** Overview of Group 16 Teflate and Selenate Compounds</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="4">O</td><td>I</td><td>F<sub>5</sub>TeOOTeF<sub>5</sub></td><td>1,5,16,204,205</td></tr><tr><td>II</td><td>O(TeF<sub>5</sub>)<sub>2</sub></td><td>4,202</td></tr><tr><td>I</td><td>F<sub>5</sub>SeOOSeF<sub>5</sub></td><td>84,99</td></tr><tr><td>II</td><td>O(SeF<sub>5</sub>)<sub>2</sub></td><td>85</td></tr><tr><td rowspan="10">S</td><td>IV</td><td>NS-OTeF<sub>5</sub></td><td>208</td></tr><tr><td>VI</td><td>F<sub>5</sub>TeOSO<sub>2</sub>F</td><td>3,4</td></tr><tr><td></td><td>F<sub>5</sub>TeOSO<sub>2</sub>Cl</td><td>4</td></tr><tr><td></td><td>(F<sub>5</sub>TeO)<sub>2</sub>SO<sub>2</sub></td><td>3,4</td></tr><tr><td></td><td>F<sub>5</sub>TeOSO<sub>3</sub>H</td><td>3,4</td></tr><tr><td></td><td>F<sub>5</sub>SeO-SO<sub>2</sub>-OSO<sub>2</sub>F</td><td>90</td></tr><tr><td></td><td>F<sub>5</sub>SeOSF<sub>5</sub></td><td>98</td></tr><tr><td></td><td>F<sub>5</sub>SeO-SO<sub>2</sub>F</td><td>99</td></tr><tr><td></td><td>cis-BrCH<sub>2</sub>-SF<sub>4</sub>-OSeF<sub>5</sub></td><td>102</td></tr><tr><td rowspan="4">Se</td><td>IV</td><td>OSe(OTeF<sub>5</sub>)<sub>2</sub></td><td>87</td></tr><tr><td></td><td>F<sub>2</sub>Se(OTeF<sub>5</sub>)<sub>2</sub></td><td>87</td></tr><tr><td></td><td>OSe(SeF<sub>5</sub>)<sub>2</sub></td><td>23,87</td></tr><tr><td></td><td>F<sub>2</sub>Se(SeF<sub>5</sub>)<sub>2</sub></td><td>87</td></tr><tr><td rowspan="10">Te</td><td>IV</td><td>Te(OTeF<sub>5</sub>)<sub>4</sub></td><td>22,28,57</td></tr><tr><td></td><td>[NR<sub>4</sub>][Te(OTeF<sub>5</sub>)<sub>4</sub>] (R = Me, Et)</td><td>209</td></tr><tr><td></td><td>[NMe<sub>4</sub>][TeF(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>209</td></tr><tr><td></td><td>F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>2</sub></td><td>87</td></tr><tr><td></td><td>[TeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>n-3</sub>] [AsF<sub>m</sub>(OTeF<sub>5</sub>)<sub>6-m</sub>]<br>(n = 0-3)</td><td>57</td></tr><tr><td>VI</td><td>Te(OTeF<sub>5</sub>)<sub>6</sub></td><td>22,28,32</td></tr><tr><td></td><td>TeF(OTeF<sub>5</sub>)<sub>5</sub></td><td>22,28</td></tr><tr><td></td><td>cis-F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>4</sub></td><td>22,28</td></tr><tr><td></td><td>trans-F<sub>2</sub>Te(OTeF<sub>5</sub>)<sub>4</sub></td><td>18,22,28,210,211</td></tr><tr><td></td><td>mer-F<sub>3</sub>Te(OTeF<sub>5</sub>)<sub>3</sub></td><td>87</td></tr><tr><td></td><td>cis-F<sub>4</sub>Te(OTeF<sub>5</sub>)<sub>2</sub></td><td>22,28</td></tr><tr><td></td><td>trans-F<sub>4</sub>Te(OTeF<sub>5</sub>)<sub>2</sub></td><td>22,28</td></tr></tbody></table>

case. Nevertheless, when comparing this set of compounds with the corresponding XF, a stronger electron-sharing interaction in the bonding is found for the teflate analogues, whereas a stronger electrostatic contribution and a higher charge separation are found in the case of XF. The electronic distribution within the systems was additionally investigated by means of electron distribution functions (EDFs), which together with the QTAIM results, indicate the greater similarity of F and OTeF<sub>5</sub> from an electronic point of view when they are bonded to less electronegative systems.

## 9.1. Hypofluorite

Synthetic attempts to prepare FOTeF<sub>5</sub>, which exists as a colorless gas at room temperature, can be traced back to a report of Seppelt et al. from 1973.<sup>5</sup> In this work, UV light treatment of FXeOTeF<sub>5</sub> and fluorination of Hg(OTeF<sub>5</sub>)<sub>2</sub> both failed to achieve the desired hypofluorite. A decade later, the work of Christe et al. showed that the reaction of HOTeF<sub>5</sub> with [NF<sub>4</sub>] [HF<sub>2</sub>] produced FOTeF<sub>5</sub> in low yields. However, yields of approximately 70% could be achieved by reacting an excess of CsOTeF<sub>5</sub> with fluorine fluorosulfate FOSO<sub>2</sub>F at -45 °C for 9 days (see eq 24).<sup>6</sup> An improved synthesis enabled a higher yield of 94% in a much shorter reaction time of 1 day by reacting B(OTeF<sub>5</sub>)<sub>3</sub> with elemental fluorine in a Monel vessel heated to 115 °C, as shown in eq 25.<sup>144</sup> The advantage of this method is the ease of isolating FOTeF<sub>5</sub> from the side products, namely BF<sub>3</sub> and HOTeF<sub>5</sub>, by fractional condensation.

$$ \mathrm{CsOTeF_5} + \mathrm{FOSO_2F} \rightarrow \mathrm{FOTeF_5} + \mathrm{CsSO_3F} \quad (24) $$

$$ \mathrm{B(OTeF_5)_3} + 3\mathrm{F_2} \rightarrow 3\mathrm{FOTeF_5} + \mathrm{BF_3} \quad (25) $$

In 1956, Dudley, Cady, and Eggers described the formation of pentafluorosulfur hypofluorite (FOSF<sub>5</sub>) through the reaction of fluorine with SO<sub>2</sub>F in the presence of AgF<sub>2</sub> at 200 °C.<sup>125</sup> Later, electron diffraction studies revealed that the sulfur atom in FOSF<sub>5</sub> is octahedrally coordinated.<sup>212</sup>

<figure><img src="image_43.png" /><figcaption>**Figure 14.** Optimized structures and NPA charges of the hypohalites FOTeF<sub>5</sub>, ClOTeF<sub>5</sub>, BrOTeF<sub>5</sub>, IOTeF<sub>5</sub> as well as the calculated electrostatic potential plotted on the electron density (BP86/def2-SVP level, isovalue: 0.025 e bohr<sup>-3</sup>) using ORCA 5.0.4,<sup>80</sup> visualized with Chemcraft.<sup>81</sup></figcaption></figure>
Building on this work, Cady et al. turned their attention to the fluorination of SeO₂ and SeOCl₂. In 1959, they reported the formation of pentafluoroselenium hypofluorite (FOSeF₅) in small yields by reacting these selenium compounds with fluorine in the presence of AgF₂.⁸⁴ This process was later improved in 1970 when Cady et al. described a more efficient synthesis. In this method, SeOF₂ was reacted with KF to produce KSeOF₃, which was then fluorinated to yield FOSeF₅.⁹⁸ With advances in OSeF₅ chemistry, Seppelt reported a new approach in 1973 to synthesize FOSeF₅ by reacting Hg(OSeF₅)₂ with fluorine.⁸⁵

## 9.2. Hypochlorite

The hypochlorite ClOTeF₅, was quantitatively obtained by reacting Hg(OTeF₅)₂ with chlorine monofluoride ClF (see eq 26) as shown by Seppelt et al. in 1973.⁵ This compound exists as a yellow liquid at room temperature and its hydrolysis products, Cl₂O and HOTeF₅, indicate the positive charge character of the chlorine in ClOTeF₅ (see Figure 14).

$$ \mathrm{Hg(OTeF_5)_2 + 2ClF \rightarrow 2ClOTeF_5 + HgF_2} \quad (26) $$

Another approach is to react B(OTeF₅)₃ with ClF, forming ClOTeF₅ and the side product BF₃.²⁷ The high volatility of BF₃ allows for a more straightforward isolation of ClOTeF₅ (see eq 27). Christe et al. reported that reacting HOTeF₅ with either ClOSO₂F or ClF allows direct access to ClOTeF₅, eliminating the need for teflate transfer reagents used in previous methods, as shown in eq 28.²¹³

$$ \mathrm{B(OTeF_5)_3 + 3ClF \rightarrow 3ClOTeF_5 + BF_3} \quad (27) $$

$$ \mathrm{HOTeF_5 + ClX \rightarrow ClOTeF_5 + HX} \\ \mathrm{X = F, OSO_2F} \quad (28) $$

In 1969, Gould et al. and Schack et al. independently reported the formation of pentafluorosulfur hypochlorite (ClOSF₅) through the reaction of SOF₄ with ClF in the presence of CsF.¹¹³,²¹⁴ Later, Kornath et al. investigated the vibrational spectra and gas-phase structure of ClOSF₅. They synthesized the compound using a similar method, starting from SOF₂, which yielded an almost quantitative formation of ClOSF₅.²¹⁵

In 1973, Seppelt reported the formation of pentafluoroselenium hypochlorite (ClOSeF₅) by the reaction of Hg(OSeF₅)₂ with ClF.⁸⁵

## 9.3. Hypobromite and Bromine Teflates

The bromine derivative BrOTeF₅, which appears as a ruby red liquid at room temperature, can be synthesized by reacting ClOTeF₅ with bromine (see eq 29).²³ The compound remains stable when handled at -20 °C and can also be stored in dry CFCl₃. However, any water present in the solution will hydrolyze BrOTeF₅, generating HOTeF₅ and precipitating Br₂O.²¹⁶ The selenate derivative BrOSeF₅ is synthesized by reacting elemental bromine with either FOSeF₅ (62% yield) or Br(OSeF₅)₃ (94% yield).⁸⁵

$$ 2ClOTeF_5 + Br_2 \rightarrow 2BrOTeF_5 + Cl_2 \quad (29) $$

A teflate compound of bromine in an oxidation state higher than +I was reported for the bromonium(III) cation in the salt [Br(OTeF₅)₂][Sb(OTeF₅)₆]. This salt is obtained by reacting [Xe(OTeF₅)][Sb(OTeF₅)₆] with a stoichiometric excess of BrOTeF₅ in SO₂ClF at -78 °C, as shown in eq 30.⁶⁷ Seppelt reported a bromine oxidation state of +III for the neutral compound Br(OSeF₅)₃ by reacting BrF₃ with 3 equiv of

HOSeF₅,⁸⁵ and as an anion in the salt Rb[Br(OSeF₅)₄] upon reacting the neutral compound with Rb[OSeF₅].²³

$$ \mathrm{[Xe(OTeF_5)][Sb(OTeF_5)_6] + BrOTeF_5 \rightarrow [Br(OTeF_5)_2][Sb(OTeF_5)_6] + Xe} \quad (30) $$

A bromine(V) compound stabilized by a teflate group was prepared from the controlled ozonization of BrOTeF₅ in CFCl₃ at low temperatures. This process results in the formation of O₂Br(OTeF₅), which yields colorless crystals upon recrystallization from CH₂Cl₂ (see eq 31 and Figure 15).

<figure><img src="image_44.png" /><figcaption>Figure 15. Molecular structure of O₂Br(OTeF₅) in the solid state.²¹⁶</figcaption></figure>

$$ \mathrm{BrOTeF_5 + O_3 \rightarrow O_2BrOTeF_5} \quad (31) $$

Partial decomposition occurs at temperatures above its melting point of -20 °C, but unlike other BrO₂ compounds such as Br₂O₅ and FBrO₂, it does not exhibit an explosive behavior.²¹⁶ This increased stability is attributed to the presence of the electron withdrawing OTeF₅ group.

## 9.4. Iodine Teflates

Before the discovery of iodine compounds with teflate groups, the selenium derivatives IOSeF₅ and I(OSeF₅)₃ were known to be unstable solids that decompose readily via autoredox pathways.⁸⁵ On the other hand, the greater stability of hexavalent tellurium made it possible to synthesize stable iodine compounds.

The formation of the hypoiodite compound IOTeF₅ as a brown solid was reported through various methods. However, characterization proved difficult because it readily converts into I(OTeF₅)₃ and other side products, as shown in eqs 32–34.²³

$$ I(OTeF_5)_3 + I_2 \rightleftharpoons 3IOTeF_5 \quad (32) $$

$$ \mathrm{ClOTeF_5 + ICl \rightarrow I(OTeF_5)_3 \rightarrow IOTeF_5 + ICl_3 + Cl_2} \quad (33) $$

$$ \mathrm{BrOTeF_5 + I_2 \rightarrow I(OTeF_5)_3 \rightarrow IOTeF_5 + IBr + Br_2} \quad (34) $$

The neutral iodine(III) teflate compound, I(OTeF₅)₃, is obtained from the reaction of ICl₃ with ClOTeF₅ in CFCl₃ (see eq 35).²³ Subsequent crystallization attempts yielded orange crystals identified as the decomposition product ClI(OTeF₅)₂, formed by the slow reaction with CFCl₃ (see eq 36). The molecular structure of ClI(OTeF₅)₂ reveals a T-shaped
coordination at the iodine center, although the {ClIO₂} skeleton is not planar due to the steric repulsion of the OTeF₅ groups. ClIO₂(OTeF₅)₂ is also produced by reacting I₂ with IF₅ and B(OTeF₅)₃ in CFCl₃ at room temperature (see eq 37). By occasionally pumping off BF₃ and slowly removing volatile components, the compound is formed as a red liquid, which crystallizes slowly at room temperature.²¹⁷

$$3\text{ClOTeF}_5 + \text{ICl}_3 \rightarrow \text{I}(\text{OTeF}_5)_3 + 3\text{Cl}_2 \quad (35)$$

$$\text{I}(\text{OTeF}_5)_3 \xrightarrow{\text{CFCl}_3} \text{ClI}(\text{OTeF}_5)_2 \quad (36)$$

$$\text{I}_2 + \text{IF}_5 + \text{exc. B}(\text{OTeF}_5)_3 \xrightarrow{\text{CFCl}_3} \text{ClI}(\text{OTeF}_5)_2 + \text{BF}_3 \quad (37)$$

The reaction of I(OTeF₅)₃ with [N(nBu)₄][OTeF₅] leads to the formation of the iodine(III) anion [I(OTeF₅)₄]⁻. In the molecular structure of the salt [N(nBu)₄][I(OTeF₅)₄] the anion adopts a nearly square planar geometry around the iodine atom (see Figure 16).²¹⁷

<figure><img src="image_45.png" /><figcaption>Figure 16. Molecular structure of the [I(OTeF₅)₄]⁻ anion in the solid state.²¹⁷ The [N(nBu)₄]⁺ cation is omitted for clarity.</figcaption></figure>

The reaction of fluorocarbon iodides with teflate transfer reagents results in the formation of RFI(OTeF₅)₂ (RFI = CF₃, C₂F₅, n-C₃F₇, i-C₃F₇) species. Naumann et al. reported the ligand exchange reaction of CF₃IF₂ with HOTeF₅, resulting in the formation of CF₃I(OTeF₅)₂. For the complete conversion, an excess of HOTeF₅ in MeCN is necessary.²¹⁸ Later, Christe et al. demonstrated that ClOteF₅ can also be used to access RFI(OTeF₅)₂ species (see eq 38). Except for the trifluoromethyl derivative, these compounds are thermally stable at room temperature.²¹⁹

$$RFI\text{IF}_2 + 2\text{ClOteF}_5 \rightarrow RFI(\text{OTeF}_5)_2 + \text{ClF} \\ RFI = \text{CF}_3, \text{C}_2\text{F}_5, \text{n-C}_3\text{F}_7, \text{i-C}_3\text{F}_7 \quad (38)$$

The teflate transfer reaction of B(OTeF₅)₃ with IF₅, which is known as a useful synthon for pentavalent iodine(V) compounds, provides access to the iodine(V) compound FI(OTeF₅)₄.²¹,³¹ Attempts to exchange the remaining fluorido ligand with the OTeF₅ group by adding B(OTeF₅)₃ resulted in the formation of OI(OTeF₅)₃ (see eq 39).²¹,³¹ The oxo compound OI(OTeF₅)₃ can be isolated as a colorless sublimable solid that is sensitive to water. The fully teflate substituted iodine(V) compound, I(OTeF₅)₅, can be synthesized by reacting I(OTeF₅)₃ with Xe(OTeF₅)₂ resulting in a colorless sublimable solid, as shown in eq 40.²¹,³¹ Based on ¹⁹F NMR spectroscopy, the observation of two inequivalent teflate groups in a 1:4 ratio suggests that the compound I(OTeF₅)₅ has a square pyramidal molecular geometry similar to that of IF₅.

However, I(OTeF₅)₅ quickly undergoes disproportionation, yielding I(OTeF₅)₃.²¹ Reacting I(OTeF₅)₅ with IF₅ results in mixed substituted fluoro-teflate iodine(V) compounds FnI-(OTeF₅)₅-n (n = 0-5) (see eq 41). The compounds FᵃₓI(OTeF₅)₄, FᵃₓFᶜₓI(OTeF₅)₃, cis- and trans-FᵃₓF₂ᶜₓI(OTeF₅)₂ and FᵃₓF₃ᶜₓI(OTeF₅) were characterized by ¹⁹F NMR spectroscopy, where each compound has a fluorine atom occupying the axial position in a square pyramidal molecular structure.²¹,³² Similar substitution was also reported by Seppelt et al. for the selenate derivatives, FₙI(OSeF₅)₅-n (n = 0-5), formed by reacting IF₅ with POF₂OSeF₅.²¹

$$\text{IF}_5 \xrightarrow[{-\frac{4}{3}\text{BF}_3}]{\frac{4}{3}\text{B}(\text{OTeF}_5)_3} \text{FI}(\text{OTeF}_5)_4 \xrightarrow[{-\text{TeF}_6}]{\text{BF}(\text{OTeF}_5)_4} \text{OI}(\text{OTeF}_5)_3 \quad (39)$$

$$\text{Xe}(\text{OTeF}_5)_2 + \text{I}(\text{OTeF}_5)_3 \xrightarrow{h\nu} \text{I}(\text{OTeF}_5)_5 + \text{Xe} \quad (40)$$

$$\text{I}(\text{OTeF}_5)_5 + \text{IF}_5 \rightleftharpoons \text{F}_n\text{I}(\text{OTeF}_5)_{5-n} \\ n = 0-5 \quad (41)$$

In contrast to the iodine(III) anion [I(OTeF₅)₄]⁻, attempts to form [N(nBu)₄][I(OTeF₅)₆] by reacting I(OTeF₅)₅ with [N(nBu)₄][OTeF₅] were unsuccessful, even with careful screening of the reaction conditions. Instead, the hydrolyzed product [N(nBu)₄][OI(OTeF₅)₄] was obtained and characterized in the solid state. The [OI(OTeF₅)₄]⁻ anion was found to have a pyramidal geometry with the oxygen atom at the apical position (see Figure 17).²¹⁷

<figure><img src="image_46.png" /><figcaption>Figure 17. Molecular structure of the [OI(OTeF₅)₄]⁻ anion in the solid state.²¹⁷ The [N(nBu)₄]⁺ cation is omitted for clarity.</figcaption></figure>

Efforts to achieve an even higher oxidation state with iodine(VII) were so far unsuccessful. The reaction of IF₇ with B(OTeF₅)₃ resulted in the formation of [IF₆][BF(OTeF₅)₃], with the cation [IF₆]⁺ being identified by Raman spectroscopy.²¹ Group 17 teflates and selenates are gathered in Table 8.

## 10. GROUP 18 TEFLATES

For the lighter noble gases (Ng) helium, neon and argon, no teflate compounds are known to date.

### 10.1. Krypton Teflates

In 1979, Seppelt et al. attempted to synthesize the first krypton teflate. However, when KrF₂ reacted with B(OTeF₅)₃ in ClO₃F at -100 °C, the formation of krypton and F₅TeOOteF₅ was observed instead.¹⁶ Another attempt using HOTeF₅ in ClO₃F together with KrF₂ at -30 °C also failed to yield Kr(OTeF₅)₂.²²⁰

A decade later, Schrobilgen et al. successfully synthesized Kr(OTeF₅)₂ by reacting KrF₂ with B(OTeF₅)₃ in SO₂ClF at
<table><caption>**Table 8. Overview of Group 17 Teflates and Selenate Compounds**</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td rowspan="2">F</td><td rowspan="2">I</td><td>FOTeF<sub>5</sub></td><td>5,6,144,175</td></tr><tr><td>FOSeF<sub>5</sub></td><td>84,85,87,98</td></tr><tr><td rowspan="2">Cl</td><td rowspan="2">I</td><td>ClOTeF<sub>5</sub></td><td>5,6,213</td></tr><tr><td>ClOSeF<sub>5</sub></td><td>85</td></tr><tr><td rowspan="5">Br</td><td>I</td><td>BrOTeF<sub>5</sub></td><td>23,58,216</td></tr><tr><td>III</td><td>[Br(OTeF<sub>5</sub>)<sub>2</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>67</td></tr><tr><td>V</td><td>O<sub>2</sub>BrOTeF<sub>5</sub></td><td>216</td></tr><tr><td>I</td><td>BrOSeF<sub>5</sub></td><td>85</td></tr><tr><td>III</td><td>Br(OTeF<sub>5</sub>)<sub>3</sub><br>Rb[Br(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>85<br>23</td></tr><tr><td rowspan="10">I</td><td>I</td><td>IOTeF<sub>5</sub></td><td>23</td></tr><tr><td>III</td><td>I(OTeF<sub>5</sub>)<sub>3</sub></td><td>23</td></tr><tr><td></td><td>ClI(OTeF<sub>5</sub>)<sub>2</sub></td><td>217</td></tr><tr><td></td><td>R<sup>F</sup>I(OTeF<sub>5</sub>)<sub>2</sub> (R<sup>F</sup> = CF<sub>3</sub>, C<sub>2</sub>F<sub>5</sub>, n-C<sub>3</sub>F<sub>7</sub>, i-C<sub>3</sub>F<sub>7</sub>)</td><td>218,219</td></tr><tr><td></td><td>[N(nBu)<sub>4</sub>][I(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>217</td></tr><tr><td>V</td><td>F<sub>n</sub>I(OTeF<sub>5</sub>)<sub>s-n</sub> (n = 0-5)</td><td>21,31</td></tr><tr><td></td><td>OI(OTeF<sub>5</sub>)<sub>3</sub></td><td>21,31</td></tr><tr><td></td><td>[N(nBu)<sub>4</sub>][OI(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>217</td></tr><tr><td>III</td><td>I(OTeF<sub>5</sub>)<sub>3</sub></td><td>85</td></tr><tr><td>V</td><td>F<sub>n</sub>I(OTeF<sub>5</sub>)<sub>s-n</sub></td><td>21</td></tr></tbody></table>

−110 °C (see eq 42). This marked the first Kr−O bond reported at this time. The compound was characterized by a distinctive AB₄ pattern in the ¹⁹F and ¹⁷O NMR spectra, which diminished as temperature increased to −90 °C, decomposing into krypton and F₅TeOOtE₅. Chemical shift comparisons indicated a more ionic nature for the teflate group in Kr(OTeF₅)₂ compared to analogous compounds like FXeOTeF₅ and Xe(OTeF₅)₂.²⁰⁴

$$3\text{KrF}_2 + 2\text{B(OtE}_F_5)_3 \rightarrow 3\text{Kr(OtE}_F_5)_2 + 2\text{BF}_3 \quad (42)$$

## 10.2. Xenon(II) Teflates

The first noble gas species involving a teflate group were initially reported by Sladky et al. in 1969, including FXeOTeF₅, Xe(OTeF₅)₂ and [XeOTeF₅][AsF₆].²²¹⁻²²³ The monoteflate substituted neutral xenon(II) compound FXeOTeF₅ appears as a pale yellow liquid under normal conditions and is produced by HF displacement in the reaction of XeF₂ with HOTEF₅ (see eq 43).²²¹ Additionally, FXeOTeF₅ can be obtained in quantitative yields by the equimolar reaction of XeF₂ with Xe(OTeF₅)₂, as shown in eq 44.¹³ The fully teflate-substituted neutral xenon(II) compound, Xe(OTeF₅)₂ is synthesized by adding a second equivalent of HOTEF₅ to the FXeOTeF₅, which is formed in situ.²²² The presence of HF and HOTEF₅ forms an equilibrium involving XeF₂, FXeOTeF₅ and Xe(OTeF₅)₂, necessitating the freeze-pump-thaw method to eliminate generated HF from the reaction mixture. This process is repeated until solid Xe(OTeF₅)₂ remains upon further addition of HOTEF₅, indicating that the liquid FXeOTeF₅ does no longer exist in the reaction mixture.²²⁴

$$XeF_2 \overset{\text{HOTEF}_5}{\underset{\text{HF}}{\rightleftharpoons}} FXeOTeF_5 \overset{\text{HOTEF}_5}{\underset{\text{HF}}{\rightleftharpoons}} Xe(OTeF_5)_2 \quad (43)$$

$$XeF_2 + Xe(OTeF_5)_2 \rightarrow 2FXeOTeF_5 \quad (44)$$

An alternative approach to synthesizing Xe(OTeF₅)₂ involves the reagents XeF₂ and B(OTeF₅)₃ in CFCI₃, resulting in

immediate precipitation of Xe(OTeF₅)₂ free from FXeOTeF₅ (see eq 45).¹,⁴⁴ This approach eliminates the formation and, consequently, the quenching of HF. Xe(OTeF₅)₂ appears as colorless crystals (see Figure 18). In the molecular structure of

<figure><img src="image_47.png" /><figcaption>**Figure 18.** Crystals of Xe(OTeF₅)₂, obtained by the reaction of XeF₂ with excess HOTEF₅.</figcaption></figure>

Xe(OTeF₅)₂ the linear arrangement of O−Xe−O with the measured angle of 180° is consistent with the AX₂E₃ VSEPR arrangement typical for xenon(II) compounds.¹²,¹⁰⁶

$$3\text{XeF}_2 + 2\text{B(OtE}_F_5)_2 \rightarrow 3\text{Xe(OtE}_F_5)_2 + 2\text{BF}_3 \quad (45)$$

An equimolar reaction of Xe(OTeF₅)₂ with Xe(OTeF₅)₂ in CFCI₃ results in the formation of the mixed compound, Xe(OTeF₅)(OTeF₅), as detected by ¹⁹F NMR spectroscopy. However, these compounds exist only in an equilibrium.⁵,¹⁰⁷ Another mixed-ligand derivative, Xe(OTeF₅)(OIOF₄), is spectroscopically identified during the synthesis of Xe(OIOF₄)₂. This was done by reacting Xe(OTeF₅)₂ with the protic acid HOIOF₄ in CFCI₃.²²⁵

Moreover, formation of a polymeric xenon(II) species was reported by Seppelt et al. in the reaction of cis-(HO)₂TeF₄ with XeF₂ in C₄F₉SO₂F at room temperature.²²⁶ The compound is formed by HF elimination and exists as a pale yellow solid with the composition (XeO₂TeF₄)ₙ. This polymeric compound is insoluble in all common solvents and decomposes by release of xenon and oxygen at temperatures above 80 °C. In contrast, likely due to its lower acidity compared to the cis-isomer, trans-(HO)₂TeF₄ does not react under the same conditions.²²⁷

In a follow-up study, reacting cis-(HO)₂TeF₄ with [Xe₂F₃][AsF₆] produced a stable yellow solid, [Xe₂FO₂TeF₄][AsF₆]. This compound is insoluble in chlorofluorocarbons and SO₂ClF. It decomposes at −70 °C in SO₂ and at −40 °C in MeCN, yet dissolves readily in HF. When [Xe₂FO₂TeF₄][AsF₆] is kept in HF for extended periods of time at low temperatures, orange crystals of the solvolysis product [HF·HOTEF₄OXe][AsF₆] form. The molecular structure of this product shows bridging cation−anion interaction between xenon and one fluorine atom of the [AsF₆]⁻ anion.²²⁷

The xenon(II) teflate cation [Xe(OTeF₅)]⁺ was initially reported by Sladky et al. as the product of the reaction of FXe(OTeF₅) with AsF₅ (see Scheme 10).²²³ The resulting salt [Xe(OTeF₅)][AsF₆] exhibits significant cation−anion interaction through a fluoride bridge, as shown by Raman spectroscopy.²²⁸ This interaction was further confirmed by the molecular structure of the salt, which shows a short Xe−F contact distance of 224(3) pm.¹⁰⁶

When [Xe(OTeF₅)][AsF₆] is dissolved in liquid SbF₅, [Xe(OTeF₅)][Sb₂F₁₁] is formed as a yellow-orange solution
Scheme 10. Overview of Teflate-Based Xenonium Ions and Their Synthesis

synthetic routes toward xenonium ions

<figure><img src="image_48.png" /></figure>

due to displacement of AsF₅ (see Scheme 10). While a fluoride-bridge interaction similar to that in [Xe(OTeF₅)][AsF₆] is expected for this salt, the Raman spectrum obtained was too complex to assign specific bands.²²⁸

In contrast, when [Xe(OTeF₅)][AsF₆] is dissolved in BrF₅ at -48 °C, it forms the cation [FXeFXeOTeF₅]⁺, characterized by the presence of a teflate group in the ¹⁹F NMR spectrum and two distinct chemical shifts in the ¹²⁹Xe NMR spectrum (see Scheme 10).²²⁸

The first fully teflate-substituted salt, [Xe(OTeF₅)][Sb(OTeF₅)₆], was synthesized during the investigation of Sb(OTeF₅)₅ by oxidatively transferring two teflate groups from Xe(OTeF₅)₂ to Sb(OTeF₅)₃. However, instead of the pentavalent antimony teflate compound, the product of the reaction was the noble gas cation [Xe(OTeF₅)]⁺ countered by the weakly coordinating anion [Sb(OTeF₅)₆]⁻ (see Scheme 10).⁴³ The salt was later characterized in the solid state, showing that it forms an adduct with the weakly basic SO₂ClF solvent molecule, rather than being a monocoordinated xenonium center.⁴³,⁶⁹ The strong two-electron oxidizing property of the salt [Xe(OTeF₅)(SO₂ClF)][Sb(OTeF₅)₆] makes it a useful reagent in the preparation of trihalomethyl carbocations.⁶⁷ Additionally, the reaction of 3 equiv of Xe(OTeF₅)₂ with Sb(OTeF₅)₃ in SO₂ClF generates the [Xe₂(OTeF₅)₃]⁺ cation, as confirmed by Raman spectroscopy of the resulting solid (see Scheme 10). The calculated molecular structure of [Xe₂(OTeF₅)₃]⁺ suggests C₂v symmetry for the free cation, similar to that of the known fluoride analogue [Xe₂F₃]⁺.⁶⁹

Considering that the Lewis acidity of the [Xe(OTeF₅)]⁺ cation is similar to that of the well-known [XeF]⁺ cation, it is noted, without much detail given, that [Xe(OTeF₅)]⁺ forms cationic adducts with nitrogen bases. These adducts include [Xe(OTeF₅)(MeCN)]⁺, [Xe(OTeF₅)(C₅F₅N)]⁺ and [Xe(OTeF₅)(C₃F₃N₃)]⁺ (C₃F₃N₃ = cyanuric fluoride), existing as salts of the [Sb(OTeF₅)₆]⁻ anion. Additionally, a selenium derivative, [Xe(OSeF₅)(NSF₃)][AsF₆], has also been reported.²²⁹

Recently, Riedel et al. reported that the Lewis superacid Al(OTeF₅)₃ is able to abstract a teflate group from Xe(OTeF₅)₂, resulting in a nonisolable intermediate salt, [Xe(OTeF₅)]⁺

[Al(OTeF₅)₄], which forms a yellow solution in SO₂ClF. This intermediate shows a complex ¹⁹F NMR spectrum, indicating rapid exchange of teflate groups between the cation and the anion. Upon adding a fluorinated nitrogen base, cationic adducts [Xe(OTeF₅)(pyF)]⁺ (pyF = C₅F₅N, C₅H₂F₃N) will be formed, countered by the weakly coordinating [Al(OTeF₅)₄]⁻ anion (see Scheme 10). These compounds were characterized using low-temperature spectroscopic methods like ¹⁹F NMR, ¹²⁹Xe NMR, IR and Raman spectroscopy. Also, the high oxidation potential of [Xe(OTeF₅)(C₅F₅N)][Al(OTeF₅)₄] has been experimentally demonstrated by the oxidation of tris(4-tribromophenyl)amine, a derivative of “magic blue”, to form the ammoniumyl radical cation.⁷⁰

Furthermore, changing the anion with [Sb(OTeF₅)₆]⁻ facilitated the growth of single crystals for the adduct salt [Xe(OTeF₅)(C₅F₅N)][Sb(OTeF₅)₆].⁷⁰ This compound represents the second example of the heteroleptic xenonium(II) teflate cation in the solid state, alongside the previously reported [Xe(OTeF₅)(SO₂ClF)][Sb(OTeF₅)₆]. Both structures confirm a nearly linear alignment at the xenonium(II) center (see Figure 19).

<figure><img src="image_49.png" /><figcaption>**Figure 19.** Molecular structures in the solid state of the [Xe(OTeF₅)(SO₂ClF)]⁺ cation (top) and the [Xe(OTeF₅)(C₅F₅N)]⁺ cation (bottom).⁴³,⁷⁰ The [Sb(OTeF₅)₆]⁻ counteranions are omitted for clarity.</figcaption></figure>

The selenate derivatives of xenon(II) species are FXeO-SeF₅,¹⁰⁶,¹⁰⁷ Xe(OSeF₅)₂,⁵,¹⁰⁶,¹⁰⁷,²³⁰ and the cation [Xe(OSeF₅)]⁺ as its [AsF₆]⁻ salt.¹⁰⁶,²²⁹

### 10.3. Xenon(IV) Teflates

The chemistry of xenon(IV) teflate compounds began with the preparation of Xe(OTeF₅)₄ by reacting XeF₄ with B(OTeF₅)₃ in perfluoro-*n*-hexane at 0 °C, as shown in eq 46.²³¹

$$ \mathrm{XeF_4} + \frac{4}{3}\mathrm{B(OTeF_5)_3} \rightarrow \mathrm{Xe(OTeF_5)_4} + \frac{4}{3}\mathrm{BF_3} \quad (46) $$

The resulting yellow crystalline compound can be sublimed under vacuum at 50 °C but is found to be less thermally stable compared to its fluorine analogue XeF₄. This difference is attributed to the formation of a relatively strong peroxide bond in F₅TeOOtE₅, which is the decomposition product of Xe(OTeF₅)₄, unlike elemental fluorine in the case of XeF₄. Moreover, Xe(OTeF₅)₄ decomposes slowly under ambient
conditions and even at low temperatures in solution.²²⁰ In its solid state, the molecular structure of Xe(OTeF₅)₄ exhibits a planar XeO₄ geometry (see Figure 20).²³²

<figure><img src="image_50.png" /><figcaption>Figure 20. Molecular structure of Xe(OTeF₅)₄ in the solid state.²³²</figcaption></figure>

A series of mixed-ligand species XeFₙ(OTeF₅)₄⁻ (n = 0-4) are produced by mixing Xe(OTeF₅)₄ with the fluorine analogue XeF₄ in CFCI₃, as shown in eq 47. These compounds were identified using ¹⁹F NMR and ¹²⁹Xe NMR spectroscopy. The findings indicate that the teflate group has a lower effective electronegativity compared to fluorine, as revealed by difference in the ¹²⁹Xe NMR chemical shifts.²³³

$$Xe(OTeF_5)_4 + XeF_4 \rightarrow 2XeF_n(OTeF_5)_{4-n} \quad (47)$$

Additionally, when Xe(OTeF₅)₄ is dissolved in SbF₅, it forms the cationic mixed-ligand species [XeFₙ(OTeF₅)₃⁻]⁺ (n = 0-2) (see eq 48). The process likely begins with SbF₅ abstracting one teflate group, generating [Xe(OTeF₅)₃]⁺, followed by a rearrangement of fluorine and teflate ligands. While mixed-ligand cations can be observed at 5 °C, the solutions gradually decompose, indicated by the gas evolution.

$$Xe(OTeF_5)_4 \xrightarrow{SbF_5} [XeF_n(OTeF_5)_{3-n}][SbF_{5-n}(OTeF_5)_{1+n}] \quad (48)$$

Although the [Xe(OTeF₅)₃]⁺ cation can be observed in SbF₅, attempts to synthesize [Xe(OTeF₅)₃][Sb(OTeF₅)₆] by reacting Xe(OTeF₅)₄ with “Sb(OTeF₅)₃” (formed *in situ* from Xe(OTeF₅)₂ with Sb(OTeF₅)₃ in SO₂ClF) have been unsuccessful so far.²³⁴

## 10.4. Xenon(VI) Teflates

The highest known oxidation state observed for Ng-teflate compounds is xenon(VI). The homoleptic compound Xe(OTeF₅)₆ was synthesized by reacting XeF₆ with B(OTeF₅)₃ in perfluoro-*n*-pentane at -40 °C (see eq 49).¹⁶ This deep red crystalline compound is highly sensitive to temperature, air as well as light and is either insoluble or decomposes in all common solvents. Due to its sensitivity, spectroscopic investigations are very limited. Nevertheless, it could be analyzed through its decomposition products Xe(OTeF₅)₄ and the peroxide F₅TeOOTeF₅. Decomposition occurs thermally at -10 °C or

when exposed to daylight, even at -230 °C. Its direct UV light decomposition results in the formation of elemental xenon and F₅TeOOTeF₅. At room temperature, Xe(OTeF₅)₆ decomposes into Xe(OTeF₅)₂ and F₅TeOOTeF₅ within 3 days.²²⁰

$$XeF_6 + 2B(OTeF_5)_3 \rightarrow Xe(OTeF_5)_6 + 2BF_3 \quad (49)$$

Compared to XeF₆, the shielding effect of the teflate group provides additional stability to Xe(OTeF₅)₆ against hydrolysis. When hydrolyzed, it forms the colorless compound OXe(OTeF₅)₄. This oxo compound can be synthesized directly by reacting XeOF₄ with B(OTeF₅)₃ in fluoro-*n*-pentane at -30 °C (see eq 50).¹⁶

$$XeOF_4 \xrightarrow[\frac{4}{3}BF_3]{\frac{4}{3}B(OTeF_5)_3} OXe(OTeF_5)_4 \quad (50)$$

When OXe(OTeF₅)₄ and XeOF₄ are dissolved in F-114 (1,2-dichloro-1,1,2,2-tetrafluoroethane), they undergo ligand scrambling, leading to the formation of various mixed-ligand species labeled as OXeFₙ(OTeF₅)₄⁻ (n = 0-4), as shown in eq 51. The existence of cis- and trans-isomers of OXeF₂(OTeF₅)₂ suggests that these species adopt a square pyramidal geometry with the oxygen atom positioned at the apex relative to the center of the base.²²⁰

$$XeOF_4 + OXe(OTeF_5)_4 \rightarrow 2OXeF_n(OTeF_5)_{4-n} \quad (51)$$

Similar to [XeFₙ(OTeF₅)₃⁻]⁺, when OXe(OTeF₅)₄ is dissolved in SbF₅ it forms cationic mixed-ligand oxo compounds [OXeFₙ(OTeF₅)₃⁻]⁺ (n = 0-2) (see eq 52). This occurs because SbF₅ abstracts one teflate group, leading to ligand scrambling in the solution.²³⁴

$$OXe(OTeF_5)_4 \xrightarrow{SbF_5} [OXeF_x(OTeF_5)_{3-x}][Sb_nF_{(5n+1)-m}(OTeF_5)_m] \quad (52)$$

To prepare dioxo-xenon(IV) teflate species, XeO₂F₂ can be reacted with B(OTeF₅)₃ in SO₂ClF. When equimolar amounts are used, O₂Xe(OTeF₅)₂ can be quantitatively obtained. Using an excess of XeO₂F₂ produces mixed-ligand compounds like O₂XeFₙ(OTeF₅)₂⁻ (n = 0-2). According to ¹²⁹Xe NMR spectroscopic studies, these species exhibit a trigonal bipyramidal molecular geometry with oxygen atoms occupying the equatorial positions around the xenon(VI) center.²³³ Attempts to crystallize OXe(OTeF₅)₄ have resulted in O₂Xe(OTeF₅)₂ due to partial decomposition. O₂Xe(OTeF₅)₂ is stable at room temperature and exhibits a pseudo trigonal bipyramidal or distorted tetragonal geometry in the solid state, influenced by strong steric hindrance from the lone pair, as shown in Figure 21.²³²

<figure><img src="image_51.png" /><figcaption>Figure 21. Molecular structure of O₂Xe(OTeF₅)₂ in the solid state.²³²</figcaption></figure>
Additionally, when OXe(OTeF<sub>5</sub>)<sub>4</sub> is dissolved in SbF<sub>5</sub>, it forms cationic xenon(VI) species [O<sub>2</sub>XeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>1-n</sub>]<sup>+</sup> (n = 0–1) as decomposition products. Although these cations have been detected via NMR spectroscopy, further decomposition yields O<sub>2</sub>, TeF<sub>6</sub>, F<sub>5</sub>TeOOteF<sub>5</sub> and the cationic species [XeF]<sup>+</sup> and [XeOTeF<sub>5</sub>]<sup>+</sup>.<sup>234</sup> Group 17 teflates and selenates are gathered in Table 9.

<table><caption>**Table 9. Overview of Group 18 Teflate and Selenate Compounds**</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td>Kr</td><td>II</td><td>Kr(OTeF<sub>5</sub>)<sub>2</sub></td><td>16,204</td></tr><tr><td rowspan="16">Xe</td><td rowspan="16">II</td><td>FXe(OTeF<sub>5</sub>)</td><td>13,107,221</td></tr><tr><td>Xe(OTeF<sub>5</sub>)<sub>2</sub></td><td>1,12,32,106,107,222,224</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)][AsF<sub>6</sub>]</td><td>14,106,221,223,228</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)][Sb<sub>2</sub>F<sub>11</sub>]</td><td>228</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>67,229,235</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(SO<sub>2</sub>ClF)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>43,69,229</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(MeCN)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>229</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>3</sub>F<sub>3</sub>N<sub>3</sub>)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>229</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>5</sub>F<sub>5</sub>N)][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>70,229</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>5</sub>F<sub>5</sub>N)][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>70</td></tr><tr><td>[Xe(OTeF<sub>5</sub>)(C<sub>5</sub>H<sub>2</sub>F<sub>3</sub>N)][Al(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>70</td></tr><tr><td>Xe(OTeF<sub>5</sub>)(OSeF<sub>5</sub>)</td><td>5</td></tr><tr><td>Xe(OTeF<sub>5</sub>)(OIOF<sub>4</sub>)</td><td>225</td></tr><tr><td>(XeO<sub>2</sub>TeF<sub>4</sub>)<sub>n</sub></td><td>226,227</td></tr><tr><td>[Xe<sub>2</sub>(OTeF<sub>5</sub>)<sub>3</sub>][Sb(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>69</td></tr><tr><td>[FXeFXe(OTeF<sub>5</sub>)][AsF<sub>6</sub>]</td><td>228</td></tr><tr><td>[FXeOTeF<sub>4</sub>OXe<sub>5</sub>][AsF<sub>6</sub>]</td><td>227</td></tr><tr><td rowspan="3">IV</td><td>Xe(OTeF<sub>5</sub>)<sub>4</sub></td><td>32,220,231,232</td></tr><tr><td>XeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>4-n</sub> (n = 0–4)</td><td>233</td></tr><tr><td>[XeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>3-n</sub>]<sup>+</sup> (n = 0–2)<sup>a</sup></td><td>234</td></tr><tr><td rowspan="10">VI</td><td>Xe(OTeF<sub>5</sub>)<sub>6</sub></td><td>16,220</td></tr><tr><td>OXe(OTeF<sub>5</sub>)<sub>4</sub></td><td>16,32,220,233</td></tr><tr><td>OXeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>4-n</sub> (n = 0–4)</td><td>220,233</td></tr><tr><td>[OXeF<sub>x</sub>(OTeF<sub>5</sub>)<sub>3-x</sub>][Sb<sub>n</sub>F<sub>(Sn-1)-m</sub>(OTeF<sub>5</sub>)<sub>m</sub>]<sup>+</sup> (x = 0–3, m = 0–5, n = 1, 2, 3...)</td><td>234</td></tr><tr><td>O<sub>2</sub>Xe(OTeF<sub>5</sub>)<sub>2</sub></td><td>232,233</td></tr><tr><td>O<sub>2</sub>XeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>2-n</sub> (n = 0–2)</td><td>233</td></tr><tr><td>[O<sub>2</sub>XeF<sub>n</sub>(OTeF<sub>5</sub>)<sub>1-n</sub>]<sup>+</sup> (n = 0–1)<sup>a</sup></td><td>234</td></tr><tr><td>Xe(OSeF<sub>5</sub>)<sub>2</sub></td><td>87,103–107</td></tr><tr><td>FXe(OSeF<sub>5</sub>)</td><td>103,106,107</td></tr><tr><td>[Xe(OSeF<sub>5</sub>)][AsF<sub>6</sub>]</td><td>106</td></tr></tbody></table>

<sup>a</sup>Cation generated in SbF<sub>5</sub> solvent; corresponding anion not specified in the reference.

## 11. d-BLOCK AND f-BLOCK METAL TEFLATES

Since the synthesis of Hg(OTeF<sub>5</sub>)<sub>2</sub> in the early 1970s,<sup>5</sup> the number of compounds containing teflate ligands coordinated to d-block metals has increased to find representatives of most of these elements. The main progress took place in the decades of the 1980s and 1990s. However, it has only been recently, that the properties of the teflate group as a ligand in coordination chemistry have been investigated, in order to ascertain its similarity in terms of electronic properties to the fluoride ligand.<sup>34,36</sup> In comparison, the related ligands OSeF<sub>5</sub> and OSF<sub>5</sub> are far less studied. In fact, whereas compounds containing the

OSeF<sub>5</sub> ligand are known but are far more scarce than teflate-based ones, the first OSF<sub>5</sub> d-metal complexes were only reported recently.<sup>110</sup>

To facilitate the analysis of such a diverse class of compounds, they will appear divided in homoleptic and heteroleptic complexes. Homoleptic complexes often entail a synthetic challenge, whereas at the same time they are extremely interesting for the investigation of fundamental properties associated with metal-ligand pairs. In the particular case of the teflate ligand, this has not been an exception and homoleptic metal teflate species constitute a group of compounds of high importance by itself, gathering species of many different characteristics and applications. Herein, we will discuss first the neutral species and afterward the anionic ones. In the case of heteroleptic compounds, it will be shown how teflate is compatible with many different ligands, allowing for coordination spheres with a large number of possibilities.

Regarding f-block metal teflates, uranium provides the only example and will be described in a separate section.

### 11.1. Homoleptic d-Block Metal Teflates

#### 11.1.1. Neutral Complexes.
Neutral metal teflate complexes are known for a handful of elements in various oxidation states. They showcase interesting features, ranging from the suitability of some as teflate transfer reagents, to the acidic properties of others. The most popular d-block metal teflate species are Hg(OTeF<sub>5</sub>)<sub>2</sub> and AgOTeF<sub>5</sub> due to their widespread use as teflate transfer reagents, as becomes clear along this review. The Hg(II) species Hg(OTeF<sub>5</sub>)<sub>2</sub> was first reported in 1973 by the group of Seppelt to be prepared by reaction of HgF<sub>2</sub> and HOTEF<sub>5</sub> in a Teflon vessel,<sup>5</sup> leading to a white residue that had to be purified via sublimation at 180 °C<sup>5</sup> or 200 °C.<sup>11</sup> An improved synthetic route by Schrobilgen et al. appeared in 2014 to produce Hg(OTeF<sub>5</sub>)<sub>2</sub> in high yield and purity without the need for further purification. This approach uses high-purity HgF<sub>2</sub>, which is reacted with a slight excess of HOTEF<sub>5</sub> at 50 °C.<sup>236</sup> Based on this highly pure compound, a more thorough characterization by <sup>19</sup>F NMR and Raman spectroscopy was possible, as well as in the solid state by single crystal X-ray diffraction. The structure of Hg(OTeF<sub>5</sub>)<sub>2</sub> consists of oxygen-bridged chains, as opposed to the three-dimensional polymeric structure of HgF<sub>2</sub>. A distorted octahedral coordination sphere at the Hg(II) center results, with the Hg-O bonds trans to each other and the Hg-O and Hg-F contacts cis to one another and to the Hg-O bonds. Computationally, both a gauche-conformation (C<sub>2h</sub> symmetry),<sup>237</sup> as well as an anti-conformation (C<sub>2</sub> symmetry)<sup>236</sup> have been optimized. Nevertheless, in the optimized gauche one, related to that of the crystal structure, there is no intermolecular interaction with other units, leading to a 4-fold coordination with two primary Hg-O bonds and two weak interactions from equatorial F atoms of the teflate ligands.<sup>237</sup> The calculated gas-phase structure of the unknown trimeric [Hg(OTeF<sub>5</sub>)<sub>2</sub>]<sub>3</sub> species retains the gauche conformation of the solid state.<sup>236</sup>

The related compound with OSeF<sub>5</sub> ligands, Hg(OSeF<sub>5</sub>)<sub>2</sub>, was reported even earlier, in 1972.<sup>88</sup> A similar synthetic route was used in this case starting from HgF<sub>2</sub> and an excess HOSeF<sub>5</sub>. The compound was obtained after sublimation at 180 °C as colorless and hydrolysis-sensitive crystals, but still, has been subsequently used for follow-up chemistry.<sup>42,86,87,90</sup>

The Ag(I) species AgOTeF<sub>5</sub> was first reported in 1975 by Mayer and Sladky as a white crystalline solid, obtained by reaction of HOTEF<sub>5</sub> and AgCN in MeCN.<sup>10</sup> The contact ion-
pair formation in the AgOTeF<sub>5</sub>/MeCN system was proved by IR and Raman spectroscopy,<sup>10</sup> based on which, the compound was better formulated as [Ag(MeCN)<sub>2</sub>]OTeF<sub>5</sub>.<sup>11</sup> Later, it was suggested to have a dimeric structure with bridging teflate ligands.<sup>168</sup> Similar reactions with AgCN using CH<sub>2</sub>Cl<sub>2</sub> as a solvent or neat HOTEF<sub>5</sub> (50 °C) do not take place.<sup>168</sup> Nevertheless, AgF and HOTEF<sub>5</sub> react in DCM at room temperature to yield [Ag(DCM)(OTeF<sub>5</sub>)]<sub>2</sub>, which can be transformed into AgOTeF<sub>5</sub> under 4 h of dynamic vacuum,<sup>168,238</sup> providing a gram-scale synthesis for this compound. The coordination of DCM is reversible, with solid AgOTeF<sub>5</sub> readily absorbing dichloromethane vapor.<sup>238</sup> Recrystallization of [Ag-(OTeF<sub>5</sub>)(DCM)]<sub>2</sub> in toluene yields [Ag(OTeF<sub>5</sub>)(tol)<sub>2</sub>]<sub>2</sub> (tol = toluene), which exhibits a dimeric structure in the solid state with bridging OTeF<sub>5</sub> ligands leading to a {Ag<sub>2</sub>O<sub>2</sub>} core.<sup>168</sup> Two η<sup>2</sup>-coordinated toluene molecules to each silver center result in a pseudo tetrahedral coordination environment at each Ag(I) center. Additionally, AgOTeF<sub>5</sub> forms [Ag(OTeF<sub>5</sub>)(DCE)]<sub>2</sub> in the presence of 1,2-dichloroethane. This dimeric compound shows again a {Ag<sub>2</sub>O<sub>2</sub>} core, in which each Ag center is additionally coordinated by one bidentate 1,2-dichloroethane molecule and stabilized by weak contacts with F and Cl atoms from neighboring dimers.<sup>238,239</sup> A rather unusual silver complex can be observed by dissolving AgOTeF<sub>5</sub> in 1,2,3-trichloropropane, [Ag(OTeF<sub>5</sub>)(1,2,3-C<sub>3</sub>H<sub>5</sub>Cl<sub>3</sub>)]<sub>2</sub>. Here, a chain-like structure is formed by the linkage of Ag<sub>2</sub>(OTeF<sub>5</sub>)<sub>2</sub> units by 1,2,3-trichloropropane via Ag–Cl bonds.<sup>240</sup>

In the case of silver, the related OSeF<sub>5</sub> species is not known, but AgOSF<sub>5</sub> was recently reported, although it was already proposed in the 1960s as an intermediate in fluorination processes.<sup>241</sup> Compound AgOSF<sub>5</sub> is synthesized by reaction of AgF with SOF<sub>4</sub> in MeCN (see eq 53), yet the compound consists in fact of solvated tetrahedral [Ag(MeCN)<sub>4</sub>]<sup>+</sup> units.<sup>110</sup> This explains why acetonitrile solutions of this species are stable at room temperature in standard glassware, yet evaporation of the solvent leads to decomposition. The compound is stable against light and its solutions can be used to transfer the OSF<sub>5</sub> unit. This way, for instance, the reaction with CuI leads to the related CuOSF<sub>5</sub> complex (eq 53), that crystallized as [Cu-(MeCN)<sub>4</sub>][OSF<sub>5</sub>]. Interestingly, to the best of our knowledge, no Cu teflate species has been reported up to now.

$$ \mathrm{AgF} + \mathrm{SOF}_{4} \rightarrow \mathrm{AgOSF}_{5(\mathrm{solv})} \xrightarrow[\mathrm{-AgI}]{\mathrm{CuI}} \mathrm{CuOSF}_{5(\mathrm{solv})} \quad (53) $$

Another neutral metal teflate complex of great significance is Au(OTeF<sub>5</sub>)<sub>3</sub>, as it triggered the first systematic study of the Lewis acidity of a teflate-containing transition metal-based Lewis acid.<sup>24</sup> The compound is actually dimeric in the solid state, with both Au(III) centers in a near square planar environment, being the first ever reported teflate-based compound containing bridging teflate ligands (see Figure 22).<sup>40</sup> As expected, the Au–O<sub>bridge</sub> distances are longer (223(4) pm and 229(4) pm) than the Au–O<sub>terminal</sub> bond lengths (178(4) pm and 182(3) pm).

The compound Au(OTeF<sub>5</sub>)<sub>3</sub>, which has been known since 1985,<sup>40</sup> was first prepared by the group of Seppelt by reaction of AuF<sub>3</sub> and B(OTeF<sub>5</sub>)<sub>3</sub> at 60 °C during 5 days (see eq 54). A newer synthetic method by Riedel et al. relies on the use of commercial AuCl<sub>3</sub> and neat ClOTeF<sub>5</sub> (see eq 55).<sup>24</sup> Compound Au(OTeF<sub>5</sub>)<sub>3</sub> could be classified as a Lewis superacid according to its FIA value (557 kJ mol<sup>-1</sup> for the monomer, 504 kJ mol<sup>-1</sup> for the dimer), the Gutmann–Beckett method and the analysis of the C≡N vibration in the CD<sub>3</sub>CN adduct.<sup>24</sup> Additionally,

<figure><img src="image_52.png" /><figcaption>Figure 22. Molecular structure in the solid state of Au(OTeF<sub>5</sub>)<sub>3</sub>, showing its dimeric nature.<sup>40</sup> Thermal ellipsoids are set at 30% probability.</figcaption></figure>

Au(OTeF<sub>5</sub>)<sub>3</sub> is able to abstract a fluoride from [PPh<sub>4</sub>][SbF<sub>6</sub>] in SO<sub>2</sub>ClF, yielding a complex mixture arising from the initial abstraction of the fluoride, formation of SbF<sub>5</sub> and further ligand scrambling. All this underlines the use of the teflate ligand in the synthesis of new Lewis superacids, since the related AuF<sub>3</sub> does not reach the acidity to belong to this category. Furthermore, Au(OTeF<sub>5</sub>)<sub>3</sub> is the only gold-based Lewis superacid after AuF<sub>5</sub>.<sup>242</sup>

$$ \mathrm{AuF}_{3} + \mathrm{B}(\mathrm{OTeF}_{5})_{3} \xrightarrow[\mathrm{-BF}_{3}]{\mathrm{}} \mathrm{Au}(\mathrm{OTeF}_{5})_{3} \quad (54) $$

$$ \mathrm{AuCl}_{3} + 3\mathrm{ClOTeF}_{5} \xrightarrow[\mathrm{-3Cl}_{2}]{\mathrm{}} \mathrm{Au}(\mathrm{OTeF}_{5})_{3} \quad (55) $$

The only known neutral homoleptic group 4 complex is Ti(OTeF<sub>5</sub>)<sub>4</sub>, which was first mentioned in the literature in 1975 as the product of the reaction of TiCl<sub>4</sub> and Hg(OTeF<sub>5</sub>)<sub>2</sub> (see eq 56).<sup>42</sup> It was described as a crystalline solid and a monomeric structure was assumed according to its high volatility despite its high molecular mass and its solubility in nonpolar solvents. The equimolar reaction of TiCl<sub>4</sub> and HOTEF<sub>5</sub> affords Ti-Cl<sub>3</sub>(OTeF<sub>5</sub>), whose dismutation eventually renders also Ti-(OTeF<sub>5</sub>)<sub>4</sub> (see eq 57).<sup>243</sup> The intermediate species Ti-Cl<sub>2</sub>(OTeF<sub>5</sub>)<sub>2</sub> and TiCl(OTeF<sub>5</sub>)<sub>3</sub> were also detected by NMR spectroscopy. Compound Ti(OTeF<sub>5</sub>)<sub>4</sub> exhibits Lewis acidic behavior, forming 1:1 and 1:2 adducts with POCl<sub>3</sub>.<sup>243</sup>

$$ \mathrm{TiCl}_{4} \xrightarrow[\mathrm{-2HgCl}_{2}]{2\mathrm{Hg}(\mathrm{OTeF}_{5})_{2}} \mathrm{Ti}(\mathrm{OTeF}_{5})_{4} \quad (56) $$

$$ \mathrm{TiCl}_{4} \xrightarrow[\mathrm{-HCl}]{\mathrm{HOTeF}_{5}} \mathrm{TiCl}_{3}(\mathrm{OTeF}_{5}) \rightarrow \mathrm{Ti}(\mathrm{OTeF}_{5})_{4} \quad (57) $$

Regarding group 5, the neutral compounds Nb(OTeF<sub>5</sub>)<sub>5</sub> and Ta(OTeF<sub>5</sub>)<sub>5</sub> were prepared by reaction of MCl<sub>5</sub> (M = Nb, Ta) with an excess of HOTEF<sub>5</sub> (see eq 58).<sup>65</sup> The reaction of the fluorides NbF<sub>5</sub> and TaF<sub>5</sub> with B(OTeF<sub>5</sub>)<sub>3</sub> was reported to lead to the same five-coordinate species (see eq 59) and not to oxo derivatives, which is often the case in such processes. These compounds were reported to have a limited solubility, to be nonvolatile, as well as to decompose into viscous oils.<sup>145</sup>
$$MCl_5 + 5HOTeF_5 \xrightarrow[-SHCl]{-SHCl} M(OTeF_5)_5 \quad (58)$$

$$MF_5 + \frac{5}{3}B(OTeF_5)_3 \xrightarrow[-\frac{5}{3}BF_3]{M = Nb, Ta} M(OTeF_5)_5 \quad (59)$$

The teflate compounds of the group 6 elements Mo and W are known for oxidation state +VI (Mo(OTeF₅)₆ and W(OTeF₅)₆), but only in the case of tungsten the compound W(OTeF₅)₅ in oxidation state +V was realized. W(OTeF₅)₅ was reported to be prepared as a yellow solid by reaction of WCl₅ and ClOTeF₅ in CFCl₃ between -30 °C and room temperature (see eq 60).²⁴⁴ Unfortunately, no single crystal XRD analysis of the molecular structure of this compound was possible, contrary to WO-(OTeF₅)₄, which arises from the decomposition of W(OTeF₅)₅ already at -78 °C (see Section 11.2.1). Highly moisture-sensitive W(OTeF₅)₆ was reported to be isolated as a colorless solid by reaction of WCl₅ and Xe(OTeF₅)₂ in CFCl₃ (see eq 61)²⁴⁴ better than between WF₅ and B(OTeF₅)₃ in 1,1,2-trichlorotrifluoroethane (F-113), from which a disproportionation process of the pentavalent tungsten intermediate should happen. The corresponding reaction starting from WF₅ and B(OTeF₅)₃ only led to partial substitution, similarly to the attempts performed from WCl₆ and Hg(OTeF₅)₂ or ClOTeF₅.¹⁴⁵ The Mo(VI) analogue, Mo(OTeF₅)₆, is prepared by means of the hypochlorite ClOTeF₅ but starting from MoCl₅ (see eq 62). This means that, in contrast to the synthesis of W(OTeF₅)₅, in this case the reaction takes place with oxidation of the metal center.

$$WCl_5 + 5ClOTeF_5 \rightarrow W(OTeF_5)_5 + 5Cl_2 \quad (60)$$

$$WCl_5 + \text{exc. Xe}(OTeF_5)_2 \rightarrow W(OTeF_5)_6 + Xe + \frac{5}{2}Cl_2 \quad (61)$$

$$MoCl_5 + \text{exc. ClOTeF}_5 \rightarrow Mo(OTeF_5)_6 + 5Cl_2 \quad (62)$$

Mo(OTeF₅)₆ could be crystallized and presents a monomeric structure, in which the Mo center exhibits an octahedral {MoO₆} core (see Figure 23).²⁴⁴ Similarly to the tungsten compound, decomposition to the oxo compound MoO-(OTeF₅)₄ also occurs, in this case at 0 °C over a period of 1 week.²⁴⁴

Apart from Ti(OTeF₅)₄, the other neutral 3d metal teflate complex known is Fe(OTeF₅)₃, which is formed by reaction of FeCl₃ with ClOTeF₅ in SO₂ClF or C₄F₉SO₂F (see eq 63).²⁵ A trigonal planar structure was assumed for this compound based on the large quadrupole splitting in the ⁵⁷Fe Mössbauer spectrum. Nevertheless, crystallization from SO₂ClF solutions renders the adduct *fac*-[Fe(OTeF₅)₃·3SO₂ClF], which shows a distorted octahedral {FeO₆} coordination environment.²⁵ Similarly, the adduct *fac*-[Fe(OTeF₅)₃·3SO₂] can be crystallized from SO₂ solutions.²⁷ The compound is extremely reactive and is only stable toward alkanes and perfluoroalkanes. Attempts of coordinating P(OEt)₃ in SO₂ClF only led to the formation of [P(OEt)₄][Te₂Cl₉] and iron fluorides.²⁵

$$FeCl_3 + 3ClOTeF_5 \rightarrow Fe(OTeF_5)_3 + 3Cl_2 \quad (63)$$

Despite the success with early transition metals, the attainment of late transition metal teflate species in high oxidation states has not been possible yet. Although reactions between IrFₓ (x = 5, 6) or PtFₓ (x = 4, 6) and B(OTeF₅)₃ were attempted, they were reported to be unsuccessful.¹⁴⁵ Furthermore, Hg(IV) teflate species, namely Hg(OTeF₅)₄ and cis-

<figure><img src="image_53.png" /><figcaption>Figure 23. Molecular structure of Mo(OTeF₅)₆ in the solid state. Thermal ellipsoids are set at 50% probability.²⁴⁴</figcaption></figure>

and trans-HgF₂(OTeF₅)₂ have been calculated to be thermchemically unstable, yet might be synthetically accessible if the activation barriers for the reductive elimination leading to decomposition are sufficiently high. In this way, suggestions for experimental preparation of such complexes are provided based on the use of the known Hg(OTeF₅)₂ and a suitable oxidizer and/or teflate transfer reagent, such as e.g. Xe(OTeF₅)₂, Xe(OTeF₅)₄ or [XeOTeF₅][Sb(OTeF₅)₆].²³⁷

**11.1.2. Anionic Complexes.** A variety of anionic teflate complexes are known, but only recently additional 3d metal complexes of Mn, Co and Ni have been reported and finally used to study the ligand properties of the OTeF₅ group in transition metal complexes. Group 4 [M(OTeF₅)₆]²⁻ dianions (M = Ti, Zr, Hf) were prepared by Strauss et al. with the objective of accessing stable, soluble and easy-to-prepare WCAs. They were obtained as their silver salts from the metal chlorides MCl₄ and AgOTeF₅ (see eq 64).⁶⁴,¹⁹² In the case of Ti, F-113 was used as a solvent, whereas for Zr and Hf, 1,2-dichloroethane or dichloromethane and higher temperatures were required. In the case of Ag₂[Ti(OTeF₅)₆], the reaction with [N(nBu)₄]Cl in dichloromethane yielded [N(nBu)₄]₂[Ti(OTeF₅)₆] (see eq 65), whereas by simply dissolving it in DCM and concentrating the solution, crystals of [Ag(CH₂Cl₂)₃]₂[Ti(OTeF₅)₆] were obtained, which readily lose the DCM at room temperature (see eq 66). The anion shows an octahedral structure, and each Ag(I) ion is coordinated in a bidentate manner by the three CH₂Cl₂ molecules. The anion and the cation only interact through two weak Ag–F contacts. Compound Ag₂[Ti(OTeF₅)₆] is also able to reversibly absorb CO gas to yield the [Ag(CO)ₙ]₂[Ti(OTeF₅)₆] salts (n = 1, 2, eq 67), as occurs also with Ag₂[Zn(OTeF₅)₄] and Ag₂[B(OTeF₅)₄].¹⁵⁰,¹⁵¹ Finally, Cs₂[Ti(OTeF₅)₆] can also be prepared by reaction of Ti(OTeF₅)₄ with 2 equiv of CsOTeF₅ in HOTeF₅ as solvent.²⁴³
$$MCl_4 \xrightarrow[\text{-4AgCl}]{\text{6AgOTeF}_5} Ag_2[M(\text{OTeF}_5)_6] \\ \qquad \text{M} = \text{Ti, Zr, Hf} \qquad (64)$$

$$Ag_2[\text{Ti(OTeF}_5)_6] \xrightarrow[\text{-2Ag}]{2[\text{N(nBu)}_4]\text{Cl}] [\text{N(nBu)}_4]_2[\text{Ti(OTeF}_5)_6] \qquad (65)$$

$$Ag_2[\text{Ti(OTeF}_5)_6] \xrightleftharpoons[\text{DCM}]{\text{AgCl}} [\text{Ag(CH}_2\text{Cl}_2)_3]_2[\text{Ti(OTeF}_5)_6] \qquad (66)$$

$$Ag_2[\text{Ti(OTeF}_5)_6] \xrightleftharpoons[\text{n = 1, 2}]{\text{CO}} [\text{Ag(CO)}_n]_2[\text{Ti(OTeF}_5)_6] \qquad (67)$$

Contrary to the $[M(OTeF_5)_6]^{2-}$ species (M = Ti, Zr, Hf), which do not perfectly fit the criteria of an ideal WCA because of their double negative charge,⁶² the $[Nb(OTeF_5)_6]^{-}$ anion can be properly considered as such. It can be prepared under the same conditions as the $[Sb(OTeF_5)_6]^{-}$ and $[Ti(OTeF_5)_6]^{2-}$ anions, starting from NbCl$_5$ and AgOTeF$_5$ in F-113 (see eq 68).⁶⁴ Dissolution of Ag[Nb(OTeF$_5$)$_6$] in CH$_2$Br$_2$ also results in the related $[Ag(CH$_2$Br$_2$)$_3$][Nb(OTeF$_5$)$_6$] (see eq 69), which consists of separate $[Ag(CH$_2$Br$_2$)$_3]^{+}$ cations (also with bidentate CH$_2$Br$_2$ ligands) and octahedral $[Nb(OTeF$_5$)$_6]^{-}$ anions, but with no significant interaction between them.¹⁹² Also this niobium-based WCA can stabilize the previously mentioned silver carbonyl cations $[Ag(CO)$_n]^{+}$ (n = 1, 2, eq 70).¹⁵⁰ Additionally, at high pressures of CO, Ag[Nb(OTeF$_5$)$_6$] can take even a third equivalent of CO, both in the solid state and in F-113 to form $[Ag(CO)$_3$][Nb(OTeF$_5$)$_6$] from which the CO ligands can be removed under vacuum at room temperature but not at -78 °C.²⁴⁵ A metathesis reaction with $[N(nBu)$_4$]Cl and CClPh$_3$ in dichloromethane yields $[N(nBu)$_4$][Nb(OTeF$_5$)$_6$] and $[CPh$_3$][Nb(OTeF$_5$)$_6$], respectively (see eq 71).¹⁹² Nevertheless, the $[Nb(OTeF$_5$)$_6]^{-}$ anion does not seem to be as useful as a WCA compared to $[Sb(OTeF$_5$)$_6]^{-}$, since contrary to the latter it is not stable in acetonitrile or in the presence of nucleophiles such as $[OTeF$_5$]$_{-}$ or $[PF$_6$]$_{-}$. By means of isotope scrambling experiments with H$^{18}$OTeF$_5$ and Ag$^{18}$OTeF$_5$, it was concluded that the $[Nb(OTeF$_5$)$_6]^{-}$ anion is more stable toward electrophiles than $[Ti(OTeF$_5$)$_6]^{2-}$ or $[B(OTeF$_5$)$_4]^{2-}$.⁶⁴,¹⁹²

$$\text{NbCl}_5 \xrightarrow[\text{-5AgCl}]{\text{6AgOTeF}_5} \text{Ag}[\text{Nb(OTeF}_5)_6] \qquad (68)$$

$$Ag[Nb(OTeF_5)_6] \xrightarrow[\text{CH}_2\text{Br}_2]{\text{AgCl}} [Ag(CH_2Br_2)_3][Nb(OTeF_5)_6] \qquad (69)$$

$$Ag[Nb(OTeF_5)_6] \xrightleftharpoons[\text{n = 1, 2, 3}]{\text{CO}} [Ag(CO)_n][Nb(OTeF_5)_6] \qquad (70)$$

$$Ag[Nb(OTeF_5)_6] \xrightarrow[\text{-AgCl}]{[\text{cat}]\text{Cl}] [cat][Nb(OTeF_5)_6] \\ \qquad \text{cat} = \text{N(nBu)}_4, \text{CPh}_3 \qquad (71)$$

Both the $[Nb(OTeF$_5$)$_6]^{-}$ and the $[Ta(OTeF$_5$)$_6]^{-}$ anions have also been prepared as their $Cs^{+}$, $[NEt$_4$]^{+}$ and $[N(nBu)$_4$] salts by reaction of the neutral M(OTeF$_5$)$_5$ complexes (M = Nb, Ta) and the corresponding $[cat][OTeF$_5$] salt (cat = Cs, $[NEt$_4$]$, $[N(nBu)$_4$]$) in HOTeF$_5$ as the solvent.⁶⁵ This synthetic method for the $[Nb(OTeF$_5$)$_6]^{-}$ anion is not as straightforward as the one using AgOTeF$_5$.¹⁹² (see eq 68) and requires an overall larger amount of HOTeF$_5$.⁶⁵ Nevertheless, it provides access to the $[Ta(OTeF$_5$)$_6]^{-}$ anion, which could not be obtained as the Ag$^{+}$ salt by using the first procedure (see eq 68).¹⁹² The

electrochemical behavior of the ammonium salts of $[Nb(OTeF$_5$)$_6]^{-}$ and $[Ta(OTeF$_5$)$_6]^{-}$ was also investigated, showing in both cases a quasi-reversible reduction of the transition metal from the oxidation state +V to +IV at -0.69 V (DCM) and -0.60 V (MeCN) in the case of Nb and at -1.52 V (DCM) and -1.42 V (MeCN) in the case of Ta. Additionally, at -1.9 V (DCM) and -2.3 V (MeCN), the 2-electron reduction of the Te atom of the teflate ligands occurs.⁶⁵ All in all, it was concluded by comparison to the analogous fluoro- and chlorometallates, that the teflate is better in stabilizing high oxidation states than the chloride, but not as good as the fluoride.⁶⁵

Four-coordinate teflate dianions have also been demonstrated to stabilize highly unstable cations. The $[Zn(OTeF$_5$)$_4]^{2-}$ dianion was prepared as its Ag$^{+}$ salt²⁴⁶ and reacts with CO reversibly, similar to the case of Ag$[B(OTeF$_5$)$_4]$ to render the $[Ag(CO)$_n]_2[Zn(OTeF$_5$)$_4]$ salts (n = 1, 2), depending on the pressure of CO.¹⁵⁰,¹⁵¹ The square planar $[Pd(OTeF$_5$)$_4]^{2-}$ anion in Ag$_2$[Pd(OTeF$_5$)$_4$] was prepared by reaction of PdCl$_2$ and AgOTeF$_5$ in dichloromethane or 1,2-dichloroethane.²³⁸,²⁴⁷ The Ag$^{+}$ cation reversibly coordinates dichloromethane or 1,2-dichloroethane. In fact, these compounds were useful to show that both dichloromethane and 1,2-dichloroethane can actually behave as coordinating solvents.²³⁸ Both salts $[Ag(CH$_2$Cl$_2$)$_2]_2[Pd(OTeF$_5$)$_4]$ and [Ag(1,2-C$_2$H$_4$Cl$_2$)$_2]_2[Pd(OTeF$_5$)$_4]$ were crystallized and their molecular structures analyzed by XRD methods. The {PdO$_4$} core exhibits a planar coordination, in which two oxygen atoms in cis are bridged by a $[Ag(solv)$_2]^{+}$ unit (solv = CH$_2$Cl$_2$, 1,2-C$_2$H$_4$Cl$_2$), or alternatively, it can be seen as the {PdO$_4$} core bridging two of the $[AgL$_2]^{+}$ units. Therefore, each Ag(I) cation is coordinated by two O atoms and four Cl atoms. When 2 equiv of AgOTeF$_5$ and 2 equiv of $[N(nBu)$_4$][OTeF$_5$] are used, the compound $[N(nBu)$_4]_2[Pd(OTeF$_5$)$_4$] is obtained.²³⁸

The $[M(OTeF$_5$)$_4]^{2-}$ anions (M = Ni, Co, Cu) were mentioned, among others, as potential WCAs already in 1993,⁵⁹ yet their use as such has not been demonstrated. The syntheses of the $[M(OTeF$_5$)$_4]^{2-}$ anions (M = Ni, Co) were reported only recently and the Cu species is still unknown. The salts $[NEt$_4]_2[Ni(OTeF$_5$)$_4]$ and $[NEt$_4]_2[Co(OTeF$_5$)$_4]$ are prepared by reaction of the corresponding $[NEt$_4]_2[MCl$_4$]$ salt (M = Ni, Co) and neat CLOTEF$_5$ as the teflate transfer reagent (see eq 72) accompanied by the release of Cl$_2$, which is removed from the reaction mixture under vacuum.³⁴,³⁶ Interestingly, with these homoleptic 3d metal complexes in hand, the properties of the teflate ligand in coordination chemistry have been now systematically investigated (see Section 2.3.3).

$$[NEt_4]_2[MCl_4] \xrightarrow[\text{-4Cl}_2]{\text{exc. CLOTEF}_5} [NEt_4]_2[M(OTeF_5)_4] \\ \qquad \text{M} = \text{Ni, Co} \qquad (72)$$

Both compounds exhibit a pseudo tetrahedral structure at the metal center, yet more distorted for the Ni ($\tau_4$ = 0.75) than for the Co ($\tau_4$ = 0.88) compound.³⁴,³⁶ [Note: The four-coordinate geometry index is used to describe the distortion of a four-coordinate complex and is defined as $\tau_4 = [360^{\circ} - (\alpha + \beta)]/141^{\circ}$, where $\alpha$ and $\beta$ are the two largest angles in the four-coordinate species. The values of $\tau_4$ range from 1.00 for a perfect tetrahedral geometry, to 0.00 for a perfect square planar geometry.²⁴⁸] In both cases, they represent analogues of the polymeric $[MF$_4]^{2-}$ species (M = Ni, Co) but with monomeric structures (see Figure 24, left), therefore rendering better solubility in organic solvents. They are similarly stable under inert conditions and start decomposing above 220 and 248 °C,
respectively. However, at ambient conditions, the high sensitivity of the Ni compound is in contrast to the highly stable Co species. Additionally, [NET₄]₂[Ni(OTeF₅)₄] dissociates all the teflate ligands when dissolved in acetonitrile, yielding [Ni(MeCN)₆][OTeF₅]₂.³⁴

Complex [NET₄]₂[Ni(OTeF₅)₄] was determined to be a high-spin Ni(II) complex with a tetrahedral structure and weak-field/medium-field ligands, whose effective magnetic moment allowed to infer this character for the teflate group (see Section 2.3.3).³⁴ Similarly, the presence of three unpaired electrons at the Co(II) center of [NET₄]₂[Co(OTeF₅)₄] could be derived from the measured effective magnetic moment, and was further supported by electronic spectroscopy and computational calculations.³⁶ Interestingly, [NET₄]₂[Co(OTeF₅)₄] is the only group 9 metal teflate prepared thus far.³⁶

More recently, some unusual geometries in homoleptic Mn species have been reported.³⁵ The high-spin complex [NET₄]₂[Mn(OTeF₅)₄] was prepared by reaction of [NET₄]₂[MnCl₄] and AgOTeF₅ (see Scheme 11).

<caption>**Scheme 11. Syntheses of High-Spin Complexes**
**[NET₄]₂[Mn(OTeF₅)₄] and [NET₄]₂[Mn(OTeF₅)₅]**</caption>

**synthetic routes toward manganese-based teflates**

<figure><img src="image_54.png" /></figure>

The anion [Mn(OTeF₅)₄]²⁻, which is the first homoleptic mononuclear Mn(II) center with a {MnO₄} core and monodentate O-donor ligands, exhibits a tetrahedral structure (τ₄ = 0.96), as found out by X-ray diffraction analysis of the [PPh₄]⁺ salt (see Figure 24, middle) and belongs to the scarce number of structurally characterized tetrahedrally coordinated teflate dianionic complexes, together with [Ni(OTeF₅)₄]²⁻, [Co(OTeF₅)₄]²⁻ and [Hg(OTeF₅)₄]²⁻. This compound can be oxidized with neat ClOTeF₅ to yield [NET₄]₂[Mn(OTeF₅)₅], a salt that can also be prepared starting from [NET₄]₂[MnCl₄] and an excess of ClOTeF₅ (see Scheme 11). A high-spin d⁴ configuration was found for this square pyramidally coordinated

Mn(III) complex anion, which is again the first homoleptic example with all monodentate O-donor ligands. As Mn(II) and Mn(III) complexes with O-donor ligands are normally heteroleptic mononuclear species or polynuclear species, these two compounds showcase unique coordination environments thanks to the combination of the similar electronic properties of the teflate and the fluoride ligands, together with the lower tendency of the teflate group to bridge metal centers (note the polymeric nature of the related fluoromanganates).³⁵

Unlike the corresponding Lewis acid, the Au anion [Au(OTeF₅)₄]⁻ was only recently synthesized by reaction of different [cat][AuCl₄] salts (cat = Cs, [NMe₄], [NET₃Me]) with neat ClOTeF₅.²⁴ The reaction proceeds via a stepwise substitution of the chloride ligands, as proven not only by quantum-chemical calculations, but also by the crystallization of the [AuCl₃(OTeF₅)]⁻ anion when the reaction is performed in DCM (see Section 11.2.1). The [Au(OTeF₅)₄]⁻ anion gives an unusual pattern in the ¹⁹F NMR spectrum with collapsed signals for the typical AB₄ spin system of the teflate group and exhibits a distorted square planar structure (τ₄ = 0.13), as found in the molecular structure in the solid state of the [NET₃Me]⁺ salt (see Figure 24, right). Targeting the stabilization of a Au(V) teflate species, both the reactions of Cs[AuF₆] with B(OTeF₅)₃ or ClOTeF₅ as the teflate transfer reagents were reported to be unsuccessful.²⁴ Another approach involving the oxidation of [NMe₄][Au(OTeF₅)₄] with strong oxidizers like Xe(OTeF₅)₂ or even diluted F₂ (10% in Ar) did not lead to any Au(V) species.²⁴

Finally, a handful of teflate anions of Hg(II) with a variety of structural motifs have been reported by the group of Schrobilgen. They were prepared using Hg(OTeF₅)₂ and [cat][OTeF₅] ([cat] = NMe₄, NET₄, Cs) as starting materials in different molar ratios in SO₂ClF (see eqs 73–76).²⁴⁹ This way, the discrete tetrahedrally distorted [Hg(OTeF₅)₄]²⁻ anion (τ₄ = 0.79) of the [NET₄]⁺ salt is isolated, whereas in compound Cs₂[Hg(OTeF₅)₄]·Hg(OTeF₅)₂ the tetrahedron is flattened (τ₄ = 0.53) due to the interactions of the anion with neutral Hg(OTeF₅)₂, resulting in fact in a chain structure. The [Hg(OTeF₅)₅]³⁻ anion was the first teflate trianion to be isolated, both with [NMe₄]⁺ and [NET₄]⁺ as counterions. A square pyramidal geometry was observed for the [NMe₄]⁺ salt (τ₅ = 0.09), whereas for the [NET₄]⁺ salt the geometries for the two crystallographic distinct anions are intermediate (τ₅ = 0.52 and 0.26) between a square pyramid and a trigonal bipyramid,

<figure><img src="image_55.png" /><figcaption>**Figure 24.** Molecular structure of [NET₄]₂[Ni(OTeF₅)₄] (left), [PPh₄]₂[Mn(OTeF₅)₄] (middle), [NET₃Me][Au(OTeF₅)₄] (right) in the solid state.²⁴,³⁴,³⁵ The cations have been omitted for clarity. Thermal ellipsoids set at 50% probability.</figcaption></figure>
which is the optimized structure in the gas phase ($\tau_5$ = 0.91). [Note: The five-coordinate geometry index is used to describe the distortion of a five-coordinate complex and is defined as $\tau_5$ = ($\beta - \alpha$)/60°, where $\alpha$ and $\beta$ are the two largest angles in the five-coordinate species. The values of $\tau_5$ range from 1.00 for a perfect trigonal bipyramidal geometry, to 0.00 for a perfect square pyramidal geometry. $^{250}$] Long Hg–F intraionic contacts *trans* to the apical oxygen were suggested to favor the observed square pyramidal geometry. The synthesis of monomeric [Hg-(OTeF$_5$)$_3$]$^-$ anion was not possible, yet the dimer [Hg$_2$(OTeF$_5$)$_6$]$^{2-}$ could be prepared as the [NMe$_4$]$^+$ salt. In the solid state it contains Hg(OTeF$_5$)$_2$ units bridged by two teflate groups, therefore rendering an overall butterfly shaped {HgO$_\mu$}$_2$ core. Finally, the [Hg$_2$(OTeF$_5$)$_7$]$^{3-}$ anion in the crystal of {Cs$_3$[Hg$_2$(OTeF$_5$)$_7$]·Hg(OTeF$_5$)$_2$}·4SO$_2$ClF was formulated as two [Hg(OTeF$_5$)$_3$]$^-$ anions bridged by one teflate anion. This compound possesses a chain structure, where Hg(OTeF$_5$)$_2$ entities alternate with [Hg$_2$(OTeF$_5$)$_7$]$^{3-}$ anions. In all non-monomeric structures, Hg centers are stabilized through additional interionic contacts with O and F atoms. The NBO analyses performed on the monomeric species [Hg(OTeF$_5$)$_4$]$^{2-}$ and [Hg(OTeF$_5$)$_5$]$^{3-}$ concluded that the Hg–O bonds are more polar than in neutral Hg(OTeF$_5$)$_2$. $^{249}$ Homoleptic transition metal compounds are gathered in Table 10.

$$ \mathrm{Hg(OTeF_5)_2 + n[cat][OTeF_5] \rightarrow [cat]_n[\mathrm{Hg(OTeF_5)_{2+n}}]} \\ \qquad \begin{array}{l} n=2, \mathrm{cat} = \mathrm{NEt_4} \\ n=3, \mathrm{cat} = \mathrm{NEt_4}, \mathrm{NMe_4} \end{array} \qquad (73) $$

$$ 2\mathrm{Hg(OTeF_5)_2 + 2[NMe_4][OTeF_5] \rightarrow [NMe_4]_2[\mathrm{Hg_2(OTeF_5)_6}]} \qquad (74) $$

$$ 2\mathrm{Hg(OTeF_5)_2 + 2CsOTeF_5 \rightarrow} \\ \mathrm{Cs_2[\mathrm{Hg(OTeF_5)_4} \cdot \mathrm{Hg(OTeF_5)_2}]} \qquad (75) $$

$$ 3\mathrm{Hg(OTeF_5)_2 + 3CsOTeF_5 \rightarrow} \\ \mathrm{Cs_3[\mathrm{Hg_2(OTeF_5)_7} \cdot \mathrm{Hg(OTeF_5)_2}]} \qquad (76) $$

## 11.2. Heteroleptic d-Block Metal Teflates

**11.2.1. Metal Teflate Complexes Containing Oxo Ligands.** By attempting to synthesize homoleptic metal teflates using the versatile approach of reacting metal fluorides with B(OTeF$_5$)$_3$, several heteroleptic metal teflate complexes containing an additional oxo ligand were discovered. The first among these compounds, MoOF$_3$(OTeF$_5$), was synthesized by the neat reaction of MoF$_6$ with B(OTeF$_5$)$_3$ at 80 °C. Initially, MoF$_4$(OTeF$_5$)$_2$ is formed, followed by the elimination of TeF$_6$ through an internal fluorination process, resulting in the formation of the oxofluoride complex (see eq 77). Starting with molybdenum oxytetrafluoride (MoOF$_4$) instead of MoF$_6$, a direct synthesis of MoOF$_3$(OTeF$_5$) was achieved by reaction with B(OTeF$_5$)$_3$ (see eq 78).$^{251}$

$$ \mathrm{MoF_6} \xrightarrow[\mathrm{BF_3}]{\mathrm{B(OTeF_5)_3}} \mathrm{MoOF_3(OTeF_5)} \qquad (77) $$

$$ \mathrm{MoOF_4} \xrightarrow[\mathrm{BF_3}]{\mathrm{B(OTeF_5)_3}} \mathrm{MoOF_3(OTeF_5)} \qquad (78) $$

The addition of F-114 as a solvent, along with a decrease of the reaction temperature to 50 °C, leads to the formation of MoO(OTeF$_5$)$_4$ from MoF$_6$ (see Scheme 12). This reactivity

<table><caption>**Table 10. Overview of Homoleptic Transition Metal Compounds**</caption><thead><tr><th>Element</th><th>Ox</th><th>Compound</th><th>Refs</th></tr></thead><tbody><tr><td>Ti</td><td>IV</td><td>Ti(OTeF<sub>5</sub>)<sub>4</sub><br>[cat]<sub>2</sub>[Ti(OTeF<sub>5</sub>)<sub>6</sub>] (cat = Ag, Ag(DCM)<sub>3</sub>, Ag(CO), Ag(CO)<sub>2</sub>, Cs, N(nBu)<sub>4</sub>)</td><td>42,243<br>64,150,151,192,243</td></tr><tr><td>Zr</td><td>IV</td><td>Ag<sub>2</sub>[Zr(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>192</td></tr><tr><td>Hf</td><td>IV</td><td>Ag<sub>2</sub>[Hf(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>192</td></tr><tr><td>Nb</td><td>V</td><td>Nb(OTeF<sub>5</sub>)<sub>5</sub><br>[cat][Nb(OTeF<sub>5</sub>)<sub>6</sub>] (cat = Ag, Ag(CH<sub>3</sub>Br)<sub>3</sub>, Ag(CO), Ag(CO)<sub>2</sub>, Ag(CO)<sub>3</sub>, CPh<sub>3</sub>, Cs, N(nBu)<sub>4</sub>, NEt<sub>4</sub>)</td><td>65,145<br>64,65,150,192,245</td></tr><tr><td>Ta</td><td>V</td><td>[Ta(OTeF<sub>5</sub>)<sub>5</sub>]<br>[cat][Ta(OTeF<sub>5</sub>)<sub>6</sub>] (cat = Cs, NEt<sub>4</sub>, N(nBu)<sub>4</sub>)</td><td>65,145<br>65,145</td></tr><tr><td>Mo</td><td>VI</td><td>Mo(OTeF<sub>5</sub>)<sub>6</sub></td><td>244</td></tr><tr><td rowspan="2">W</td><td>V</td><td>W(OTeF<sub>5</sub>)<sub>5</sub></td><td>244</td></tr><tr><td>VI</td><td>W(OTeF<sub>5</sub>)<sub>6</sub></td><td>145,244</td></tr><tr><td rowspan="2">Mn</td><td>II</td><td>[cat]<sub>2</sub>[Mn(OTeF<sub>5</sub>)<sub>4</sub>] (cat = NEt<sub>4</sub>, PPh<sub>4</sub>)</td><td>35</td></tr><tr><td>III</td><td>[NEt<sub>4</sub>]<sub>2</sub>[Mn(OTeF<sub>5</sub>)<sub>5</sub>]</td><td>35</td></tr><tr><td>Fe</td><td>III</td><td>Fe(OTeF<sub>5</sub>)<sub>3</sub></td><td>25</td></tr><tr><td>Co</td><td>II</td><td>[NEt<sub>4</sub>]<sub>2</sub>[Co(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>36</td></tr><tr><td>Ni</td><td>II</td><td>[NEt<sub>4</sub>]<sub>2</sub>[Ni(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>34</td></tr><tr><td>Pd</td><td>II</td><td>[cat]<sub>2</sub>[Pd(OTeF<sub>5</sub>)<sub>4</sub>] (cat = Ag(DCM)<sub>2</sub>, Ag(DCE)<sub>2</sub>, N(nBu)<sub>4</sub>)</td><td>238,247</td></tr><tr><td>Cu</td><td>I</td><td>CuOSF<sub>5(solv)</sub></td><td>110</td></tr><tr><td rowspan="2">Ag</td><td>I</td><td>AgOTeF<sub>5</sub></td><td>10,168,238</td></tr><tr><td></td><td>AgOSF<sub>5(solv)</sub></td><td>110</td></tr><tr><td rowspan="2">Au</td><td>III</td><td>Au(OTeF<sub>5</sub>)<sub>3</sub></td><td>24,40</td></tr><tr><td></td><td>[cat]Au(OTeF<sub>5</sub>)<sub>4</sub> (cat = Cs, NMe<sub>4</sub>, NEt<sub>3</sub>Me)</td><td>24</td></tr><tr><td>Zn</td><td>II</td><td>[cat][Zn(OTeF<sub>5</sub>)<sub>4</sub>] (cat = Ag, Ag(CO), Ag(CO)<sub>2</sub>)</td><td>150,181,246</td></tr><tr><td rowspan="5">Hg</td><td>II</td><td>Hg(OTeF<sub>5</sub>)<sub>2</sub></td><td>5,11,236</td></tr><tr><td>[NEt<sub>4</sub>]<sub>2</sub>[Hg(OTeF<sub>5</sub>)<sub>4</sub>]</td><td>249</td></tr><tr><td>[cat]<sub>3</sub>[Hg(OTeF<sub>5</sub>)<sub>5</sub>] (cat = NMe<sub>4</sub>, NEt<sub>4</sub>)</td><td>249</td></tr><tr><td>[NMe<sub>4</sub>]<sub>2</sub>[Hg<sub>2</sub>(OTeF<sub>5</sub>)<sub>6</sub>]</td><td>249</td></tr><tr><td>Cs<sub>2</sub>[Hg(OTeF<sub>5</sub>)<sub>4</sub>·Hg(OTeF<sub>5</sub>)<sub>2</sub>]</td><td>249</td></tr><tr><td></td><td>Cs<sub>3</sub>[Hg<sub>2</sub>(OTeF<sub>5</sub>)<sub>7</sub>·Hg(OTeF<sub>5</sub>)<sub>2</sub>]</td><td>249</td></tr></tbody></table>

differs from the behavior of the heavier homologue WF$_5$, which, under similar conditions, forms the homoleptic W(OTeF$_5$)$_6$ (see Section 11.1).$^{145}$ The tetragonal pyramidal coordination sphere observed around the central molybdenum atom in MoO(OTeF$_5$)$_4$ is surprising when compared to the trigonal bipyramidal structure of the analogous MoO[OC(CF)$_3$]$_4$. $^{145,252}$ This unexpected structural difference may be attributed to an
<figure><img src="image_56.png" /><figcaption>Scheme 12. Syntheses of Various Metal Teflate Complexes Containing Oxo Ligands</figcaption></figure>

intermolecular Mo—F interaction that complete the octahedral coordination.¹⁴⁵ Furthermore, MoO(OTeF₅)₄ is the decomposition product obtained when storing Mo(OTeF₅)₆ above –30 °C.

Likewise, W(OTeF₅)₅ decomposes over time, even at –78 °C, to yield the structurally similar compound WO(OTeF₅)₄. Nevertheless, a more efficient method for obtaining the latter involves reacting WOCl₄ with ClOTeF₅.²⁴⁴

Similarly, when reacting ReF₇ or ReF₆ with B(OTeF₅)₃ in CClCl₃, the desired homoleptic rhenium complexes could not be obtained; instead, the yellow liquid ReO₂(OTeF₅)₃ was formed (see Scheme 12).¹⁴⁵ Pure ReO₂F₃ also produces the aforementioned compound in a reaction with B(OTeF₅)₃ in F-114 (see Scheme 12).²⁵³ The molecular structure of this compound in the solid state could not be determined, but the teflate groups were shown to be fluxional on the NMR time scale, indicative of a trigonal bipyramid where the oxygen atoms occupy the equatorial plane. Additionally, recorded vibrational spectra are in agreement with the proposed structure. The acceptor ability of the rhenium center was probed by addition of [NMe₄][OTeF₅], leading to the formation of [NMe₄][ReO₂(OTeF₅)₄] (see Scheme 12). The structure exhibits an approximately octahedral coordination around the rhenium, exclusively adopting the cis-dioxo isomer both in solution and in the solid state (see Figure 25).²⁵³ Both ReCl₅ and ReOCl₄ react with ClOTeF₅ to form ReO(OTeF₅)₄ (see Scheme 12), a compound structurally similar to MoO(OTeF₅)₄. ReO(OTeF₅)₄ can be further oxidized by Xe(OTeF₅)₂ to yield the Re(VII) species ReO(OTeF₅)₅.²⁵⁴ The structure of this compound exhibits the anticipated octahedral coordination and serves as another example of a bonding scenario that can otherwise only be achieved with fluorido ligands.

For osmium, a preferred coordination number of 5 seems evident, as OsF₆ and OsO₂F₃ both react with B(OTeF₅)₃ to render OsO(OTeF₅)₄ (see Scheme 12). However, the similar reaction of octavalent OsO₃F₂ only leads to the formation of OsO₄. In the molecular structure in the solid state of OsO(OTeF₅)₄, similarities with MoO(OTeF₅)₄ and ReO(OTeF₅)₄ are observed. Nevertheless, TeF₄ cocrystallizes, providing a bridging fluorine atom to complete the octahedral coordination at the Os center. OsO(OTeF₅)₄ can be oxidized by

Xe(OTeF₅)₂, similarly to the related Re species, likely resulting in the OsOF₅ analogue OsO(OTeF₅)₅, although isolation of the latter has been elusive (see Scheme 12).¹⁴⁵,²⁵⁴

The early transition metal pentafluorides of vanadium (VF₅) and chromium (CrF₅) react with B(OTeF₅)₃ to form the corresponding oxo compounds VO(OTeF₅)₃ and CrO₂(OTeF₅)₂, exhibiting a more stable coordination number of 4 (see Scheme 12). Both complexes are obtained in solution, colorless VO(OTeF₅)₃ being in contrast with the dark red color of CrO₂(OTeF₅)₂.¹⁴⁵ The lighter chalcogen homologues of these two compounds, VO(OSeF₅)₃ and CrO₂(OSeF₅)₂, can be synthesized by reacting the respective chloride precursors with Hg(OSeF₅)₂.⁴²

### 11.2.2. Metal Teflate Complexes Containing Halide Ligands.
Another compound class that was predominantly observed as the main or side product during attempts of synthesizing homoleptic metal teflates are the mixed metal

<figure><img src="image_57.png" /><figcaption>**Figure 25.** Molecular structure in the solid state of [NMe₄][ReO₂(OTeF₅)₄].²⁵³ Thermal ellipsoids are set at 25% probability.</figcaption></figure>
teflate halide complexes, as for instance TiCl₄-(n)(OTeF₅)ₙ (n = 1–3) and W(OTeF₅)ₙX₆-n (n = 1, 2, 4, 5; X = F, Cl). The first example of such complexes was observed during the reaction of WCl₆ with Hg(OTeF₅)₂, targeting the homoleptic W(OTeF₅)₆ in a reaction analogous to that of TiCl₄ (see eq 56). However, the reaction resulted in WCl(OTeF₅)₅, a viscous, distillable liquid (see eq 79). At this time, this compound represented the first complex with more than four pentafluoroorthochalcogenate ligands.⁴² The usage of ClOTeF₅ as a teflate transfer reagent instead of Hg(OTeF₅)₂ yields the same product.¹⁴⁵ The reaction of the fluorinated starting material WF₆ with B(OTeF₅)₃ results, depending of the stoichiometry of the reaction, in the formation of either the one or the two times substituted compounds WF₅(OTeF₅) or WF₄(OTeF₅)₂ (see eq 80).²⁵⁵ A second addition of B(OTeF₅)₃ to WF₄(OTeF₅)₂ yields WF₂(OTeF₅)₄ (see eq 80).¹⁴⁵

$$WCl_6 \xrightarrow[{-Cl_2 \text{ or } HgCl}_2]{\text{ClOTeF}_5 \text{ or } Hg(\text{OTeF}_5)_2} WCl(\text{OTeF}_5)_5 \quad (79)$$

$$WF_6 \xrightarrow[{-BF_3}]{B(OTeF_5)_3} WF_{6-n}(OTeF_5)_n \quad (80)$$

The mixed titanium teflate compound TiCl₃(OTeF₅) can be synthesized in a unimolecular reaction between TiCl₄ and HOTeF₅ with a conversion of 30%. The removal of formed HCl and the addition of new teflic acid leads to the complete conversion of the starting material (see eq 81). In an attempt to remove TiCl₄ via distillation from the reaction mixture, dismutation equilibria result in the homoleptic Ti(OTeF₅)₄ (see eq 56).²⁴³ In the case of gold, [AuCl₃(OTeF₅)]⁻ (see eq 82) was observed in the reaction of [AuCl₄]⁻ with ClOTeF₅ in CH₂Cl₂, which underlines the stepwise ligand substitution in the formation of [Au(OTeF₅)₄]⁻ (see Section 11.1).²⁴

$$TiCl_4 \xrightarrow[{-HCl}]{HOTeF_5} TiCl_3(OTeF_5) \quad (81)$$

$$[NMe_4][AuCl_4] \xrightarrow[{-Cl_2}]{ClOTeF_5} [NMe_4][AuCl_3(OTeF_5)] \quad (82)$$

Furthermore, related to this kind of species, NgF₂ adducts (Ng = Kr, Xe) of teflate complexes have been also reported. The first complex with a bridging KrF₂ moiety was reported by Schrobilgen et al. to be [Hg(OTeF₅)₂·1.5 KrF₂], which was observed by reacting the neutral Hg(OTeF₅)₂ with KrF₂ at low temperatures in SO₂ClF (see eq 83).

$$Hg(OTeF_5)_2 + 1.5NgF_2 \rightarrow [Hg(OTeF_5)_2 \cdot 1.5NgF_2] \quad (83)$$

The Hg(OTeF₅)₂ units have *gauche* conformations with intramolecular Hg–O and Hg–F contacts leading to a chain-like structure due to the bridging KrF₂ units (see Figure 26). An isostructural complex is formed by using XeF₂ instead of KrF₂, and shows shorter Hg–F contacts due to the greater ionic character of the Xe–F bond.²³⁶ It is surprising that the strong oxidizer KrF₂ is not able to oxidize the mercury further to its highest oxidation state +IV, as shown by isolated HgF₄ in cryogenic rare gas matrices.²⁵⁶

### 11.2.3. Metal Teflate Complexes Containing O-, N- and P-Donor Ligands.
Metal teflate complexes containing additionally O-, N- or P-donor ligands are only known for middle and late d-block metals. For iron there are two different types of porphyrin complexes known, namely [Fe(OEP)(OTeF₅)] and

<figure><img src="image_58.png" /><figcaption>**Figure 26.** Molecular structure of [Hg(OTeF₅)₂·1.5 KrF₂] in the solid state.²³⁶ Thermal ellipsoids are set at 50% probability.</figcaption></figure>

[Fe(TPP)(OTeF₅)].²⁵⁷ They are synthetically accessible as reported by Strauss et al. through the reaction of either [Fe(TPP)]₂O or [Fe(OEP)]₂O with teflic acid, whereby water is eliminated (see Scheme 13). Additionally, through treatment of [Fe(TPP)(OTeF₅)] with tetrahydrofuran (THF) the complex [Fe(TPP)(OTeF₅)(THF)] could be observed, which is a rare example of a six-coordinate high-spin Fe(III) porphyrin complex. The temperature-dependent ¹H NMR spectra of the five-coordinate Fe complexes were studied and compared to related chlorido compounds.²⁵⁸

For group 10 metals, the first of such complexes, [Pd-(PhCN)₂(OTeF₅)₂], was published in 1987 and prepared by reaction of [PdCl₂(PhCN)₂] with AgOTeF₅ (see Scheme 13). In this work, the incompatibility of the teflate and triphenylphosphine ligands was discussed, arising from the oxidation of PPh₃ to OPPh₃ and further decomposition to P–F species and elemental tellurium.²⁵⁹ However, it was shown that the incompatibility is only in the case of PPh₃, since the reaction of cis- or trans-[PtCl₂(PEt₃)₂] with HOTeF₅ yielded the corresponding mixed cis- or trans-[Pt(OTeF₅)₂(PEt₃)₂] (see Scheme 13).²⁶⁰ Additionally, the PPh₃ incompatibility seems to be metal specific, since [Re(CO)₄(OTeF₅)(PPh₃)] could be isolated (see Section 11.2.4) from the reaction between [Re(CO)₄Me(PPh₃)] and HOTeF₅.²⁶¹

The first nickel teflate complex, [Ni(Hacac)₂(OTeF₅)₂], was prepared by Riedel et al. by reaction of Ni(acac)₂ (acac = acetylacetonate) with HOTeF₅ (see Scheme 13). Both teflate ligands can be easily replaced by stronger coordinating agents, e.g. MeCN, and already small amounts of water lead to the bridged compound [{Ni(Hacac)(H₂O)(OTeF₅)(µ-OTeF₅)}₂].²⁶² Further nickel(II) complexes containing the teflate ligand were observed after treatment of the homoleptic [Ni(OTeF₅)₄]²⁻ with either diethyl ether or 2,2'-dimethyl-6,6'-bipyridine (bpyMe₂) to form [trans-Ni(Et₂O)₂(OTeF₅)₄]²⁻ or [Ni(bpyMe₂)(OTeF₅)₃]⁻, respectively (see eqs 84 and 85). Notably, using 2,2'-bipyridine does not lead to the desired similar product, but forms instead a mixture of species, which could not be separated, and which included the [Ni(bpy)₃]²⁺ dicationic complex. This underlines the easiness of dissociation of the weakly coordinated teflate groups in the homoleptic species.³⁴

$$[NEt_4]_2[Ni(OTeF_5)_4] \xrightarrow[Et_2O]{Et_2O} [NEt_4][Ni(OTeF_5)_4](OEt_2)_2 \quad (84)$$