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# Computational Experiments Probing the Adaptability of the [NCCH 2 ] – Electronic Structure to Various Bonding Environments

Jordan Rio, Jean-françois Brière, Hélène Gérard

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Jordan Rio, Jean-françois Brière, Hélène Gérard. Computational Experiments Probing the Adaptability of the [NCCH 2 ] – Electronic Structure to Various Bonding Environments. ChemPhysChem, 2026, 27 (2), pp.e202500580. 10.1002/cphc.202500580. hal-05490869

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https://hal.sorbonne-universite.fr/hal-05490869v1

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# Computational experiments probing the adaptability of the [NCCH₂]⁻ electronic structure to various bonding environments

Jordan Rio,*[a], Jean-François Brière[b], Hélène Gérard[a]

Using combined geometry optimisation and electronic analyses, we examine how metal nature (alkali and Cu(I)), solvation (THF), ligands and aggregation modulate the N- vs. C-bonding balance in metalated acetonitrile. C-binding is energetically favoured in covalent Cu(I) complexes, while lithiated species prefer N-binding. Surprisingly, N-metalated species do not all exhibit the expected ketenimine-like character (C=C=N, lone pair on N), but a nitrile-like one (C<sup>b</sup>-C≡N, lone pair on C<sup>b</sup>) also emerges from the NBO analyses. Ketenimines are stabilised by polarising or covalent M–N bonds, while nitriles are obtained with weakly coordinating cations or in anionic species. Notably, an external electric field can induce a similar electronic reorganisation, thus revealing the electronic flexibility of metalated nitriles.

## Introduction

Metalated nitriles are versatile synthetic tools for organic chemists. An important use of metalated nitriles are cyanoalkylation and cyanoarylation reactions.[1–8] In these reactions, the metalated nitriles are typically generated in situ by deprotonation, halogen- or sulfoxide-metal exchange reactions.[9] While these preparation methods are well established, the nature of the metal plays a critical role in dictating the electronic properties of the resulting metalated species, and the reactivity derived thereof, as testified by an insightful review on this topic.[9]

In the absence of metal coordination, the acetonitrile anion [NCCH₂]⁻, obtained by deprotonation of acetonitrile, predominantly exhibits a negative charge localised at the carbon atom adjacent to the cyanide functional group (C<sup>b</sup> in Scheme 1a) rather than at the nitrogen N atom of the ketenimine form.[10] Ab initio calculations and QM/MM simulations of cyanoalkane deprotonation in water suggest that acetonitrile has a pKₐ of 28.9, with its anionic form being stabilised primarily through inductive effect rather than by the mesomeric withdrawal capability of the cyanide functional group. In principle, the resonance forms of [NCCH₂]⁻

allow for two possible metalated structures, namely the N-metalated ketenimine and C<sup>b</sup>-metalated nitrile derivatives (Scheme 1b). However, this simplistic representation is incomplete, considering that some metalated species are indeed best described as N-metalated nitriles (Scheme 1c).[9] The coordination of the metalated entities is dictated by multiple factors, with the nature of the metal cation and its chemical environment playing a significant role.

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Resonance structures of [CH₂CN]⁻ a), and structural continuum of metalated nitriles (b and c). M = alkali-metal or transition-metal catalyst.</figcaption></figure>

Hence, solid-state X-ray diffraction (XRD) characterisation provides a valuable picture of the variety of binding modes in metalated nitrile structures.[9,11,12] Many of the reported structures suggest a strong preference for the formation of the N-metalated species among alkali metals, as evidenced by N-lithiated monomeric,[13] homo-bimetallic complexes,[14–16] polymers,[17] and even a homo-tetrametallic N-sodiated species.[18] In contrast, Zn(II), Cu(II) and Ni(II) frequently furnish C-metalation.[19–26] However, the metalation site is not determined solely by the nature of the metal. As an illustration, N-metalation of electron-deficient tricyanomethanide (C(CN)₃⁻) with Ni(II) has also been characterised by X-ray crystallography.[27]

Though XRD reveals solid-state preferences, it may not reflect solution behaviour. Lithiated nitriles can exist as either solvent-separated[28], mono-nuclear,[29,30] or polynuclear species,[30,31] illustrating their complex solution spe-

<footnote>[a] J. Rio, H. Gérard
Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, F-75005 Paris, France
E-mail: jordan.rio@sorbonne-universite.fr</footnote>

<footnote>[b] J.-F. Brière
CNRS, INSA Rouen Normandie, Univ Rouen Normandie, Univ Caen Normandie, ENSICAEN, Institut CARMEN UMR 6064, F-76000 Rouen, France</footnote>
ciation. Their structure and reactivity depend on ligand denticity and stericity, [32,33] solvent, [28,30,31,34] substrate nature, [35] and even temperature, [34] making direct extrapolation from solid-state data hazardous.

Computational chemistry can be a tool in unravelling solution structure. [36–46] Additionally, it allows completing the structural characterisation by analysis of electronic properties. In this context, the combination of geometry optimisation and Natural Bond Orbital (NBO) analysis was recently used to clarify the roles of alkali metal nature and solvent coordination in governing the bonding to biphenylene. [47] We thus resort on a similar approach to unravel the factors that control the balance between C- and N-metalation of acetonitrile by alkali metals and Cu(I) as a function of the nature of the metal coordination sphere.

## Results and Discussion

**Defining geometrical and electronic references.** The longstanding debate surrounding charge delocalisation in the acetonitrile anion [NCCH₂]⁻ has been extensively documented in the literature. [10,48,49] To establish a foundation for the forthcoming investigation of metalated acetonitrile, the geometry and electronic structure of [NCCH₂]⁻ in the absence of a counter-ion had to be recomputed at our computational level, using NBO calculations (Figure 1a). This will serve as a reference point for assessing the influence on the cation nature on the electronic structure of [NCCH₂]⁻. This anion exhibits the expected feature of a cyanide functional group, that is a short terminal Cª ≡ N triple bond (d(Cª-N) = 1.191 Å, see Table 1, entry 1) adjacent to a longer Cª-Cᵇ single bond (d(Cª-Cᵇ = 1.379 Å). It also features two lone pairs: a fully occupied sp lone pair at the nitrogen atom and a partially occupied (1.615) p lone pair at the α position of the cyanide group (Cᵇ atom). In line with previous computational and experimental studies, [10,48,49] the negative NPA charge is primarily localised on the Cᵇ atom (q(Cᵇ) = -1.029 e). Structurally, this deprotonated form exhibits a small pyramidalisation at the Cᵇ centre (dihedral D(Cᵇ-Cª-H¹-H²: 157°, see Figure 1a). This anion can serve as a model system for solvent-separated ion pairs, in which the [NCCH₂]⁻ anion and its counter-ion coexist in independent solvation spheres (e.g., lithiated nitriles in highly coordinating solvents or solvent mixtures). [28]

To evaluate the impact of covalent bonding on the electronic structure of metalated nitriles, we next investigated the topology of two tautomeric protonated species (Figure 1b and Table 1, entry 2), featuring Cᵇ-H (nitrile form CH) or N-H (ketenimine form NH) bonds. Expectedly, acetonitrile CH is thermodynamically favoured by an electronic energy difference (ΔE) of 25.9 kcal mol⁻¹ relative to the ketenimine NH. In the C-protonated acetonitrile molecule CH, the electronic structure remains very similar to [NCCH₂]⁻, the lone pair at Cᵇ now forming a covalent bond with H. In contrast, formation of the N–H bond is associated to a geometrical rearrangement (d(Cª-Cᵇ): 1.313 Å, d(Cª-N):1.229 Å in Table 1, entry 2), with formation of two double bonds (Cᵇ=Cª=N). Concerning the N lone pair, higher p character and smaller population are observed in the ketenimine tautomer NH, (sp²: N 39% s, 61% p, 1.835 e, ) than in the acetonitrile (sp: 54% s, 46% p, 1.970 e). These two clearly defined tautomer forms will serve as references for strongly localised structures.

### a) Acetonitrile anion

<figure><img src="image_4.png" /></figure>

### b) Acetonitrile and ketenimine

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Electronic structures of a) acetonitrile anion and b) acetonitrile and ketenimine. Occupancies of the lone pairs and anti-bonding orbitals are given just below the structures. Atomic NPA charges (q, in e) are provided in plain text, with charges for CH₂ groups shown in parentheses.</figcaption></figure>

**Lithiated species.** We investigated how decreasing the covalency of the C-M or N-M bond would affect the bonding of [NCCH₂]⁻. Numerous results from the literature suggest that lithium cation preferentially coordinates the nitrogen atom, to form monomeric or dimeric complexes. [9] We first examined the coordination of a ‘naked’ lithium cation with the N or Cᵇ atoms (Figure 2a). While such a species is unrealistic in solution, it will serve the discussion as an extreme case, in which cation polarisation is unaffected by solvent coordination. Using a Li⁺, the N-lithiated form (NLI) is found to be slightly favoured over the C-lithiated one (CL) (ΔE = +2.4 kcal mol⁻¹). Notably, in NL, lithium interacts with the nitrogen atom along the Cª-N axis (Cª-N-Li: 180°, in Table 1, entry 3). The short Cª-Cᵇ distance (d(Cª-Cᵇ) = 1.352 Å) is closer to that observed in the protonated ketenimine NH (d(Cª - Cᵇ) = 1.313 Å, entry 2). The NBO analysis of NL reveals that two lone pairs (p(N): 100% p, 1.530 e, and sp(N): 50 %s, 50 %p, 1.961 e) are located at the nitrogen atom, whereas both Cᵇ=Cª and Cª=N are analysed as double bonds. In contrast, C-lithiation results in the formation of a C-metalated nitrile CL, where the lone pairs are distributed as sp(N) (16 %s, 84% p, 1.734 e) and p(Cᵇ) (53%s, 47%p, 1.966 e), with a single C–C and a triple C≡N bond, similar to [NCCH₂]⁻ (Figures 1 and 2a). This change in the location of the lone pairs is interesting, as it suggests that the lithium cation is polarising enough to displace the p(Cᵇ) lone pair of [NCCH₂]⁻ on the N atom in the N-bonded structure.

Nevertheless, numerous studies from the literature have evidenced that organolithium compounds are highly sensitive to chemical environment effects – and particularly solvent coordination. [50–54] Therefore, we investigated the impact of THF coordination on the electronic structure of lithiated species (Figure 2b and Table 1, entry 4). Bond distances and NBO analyses of mono-solvated N-and C-lithiated complexes NL(THF) and CL(THF) reveal geometries and electronic structures nearly identical to their "naked" counterparts. Strikingly, coordinating a second
<table><caption>Table 1. Impact of explicit solvent coordination on the structure of lithiated nitriles. Distances *d* are given in Å, angles *A* are given in °.</caption><thead><tr><th>Entry</th><th>Y<sup>MX</sup></th><th>d(C<sup>a</sup>-C<sup>b</sup>)</th><th>d(C<sup>a</sup>-N)</th><th>A(C<sup>a</sup>-N-M)</th><th>C<sup>MX</sup></th><th>d(C<sup>a</sup>-C<sup>b</sup>)</th><th>d(C<sup>a</sup>-N)</th><th>A(C<sup>a</sup>-C<sup>b</sup>-M)</th></tr></thead><tbody><tr><td>1</td><td>Acetonitrile anion</td><td>1.379</td><td>1.191</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2</td><td>N<sup>H</sup></td><td>1.313</td><td>1.229</td><td>116</td><td>C<sup>H</sup></td><td>1.451</td><td>1.162</td><td>110</td></tr><tr><td>3</td><td>N<sup>Li</sup></td><td>1.352</td><td>1.197</td><td>180</td><td>C<sup>Li</sup></td><td>1.409</td><td>1.179</td><td>105</td></tr><tr><td>4</td><td>N<sup>Li</sup>(THF)<sup>1</sup></td><td>1.354</td><td>1.196</td><td>180</td><td>C<sup>Li</sup>(THF)<sup>1</sup></td><td>1.406</td><td>1.189</td><td>102</td></tr><tr><td>5</td><td>N<sup>Li</sup>(THF)<sup>2</sup></td><td>1.357</td><td>1.195</td><td>179</td><td>C<sup>Li</sup>(THF)<sup>2</sup></td><td>1.401</td><td>1.182</td><td>94</td></tr><tr><td>6</td><td>N<sup>Li</sup>(THF)<sup>3</sup></td><td>1.361</td><td>1.194</td><td>153</td><td>C<sup>Li</sup>(THF)<sup>3</sup></td><td>1.396</td><td>1.184</td><td>100</td></tr><tr><td>7</td><td>Bim<sup>NN</sup></td><td>1.342</td><td>1.205</td><td>140</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>8</td><td>Bim<sup>NN</sup>(THF)</td><td>1.346</td><td>1.201</td><td>143</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>9</td><td>Bim<sup>NC</sup></td><td>1.385</td><td>1.185</td><td>137</td><td>-</td><td>-</td><td>-</td><td>96</td></tr><tr><td>10</td><td>Bim<sup>NC</sup>(THF)</td><td>1.384</td><td>1.184</td><td>142</td><td>-</td><td>-</td><td>-</td><td>98</td></tr><tr><td>11</td><td>N<sup>Cs</sup></td><td>1.367</td><td>1.195</td><td>175</td><td>C<sup>Cs</sup></td><td>1.193</td><td>4.008</td><td>57</td></tr><tr><td>12</td><td>N<sup>K</sup></td><td>1.362</td><td>1.196</td><td>179</td><td>C<sup>K</sup></td><td>1.193</td><td>3.094</td><td>64</td></tr><tr><td>13</td><td>N<sup>Na</sup></td><td>1.358</td><td>1.196</td><td>179</td><td>C<sup>Na</sup></td><td>1.403</td><td>1.181</td><td>103</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Figure 2. Electronic structures of a) monomeric alkali complexes, b) monomeric THF-coordinated lithiated complexes and c) homobimetallic lithiated complexes. Occupancies of the lone pairs and selected antibonding orbitals are given just below the structures. Atomic NPA charges (*q*<sub>ij</sub>, in *e*) are provided in plain text, with charges for CH<sub>2</sub> groups shown in parentheses. The difference in electronic energy (Δ*E*<sub>ij</sub>, in kcal mol<sup>-1</sup>) between *N*-and *C*-metalation is given above the arrows, using *N*-metalated complex as a reference.</figcaption></figure>
THF to N-lithiated ketenimine shifts the *p*(*N*) lone pair (100% *p*, 1.521 e in N<sup>Li(THF)1</sup>) toward C<sup>b</sup> atom (100% *p*, 1.534 e in N<sup>Li(THF)2</sup>). This displacement is also associated to the formation of a single C–C and a triple C≡N bonds, consistent with the electronic structure of a nitrile group (Figure 2b). Despite these different electronic populations, N<sup>Li(THF)1</sup> and N<sup>Li(THF)2</sup> display quasi identical geometries (distances in Table 1 change within 0.003 Å, see entries 4 and 5). Notably, the short *d*(C–N) distance (1.357 Å) lies closer from the protonated ketenimine N<sup>H</sup> (1.313 Å) than from the protonated nitrile C<sup>H</sup> (1.451 Å) (Figure 1, entry 2), thus creating a discrepancy between the nitrile-like electronic structure and the ketenimine-like geometry in species N<sup>Li(THF)2</sup>. The coordination of additional THF molecules has no more impact on the geometry and NBO structures, with the exception of the C<sup>a</sup>–N–Li bond angle that deviates from linearity, decreasing to 153° (see Table 1, entry 6 and cartesian coordinates in Supporting Information).

Energetically, the influence of solvent coordination on the N- vs. C-lithiation balance appears limited (Figure 2b). Though N-lithiation remains systematically favoured, this slight preference is gradually diminished by THF coordination ($\Delta E = -2.4, -2.6$ and $-1.6$ and $-1.1$ kcal mol<sup>-1</sup>, for the coordination of 0 to 3 THF).

The ability of lithiated species to form polymetallic aggregates has been evidenced by numerous studies.[14-17,30,31] The structure and electronic properties of N-lithiated homobimetallic aggregates were also investigated (Figure 2c and Table 1, entries 7-10). Geometric data and NBO calculations (i.e. two double C=C and C=N bonds, as well as the presence of sp(*N*) and *p*(*N*) lone pairs, see Table 1 entry 7 and Figure 2c) indicate that the ketenimine pattern observed in N<sup>Li</sup> is retained in the dimeric form Bim<sup>NN</sup>. An alternative dimeric structure Bim<sup>NC</sup> where each lithium coordinates both a C<sup>b</sup> atom and a nitrogen atom can also be proposed. In this dimer, a cyanide functional group is suggested based on NBO analysis (see Figure 2), whereas geometrical data remain inconclusive. Energetically, this mixed N,C-coordinated complex is favoured over Bim<sup>NN</sup> by $\Delta E = -5.3$ kcal mol<sup>-1</sup>. Coordination of solvent molecules to Bim<sup>NN</sup> and Bim<sup>NC</sup> has no effect on the structure (Table 1, entries 9 and 10) and bonding characteristics of the complexes (Bim<sup>NN</sup>(THF) and Bim<sup>NC</sup>(THF)), which become isoenergetic.

As a conclusion, the chemical environment surrounding the Li centre (i.e. solvent coordination, aggregation state) plays a critical role in shaping the structure of the N-lithiated complexes. When up to one solvent molecule is coordinated, as well as in homo-bimetallic complexes where Li<sup>+</sup> coordinates exclusively to nitrogen atoms, the geometry and electronic structure resemble those of N-lithiated ketenimines. In contrast, the coordination of more than two THF leads to the formation of a cyanide functional group despite the unaffected N-coordination, which is also observed in homo-bimetallic complexes where Li<sup>+</sup> interacts with both C<sup>b</sup> and N atoms.

**Comparison with other alkali cations.** In order to further elucidate how cation polarisation modulates the balance between N- and C-metalation, we examined the effects of larger alkali cations M<sup>+</sup> (M = Na, K, Cs) on the electronic structure of [NCCH<sub>2</sub>]<sup>-</sup>. In the C-metalated series, the N–C<sup>a</sup>–M angle varies with the nature of the cation (see N<sup>M</sup> complexes in Figure 2a, and entries 11-13 in Table

1). This variation reflects a change in coordination modes, shifting from an $\eta^1$ mode with Li<sup>+</sup> and Na<sup>+</sup> (105° and 103°) to a bridging mode with K<sup>+</sup> and Cs<sup>+</sup> (64° and 57°).

In contrast, N-metalation (see N<sup>M</sup> complexes in Figure 2a) consistently occurs along the C<sup>a</sup>–N axis in. However, NBO analyses reveal that these softer cations M<sup>+</sup> are not polarising enough to change the position of the lone pairs, and C≡N bonds are consequently retained, whatever the metal bonding site. From a geometrical point of view, the bond lengths *d*(C<sup>a</sup> – C<sup>b</sup>) (1.367-1.358 Å, Table 1 entries 11-13) and *d*(C<sup>a</sup> – N) (1.195-1.196 Å) distances are nearly invariant across the series, and closely resembles those in the ketenimine complex N<sup>Li</sup> (*d*(C<sup>a</sup> – C<sup>b</sup>): 1.352 Å, *d*(C<sup>a</sup> – N): 1.197 Å, see Table 1 entry 3). A discrepancy in the attribution of nitrile / ketenimine thus appears between geometry and NBO structure. The NBO analyses provide herein a valuable complement to structural data, enabling discrimination between N-metalated nitriles and ketenimines where geometry alone is inconclusive.

The equilibrium between N- and C- metalated species reflects the softening of the cation binding (Figure 2b). In line with a $\eta^1$-localized C-binding mode, Li<sup>+</sup> and Na<sup>+</sup> display a significant preference for this structure. In contrast, the equilibrium between the delocalized $\eta^3$-binding and N-only binding of Cs<sup>+</sup> and K<sup>+</sup> is quasi neutral.

From these results, it seems that Li<sup>+</sup> is able to displace the lone pair from the C<sup>b</sup> to the N. The differences observed for the N-H and N-Li bonds both geometrically (C<sup>a</sup>-N-M angle value equal to 116° in N<sup>H</sup> and 180° in N<sup>Li</sup>) and electronically<sup>55</sup> show that a covalent bond between N and H is formed in the case of N<sup>H</sup> whereas none is obtained for N<sup>Li</sup>. We thus propose that the lone pair localization at N in N<sup>Li</sup> is associated to the strong polarising effect of the Li<sup>+</sup> highly localised positive charge.

**Analogy with an electric field.** To further illustrate the critical impact of cation polarizing effect on the mesomeric balance in metalated species, we investigated the effect of an external electric field (*F*<sub>x</sub>) applied in the same direction as the one created by the charge, that is along the C<sup>a</sup>–N bond (*x* axis, Figure 3) in [CH<sub>2</sub>CN]<sup>-</sup>. This computational experiment provides a convenient conceptual framework to probe the influence of a polarizing environment on resonance between nitrile and ketenimine forms. Applying a positive electric field slowly and continuously increases the occupancy of the *p*(C<sup>b</sup>) orbital (from 1.615 e with no field to 1.868 e with 0.25 a.u. field, the maximum applied) while reducing the population of the antibonding $\pi^*(C = N)$ orbital (from 0.373 e with no field, to 0.122 e for *F*<sub>x</sub> = 0.250 a.u.). This effect is analogous to that observed when hard cations, such as lithium (*p*(C<sup>b</sup>): 1.734 e in N<sup>Li</sup>) and sodium (*p*(C<sup>b</sup>): 1.691 e in N<sup>Na</sup>), coordinate to the C<sup>b</sup> atom of [CH<sub>2</sub>CN]<sup>-</sup> C<sup>Li</sup> (Figure 3, region *i*)). Interestingly, this observation also holds for all the THF-coordinated C-lithiated complexes that were studied. In contrast, applying a moderate negative electric field (*F*<sub>x</sub> > -75 a.u.) along the C<sup>a</sup>–N bond initially depletes the lone pair *p*(C<sup>b</sup>) (down to 1.521 e), leading to progressive filling of the anti-bonding $\pi^*(C = N)$ orbital (0.465 e, for *F*<sub>x</sub> = -0.075 a.u.). This behaviour mirrors the effect of coordinating Na<sup>+</sup> (1.537 e), K<sup>+</sup> (1.553 e), or Cs<sup>+</sup> (1.576 e) cations to the nitrogen atom (Figure 3, region *ii*)) A similar trend is also observed in di- and tri-solvated N-lithiated complexes - all of them featuring a cyanide functional group. For a more negative electric field (*F*<sub>x</sub> < -0.075 a.u.), a shift toward the electronic structure
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Impact of an electric field *F*(*x*) applied along the *x* axis on the populations of the lone pairs (upper part), and comparison with the populations obtained for mono-and bi-metallic metalated nitriles (lower part). The straight black line (*F*<sub>x</sub> = 0 a.u., *p*(*C*<sup>b</sup>): 1.615 e) corresponds to the acetonitrile anion in the absence of any electric field. Region *i*) shows an increase in the *p*(*C*<sup>b</sup>) lone pair population, region *ii*) a decrease, and region *iii*) a progressive filling of the *p*(*N*) lone pair.</figcaption></figure>

of the ketenimine form occurs. The *p*(*N*) lone pair forms and continues to populate as the field strength increases in magnitude, consequently depleting the π*(*C* = *C*) orbital. This effect is similar to what is observed in the case of naked and mono-solvated *N*-lithiated complexes, which are identified as ketenimine-like structures (Figure 3, region *iii*). Overall, these results demonstrate that the application of an electric field can modulate the electronic structure of [CH<sub>2</sub>CN]<sup>−</sup>, in a similar manner to that of cation coordination to C<sup>b</sup> or N. We propose that the strength of the electric field induced by the cations is the major factor in shaping the electronic structure of *N*-metalated nitriles and ketenimines when binding alkali metals. The concept of controlling bond polarity and electronic structure by manipulating local electric fields is not limited to metalated species. Similar effects have been recently observed for imine bonds in donor–acceptor covalent organic frameworks, where adjusting the polarity of C=N bonds through chemical environ-

ment or applied fields was shown to control charge separa-
tion and enhance photocatalytic performance.[56] The mod-
ulation of this effect when resorting on a non-alkali metal is
next examined.

**N**-and **C**-metalated complexes of **Cu(I)**. Next, we chose to investigate the bonding characteristics of Cu(I) complexes (Figures 4 and 5). With this 3d¹⁰4s⁰ cation, bonding primarily involves 4s orbitals (i.e. as for alkali metals) but exhibits significantly greater covalency. Using a single Cu⁺ cation as a ligand-free model, two different binding modes could be obtained, the *C*-metalated form being largely favoured (Δ*E* = −16.1 kcal mol⁻¹, relative to *N*<sup>Cu</sup>, Figure 4). Coordination to the nitrogen atom (*N*<sup>Cu</sup>) shows a *C*<sup>b</sup>-N-Cu value of 135.5°. A ketenimine-like electronic structure is observed, with a *sp*(*N*) lone pair and a moderately populated Cu–N bond (1.620 electrons involved, 1.539 coming from N). Alternatively, coordination of the Cu⁺ cation with the *C*<sup>b</sup> atom (*C*<sup>Cu</sup>) yields the electronic structure of a *C*-metalated nitrile, with a single *sp*(*N*) lone pair and a considerably more populated Cu–*C*<sup>b</sup> bond (1.864 e) compared to the *N*-metalated ketenimine *N*<sup>Cu</sup>. Nevertheless, this simplified Cu(I) model does not take chemical environment effects into account, which will of course modulate the properties of the system.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Electronic structures of ligand-free copper complexes. Occupancies of the lone pairs and selected antibonding orbitals are given just below the structures.</figcaption></figure>

Hence, we examined how the coordination of L- or X-type
ligands to the Cu(I) centre shift this equilibrium. The re-
sults obtained for all the stable metalated species are shown
in Figure 5. Only the THF-containing N-metalated struc-
ture, which features the least σ-donating L-ligand, retains
a ketenimine-like electronic structure. The NBO analysis
does not yield a covalent Cu–N bond but the population
in the N lone pair (1.542 e) is nearly identical to the pop-
ulation in the Cu–N bond in N<sup>Cu</sup> (1.539 e). All other
structures, either N- or C-coordinated, exhibit a nitrile-like
electronic structure, with a triple C≡N bond and either a
true lone pair at C or a Cu–C bond highly polarised at C.
Most notably, Cu(PMe<sub>3</sub>) and Cu(Me) yield similar results
despite a different global charge and thus a different electric
field. Independently of the ligand nature, the equilibrium
is shifted toward C-metalation. Expectedly, the preference
for C-metalation can be diminished by coordinating strong
σ-donor ligands (L=PMe<sub>3</sub>, X = Me) in trans-position to
[CH<sub>2</sub>CN]<sup>−</sup>. These results illustrate that the chemical envi-
ronment at the metal can be adjusted - through the choice
of the solvent, ligands and possibly additives - not only to
decrease the preference for C-coordination, but also and in-
dependently, to favour electronic structure with a lone pair
at the terminal C.
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Electronic structures of LCu- and XCu-complexes. Occupancies of the lone pairs and selected antibonding orbitals are given just below the structures. Electronic energies (ΔE) are indicated above the arrows, and are given in kcal mol⁻¹.</figcaption></figure>

## Conclusion

Computational studies have provided a clear illustration of the importance of chemical environment in modelling structures of metalated complexes. Consistently with previous reports, [9] N-lithiation is energetically preferred over C-lithiation. For other alkali metals, a delocalised coordination to the whole π system is obtained with no significant site preference. However, in the fringes of the geometric continuum, a decorrelated electronic duality has emerged from NBO analyses of the N-coordinated structures. In most cases, the electronic structure of the free anion [NCCH₂]⁻ with localisation of a lone pair at the terminal carbon is observed even for N-coordinated species. However, a strongly polarising cation (such a Li⁺) can induce transfer of the lone pair to the terminal N to form a ketenimine-like electronic structure (see Scheme 2). Hence, lithium exhibits a behaviour that contrasts with other alkali metals — for which a cyanide functional group is systematically observed. In addition, solvent coordination on the N-lithiated species induces a shift back from ketenimine to nitrile mesomeric forms, thus playing a crucial role in shaping its electronic structure. Formation of a covalent bond to N (such as N-H or N-Cu bonds) can also induce a ketenimine electronic structures in N-bonded complexes (see Scheme 2), a shift strongly sensitive to the nature of the ligand at Cu. These covalently bonded structures are all associated to a preference for C-coordination but it can be significantly tuned by the nature of the ligands, independently of the electronic structure observed.

## Computational details

All calculations were performed using the Gaussian 16 software (revision C.01). [57] Geometries were optimised via frequency calculations at the B3PW91 level of theory, [58,59] incorporating Grimme D3 dispersion correction with Becke-Johnson damping (GD3BJ). [60] A benchmark of computational methods is provided in Supporting Information. A continuum solvation model based on density (SMD) for

<figure><img src="image_10.png" /><figcaption>**Scheme 2.** Shift of the electronic continuum from N-metalated nitriles to N-metalated ketenimines.</figcaption></figure>

THF was applied in all calculations. [61] Explicit solvation is needed for properly modelling lithiated nitriles. [62] Hence, for lithiated structures, additional calculations involving a mixed explicit-implicit representation of the solvent are carried out and reported. In this case, the THF molecules were pre-optimised, and then coordinated with the lithium one by one. The system was optimised again after each addition. No further conformational sampling was performed. For other alkali metals, the coordination of a THF molecule was found to be less exergonic and directional (see the Supporting Information for details), and only implicit solvation was used to discuss the impact of the nature of alkali-metal on the balance between N- and C-metalation. [63] For the choice of basis sets, a split-valence Pople basis set (6-31++G**) was employed for light elements (C, H, N, O, Li, Na, P, Cl and K), [64–66] while fully-relativistic Stuttgart-Dresden pseudopotentials (SDD) with their associated basis sets were used for elements heavier than K (not included, i.e. Cu and Cs). [67–69] Electronic structure analyses were conducted using the NBO7 package within Gaussian. [70] All species examined in this study were generated through manual sampling, with particular attention given to exploring
alternative coordination modes. Optimised geometries and associated energies are provided in an external file (structures.xyz). All the output files discussed in this article are available at https://doi.org/10.19061/iochem-bd-6-582.

## Acknowledgements

This work was carried out at Laboratoire de Chimie Théorique (Sorbonne Université), and supported by the french ANR funding program (project NICKELATE, ANR-23-CE07-0014). This work has been partially supported by the University of Rouen Normandy, INSA Rouen Normandy, the Centre National de la Recherche Scientifique (CNRS), European Regional Development Fund (ERDF), Région Normandie, Labex SynOrg (ANR-11-LABX-0029), Carnot Institute I2C, and the Graduate School of Research XL-Chem (ANR-18-EURE-0020 XL CHEM).

## Conflict of Interest

The authors declare no conflict of interest.

**Keywords:** Metalation • Ligand effects • Molecular modeling
• Solvent effects • Electronic structure

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Entry for the Table of Contents

<figure><img src="image_11.png" /></figure>

Structural continuum
of metalated nitriles
in solution

<figure><img src="image_12.png" /></figure>

Nature of the metal cation?
Chemical environment effects ?

Metalated nitriles are organometallic chameleons in solution, existing as a structural duality of *N*- and *C*-metalated species exhibiting specific reactivities. With DFT calculations and NBO analyses, we explored how the metal nature (Li, Na, K, Cs, Cu(I)) as well as the chemical environment (ligand, solvation) impact this continuum. A divergence between metalation site and formal charge localization is evidenced.