# Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State

Kinga Potempa, Krystyna Deresz, Joanna Jankowska, Katarzyna Jarzembska, Adam Krówczyński, Artem Mikhailov, Dominik Schaniel, Radosław Kamiński

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Kinga Potempa, Krystyna Deresz, Joanna Jankowska, Katarzyna Jarzembska, Adam Krówczyński, et al. Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State. Chemistry - A European Journal, 2023, 29 (72), 10.1002/chem.202302629. hal-04568705

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# Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State

Kinga Potempa,[a]# Krystyna A. Deresz,[a]# Joanna Jankowska,[a]

Katarzyna N. Jarzembska,[a]# Adam Krówczyński,[a] Artem Mikhailov,[b]

Dominik Schaniel,[b] Radosław Kamiński [a]

**Abstract:** An efficient nitrite nickel(II) photoswitch, with the 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propanone moiety used as the ancillary ligand, is reported. In the ground-state ('dark') crystal structure, the studied compound exists predominantly as the nitro-(η¹-N(O)₂) isomer, however, traces of the *exo*- and *endo*-nitrito-(η¹-ONO) forms are detected both at 100 K (4–5% each) and under ambient conditions (~9% each). When excited with the 405–530 nm LED light, the nitro-to-nitrito isomerization takes place. The total conversion exceeds 90%. The *exo*-nitrito linkage isomer constitutes the dominant photo-generated form, whereas the relative population of both nitrito species depends on temperature. The reaction is fully reversible and reproducible. The photo-products are stable up to 200 K. The system constitutes a good model case for the reaction mechanism studies. Thus, experimental and theoretical investigations on the photo-isomerism were conducted and are presented in detail. Eventually, the nitro → *exo*-nitrito → *endo*-nitrito reaction pathway is proposed.

**Keywords:** photoisomerization, linkage isomers, nitrite nickel coordination compounds, isomerization mechanism, DFT calculations

<footnote>[a] K. Potempa, K. A. Deresz, Dr. J. Jankowska, Prof. K. N. Jarzembska, Prof. A. Krówczyński, Dr. R. Kamiński
Department of Chemistry
University of Warsaw
Żwirki i Wigury 101, 02-089 Warsaw. Poland
E-mail: katarzyna.jarzembska@uw.edu.pl
Webpage: www.photocrystallography.eu</footnote>

<footnote>[b] Dr. A. Mikhailov, Prof. D. Schaniel
Université de Lorraine, CNRS, CRM²
54500 Nancy, France</footnote>

<footnote># Both authors contributed equally</footnote>
Introduction

Molecules that can undergo structural transformations upon exposure to light are classified as photoswitches.[1, 2, 3] Thanks to their capability to interconvert between two or more (meta)stable states, they may find wide applications in materials science, optoelectronics, or biology.[4] Molecular photoswitches are typically organic molecules with extended aromatic systems responsive to light.[5] Nevertheless, transition-metal complexes possessing ambidentate ligands, such as NO, NO₂, or SO₂[2, 6, 7, 8-11, 12-15] constitute a promising and readily modifiable group of this kind of compounds.[9] The ambidentate ligands are the key fragments of these molecules because they can bind to a metal center in multiple ways which can be controlled optically.[16]

In the view of the above, the nitrite group constitutes an interesting moiety as it may adopt up to nine different binding modes,[17] some of which can interconvert one into another under certain conditions. In the case of mono-nuclear transition-metal complexes, four linkage isomers can be observed, i.e. nitro-(η¹-N(O)₂), *exo*-nitrito-(η¹-ONO), *endo*-nitrito-(η¹-ONO), and κ-nitrito-(η²-O₂N) forms (Scheme 1).[9, 17] Up to date, numerous photoswitchable nitrite transition-metal coordination systems have been reported.[6, 8, 11, 15, 18-20, 21] There are, though, rather limited examples of high-conversion photoswitches of this kind, and/or complexes which work at higher temperatures.[10, 11, 13-15, 22-24] Furthermore, an aspect which is still under debate is the mechanism of light- and thermally-induced transformations between linkage isomers. There are few papers which undertake this issue supporting experimental presumptions with theoretical modelling. The articles published by Muya *et al.*[25] and Skelton *et al.*[26] are devoted to octahedral nitro complexes of cobalt(III) and nickel(II), respectively, whereas there is only one paper discussing a square-planar nitro Ni¹¹ complex.[22] In the case of the octahedral Ni¹¹ compound, which exhibits a triplet electronic configuration in the ground state, theoretical modelling results indicated that the nitro-to-*endo*-nitrito transformation occurs first and then the *endo*-nitrito product can isomerize further, with lower energy barrier, to the *exo*-nitrito form. There was no evidence of a direct reaction pathway connecting the *exo*-nitrito and nitro forms, the reasons for which were not clear. For the analyzed singlet octahedral Co¹¹¹ complex, the most probable isomerization mechanism also seems to be the transformation of the nitro binding mode to the *exo*-nitrito form through the *endo*-nitrito isomer. However, in that case both direct nitro-to-*endo*-nitrito and nitro-to-*exo*-nitrito pathways were found. The fact that experimentally only the nitro
and *exo*-nitrito isomers were observed was explained by the kinetics of considered processes. On the other hand, as far as the square-planar Ni¹¹ photoswitchable system studied by Warren *et al*. is concerned, a different mechanism is proposed following the theoretical considerations. Here, the nitro-to-*exo*-nitrito transformation is postulated to occur prior to the *endo*-nitrito isomer formation. Nevertheless, it should also be mentioned that for a photoswitchable square-planar Pd¹¹ complex recently studied by some of us, again the nitro ⇌ *endo*-nitrito ⇌ *exo*-nitrito mechanism was proposed based on detailed experimental analyses, but lacking theoretical support.[27]

<figure><img src="image_2.png" /></figure>

**Scheme 1.** Schematic diagram of possible NO₂-group binding modesin mono-nuclear complexes (M = metal center). Note the *κ*-nitrito isomer was not observed in this study.

In the current paper a new photoswitchable square-planar Ni¹¹ nitro complex is introduced (Figure 1a), hereafter referred to as **Ni-1d'**. It consists of a 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propanone moiety as an (*N*,*N*,*O*)-donor supporting ligand coordinating the metal center and the ambidentate nitrite group. The system has been comprehensively analyzed, including computational modelling conducted so as to rationalize the experimental observations, and to fill in the mentioned niche regarding the isomerization reaction mechanism investigations. The reported research constitutes a part of our wider project dedicated to photoswitchable nitrite complexes of transition metals.[6, 10, 18, 19, 24]

**Experimental section**

**Synthesis.** All solvents and substrates were purchased from the chemical companies and used without further purification. The synthesis is analogous to that described in our previous contribution[6] (reaction scheme is shown in the Supporting Information, Scheme S1). 6.5 mmol of sodium in 85 mL of Et₂O was added to a mixture of 4.0 mmol of propiophenone in 8.0 mmol of ethyl formate and stirred for 12 hours. After solvent evaporation, the obtained hydroxymethyleneketone sodium salt was dissolved in 30 mL of AcOH, and then the 1.5 mmol of
*N,N*-dimethylethylenediamine in 15 mL of AcOH was added. The obtained intermediate product (Supporting Information) was not isolated; its solution was brought to boil followed by an addition of 0.5 mmol of Ni(OAc)₂·4H₂O in 10 mL of MeOH, respectively. Then, the solution was not heated anymore while continued to be stirred for 2 hours. The dark-brown mixture was purified by filtration and then *ca.* 1.0 mmol of LiNO₂ in MeOH was added. The resulting mixture containing the final product was cooled down using an ice bath, and the product was filtered. Yield: 0.836 g (47%). Deep-red crystals of **Ni-1d'** suitable for single-crystal X-ray diffraction experiments were grown via slow evaporation of solvent from the chloroform solution of the studied compound.

Nuclear magnetic resonance (NMR) spectra were measured with an Agilent NMR 400 MHz Varian spectrometer; ¹H chemical shifts are given relative to TMS by using residual solvent resonances. The ¹H NMR spectra in CDCl₃ show broadened signals, which may attributed to a reversible NO₂ group dissociation in the presence of small amounts of water. Elemental analyses were carried out with an Elementar Vario EL III analyzer. ¹H NMR (400 MHz, acetone-*d*₆) δ = 7.82 (d, *J* = 7.7 Hz, 2H), 7.55 (t, *J* = 7.4 Hz, 1H), 7.43 (t, *J* = 7.6 Hz, 2H), 4.60 – 3.70 (m, 1H), 3.25-2.40 (m, 9H) ppm. Elemental analysis: C₁₃H₁₆N₄NiO₄ (350.98); calculated: C 44.48%, H 4.60%, N 15.97%; found: C 44.11%, H 4.47%, N 15.89%.

**X-ray diffraction.** All X-ray diffraction experiments (including the preliminary ones) were carried out on a Rigaku Oxford Diffraction SuperNova single-crystal diffractometer equipped with a CCD detector, a copper microfocus X-ray source, a low-temperature nitrogen gas flow Oxford Cryosystems device, and our home-made light-delivery device, which allows *in situ* photocrystallographic experiments.[28] Single crystals were irradiated with the 530 nm LED light (*ca.* 290 mW·cm⁻²; note the beam is larger than the crystal). The optimal data-collection strategy took into account the mounted light delivery device and was prepared using the native diffractometer software. In the case of photocrystallographic experiments the same strategy was used for all data collected for a given crystal (only the exposure time was adjusted for various temperatures). All data collections were carried out in complete darkness (the sample mounting and centering was done at room temperature prior to any further data collection, and all experiments were performed with all the diffractometer lights permanently switched off). For details concerning photocrystallographic data collection codes and the measurement sequence for each sample, see the Supporting Information (Table S1). Further data processing (i.e., unit-cell determination, raw diffraction-frame integration, absorption correction, and scaling) was the same for all data sets collected. All structures were solved using an intrinsic phasing method as implemented in the *SHELXT* program[29] and refined with the *JANA* package[30] within the independent atom model approximation.[31] Deposition numbers 2287053–103 (for **Ni-1d'**) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre[32] and Fachinformationszentrum Karlsruhe[33] Access Structures service. The CIF files can also be retrieved from the Supporting Information.

**Spectroscopy.** All infrared (IR) measurements were performed using a Nicolet 5700 FTIR spectrometer (spectral resolution of 2 cm⁻¹ in the range of 360–4000 cm⁻¹) equipped with a closed-cycle cryostat (Oxford Optistat V01). The sample was ground, mixed with spectroscopic-grade KBr, pressed into pellets, and glued to the coldfinger of the cryostat using a silver-paste thermal adhesive. During measurements, the sample was kept in a vacuum inside the cryostat. Irradiation of the sample was achieved through the cryostat window using various LEDs (Thorlabs L and LP series), the central wavelengths of which covered the range from violet to red (from 385 to 735 nm). LED powers are presented in the Supporting Information (Table S10).
Room-temperature UV-Vis absorption measurements were performed with a Shimadzu UV-2600i spectrometer equipped with the ISR-2600Plus integrating sphere using the diffuse reflectance method (300-800 nm wavelength range in 1 nm intervals). The sample holder was loaded with the uniform mixture of the title compound and BaSO₄ as the white reference. The obtained spectrum is presented in the Supporting Information (Figure S4).

**Calorimetric measurements.** Differential scanning calorimetry (DSC) measurements were performed on a Mettler-Toledo DSC1 instrument equipped with a high-sensitivity DSC HSS8 sensor. The crystal sample was homogenously ground and placed in an aluminum pan to form a thin layer covering the pan's surface. The sample in the open pan was placed in the instrument, cooled down to 123 K and then irradiated with a chosen LED for a given time. LED powers are presented in the Supporting Information (Table S10). After irradiation the standard DSC measurement was performed. For studying the kinetics of the isomerization reaction the NETZSCH DSC 204 F1 Phoenix software was used.

**Computations.** Isolated-molecule and quantum-mechanics/molecular-mechanics (QM/MM) geometry optimizations, dimer interaction energies, and normal-mode frequencies were calculated using the density functional theory (DFT) at the DFT(B3LYP)/6-311++G** level of theory[34] using the *GAUSSIAN* package (ver. 16).[35] For harmonic-mode computations, no imaginary frequencies were found. All optimized molecular energies were corrected for the zero-point energy (ZPE). In the case of interaction energy calculations, the Grimme empirical dispersion correction[36] modified by the Becke-Johnson damping function[37] and a correction for basis set superposition error[38] were applied. As far as the used QM/MM method is concerned, the crystal environment was modelled by cutting out a shell with a radius of 15 Å around the central molecule from the examined experimental crystal structure, the C-H distances of which were set to the neutron-normalized values.[39] The DFT(B3LYP)/6-311++G** level of theory was applied for the optimization of the central molecule, whereas the molecular shell was kept fixed and approximated with the Universal Force Field[40] using Hirshfeld atomic charges[41] initially derived at the same level of theory. The semi-automatic generation of input files was accomplished with the *CLUSTERGEN* program.[42] Additionally, unit-cell parameters and atomic positions were optimized using the periodic approach implemented in the *CRYSTAL* program (ver. 17)[43] applying the sol-def2-mSVP functional and the PBESOL0-3C basis set for all atoms.[44]

Potential energy profiles (PEPs) in the ground (S₀) and lowest excited (S₁) singlet states were determined with the time-dependent methods of the pure DFT (TDDFT) and the Tamm-Dancoff approximation (TDA) levels.[45] In these calculations, the B3LYP functional with Grimme dispersion correction was used, with the 6-311++G** basis set applied for all atoms but the nickel, for which the 6-31G** basis set[46] was employed; the same level of theory was also used for the UV-Vis absorption spectrum calculations. All reaction-coordinate-scan calculations have been performed with *TURBOMOLE* suite of programs (ver. 7.1).[47] The vibronically-corrected UV-Vis spectra were obtained with the *NEWTON-X* program (ver. 2.4)[48] interfaced with *TURBOMOLE*. In this case, the Wigner energy distribution was employed to account for the zero-point energy (ZPE) vibrational effects. The associated natural transition orbitals (NTOs) have been calculated with the *THEODORE* code (ver. 3).[49] The approximate minimum-energy conical intersection (MECI) optimization has been performed with the *ORCA* software (ver. 5.0.4)[50] at the TDDFT level [51]. In this case, the B3LYP functional was employed with the all-atom 6-31G basis set.[52]
## Results and discussion

**General structural remarks.** The examined Ni-1d' complex is a modification of the previously studied photoswitchable compounds.[6, 19] In this case, the aliphatic amine used in the synthesis exhibits a more basic character when compared to the respective substrates applied before (Supporting Information). Therefore, once reacted and coordinated, it should push more of electron density to the nickel center, thus weakening the bond between Ni and the ambidentate nitrite ligand. This may further contribute to a more efficient photoswitching. Such a manipulation should decrease the activation energy for the nitrite fragment isomerization, whereas the overall bulky (N,N,O)-donor ligand is expected to secure the space facilitating the transformation, eventually leading to smaller unit cell changes during the process. It has appeared that the chosen amine also influenced the typical reaction route (Supporting Information) and, in consequence, a new type of compound was obtained with the NO (N4=O3) fragment incorporated in the (N,N,O)-donor ligand (Figure 1a), located in the coordination sphere of the nickel atom. In turn, the carbonyl group (C5=O4) in the molecular structure is not coordinated to the metal center.
<figure><img src="image_3.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_6.png" /></figure>

**Figure 1.** (a) Molecular structure of the studied **Ni-1d′** compound derived from the X-ray diffraction data collected in complete darkness at 100 K. Atomic thermal motion is represented as ellipsoids at the 50% probability level. (b) 2-Dimensional difference electron density map of the ground state structure at 100 K. The presence of the *exo-* and *endo*-nitrito isomers is indicated by additional peaks in the vicinity of the nitro ligand. (c) Fingerprint plot generated for the studied crystal structure using the *CRYSTALEXPLORER* program.[53] (d) Reaction cavity calculated and generated with the *MERCURY* program (probe radius of 1.2 Å and grid spacing of 0.1 Å).[54]
<table><caption>**Table 1.** Selected crystal structure parameters of the studied complex 'dark' crystal structure at 100 K (2nd column), crystal structures after irradiation at 100 K (3rd column), at 150 K (4th column), and at 200 K (5th column) (for more details see the Supporting Information, Table S1).</caption><thead><tr><th>Data set<br>collection code</th><th>Ni-1dprim-<br>100K-dark-<br>xtal-2</th><th>Ni-1dprim-<br>100K-irr-1-<br>100K-xtal-3</th><th>Ni-1dprim-<br>150K-irr-4-<br>150K-xtal-3</th><th>Ni-1dprim-<br>200K-irr-1-<br>200K-xtal-4</th></tr></thead><tbody><tr><td>Moiety formula</td><td colspan="4">C<sub>13</sub>H<sub>16</sub>N<sub>4</sub>Ni<sub>1</sub>O<sub>4</sub></td></tr><tr><td>Moiety formula mass,<br>M<sub>r</sub> / a.u.</td><td colspan="4">350.98</td></tr><tr><td>Crystal system</td><td colspan="4">triclinic</td></tr><tr><td>Space group</td><td colspan="4">P<span style="text-decoration: overline">1</span> (No. 2)</td></tr><tr><td>Z</td><td colspan="4">2</td></tr><tr><td>F<sub>000</sub></td><td colspan="4">364</td></tr><tr><td>Crystal color &amp; shape</td><td colspan="4">red block</td></tr><tr><td>T / K</td><td>100</td><td>100</td><td>150</td><td>200</td></tr><tr><td>Irradiation time,<br>t<sub>irr</sub> / min</td><td>0 ('dark' set)</td><td>360</td><td>360</td><td>360 ¶</td></tr><tr><td>R<sub>mrg</sub> (or R<sub>int</sub>)</td><td>3.2%</td><td>7.4%</td><td>8.2%</td><td>3.9%</td></tr><tr><td>a / Å</td><td>7.619(2)</td><td>7.702(2)</td><td>7.698(2)</td><td>7.669(2)</td></tr><tr><td>b / Å</td><td>8.651(2)</td><td>8.456(2)</td><td>8.511(2)</td><td>8.625(2)</td></tr><tr><td>c / Å</td><td>11.176(2)</td><td>11.600(2)</td><td>11.589(2)</td><td>11.533(2)</td></tr><tr><td>α / °</td><td>78.93(3)</td><td>78.12(3)</td><td>78.14(3)</td><td>78.08(3)</td></tr><tr><td>β / °</td><td>82.58(3)</td><td>78.33(3)</td><td>78.49(3)</td><td>78.80(3)</td></tr><tr><td>γ / °</td><td>89.68(3)</td><td>90.62(3)</td><td>90.62(3)</td><td>90.48(3)</td></tr><tr><td>V / Å<sup>3</sup></td><td>716.7(3)</td><td>722.9(3)</td><td>727.1(3)</td><td>731.3(3)</td></tr><tr><td>d<sub>calc</sub> / g·cm<sup>-3</sup></td><td>1.6263</td><td>1.6125</td><td>1.6032</td><td>1.5939</td></tr><tr><td>R[F] (I > 3σ(I))</td><td>3.51%</td><td>5.90%</td><td>6.51%</td><td>4.90%</td></tr><tr><td>R[F] (all data)</td><td>4.48%</td><td>10.50%</td><td>11.40%</td><td>6.79%</td></tr><tr><td>Q<sup>min/max</sup><sub>res</sub> / e·Å<sup>-3</sup></td><td>-0.70 / +1.18</td><td>-0.69 / +0.69</td><td>-1.23 / +1.20</td><td>-0.92 / +0.83</td></tr><tr><td>CCDC code</td><td>2287080</td><td>2287094</td><td>2287102</td><td>2287062</td></tr></tbody></table>

¶ 360 min. of irradiation prior to X-ray crystallographic measurement and subsequent continuous irradiation during experiment (ca. 1.5 hour duration).

The **Ni-1d'** coordination compound crystallizes in the *P*<span style="text-decoration: overline">1</span> space group with one molecule in the asymmetric unit (Figure 1a). The nickel(II) center adopts a typical slightly distorted square-planar geometry which is imposed by the spatial arrangement of the nitrogen and oxygen atoms from the rather stiff (*N*,*N*,*O*)-donor ancillary ligand. The fourth coordination site is occupied by the ambidentate nitrite moiety. As expected, the nitro isomer is dominant in the 'dark' crystal structure of **Ni-1d'**, however, some traces of both, *endo*-nitrito and *exo*-nitrito forms (ca. 3.5–4.5% population at 100 K) were detected based on the residual electron density map (Figure 1b). To date, there are only four already
reported nickel(II) systems of this kind for which all the three isomers were observed experimentally[6, 10, 15, 22] and the recently reported palladium(II) complex.[27] Molecules in the crystal structure of **Ni-1d′** are relatively sparsely arranged. This is well-illustrated on the Hirshfeld fingerprint plot (Figure 1c) which shows that interatomic contacts in the crystal structure are rather distant with the lack of characteristic sharp spikes indicating strong and direct hydrogen bonds. Extensive free space available within the crystal structure is reflected in a large reaction cavity (ca. 43.7 Å³) when compared to other literature-reported photoswitchable compounds of this kind (Figure 1d).[10, 12, 14, 19]

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Main dimeric motifs involving the nitrite ligand interactions encountered for each isomeric form in the **Ni-1d'** crystal structure.</figcaption></figure>
<table><caption>**Table 2.** Major dimeric motifs and respective interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory based on the *CRYSTAL*-optimized crystal structures for each considered linkage isomer. Corrections for dispersion and BSSE were applied. **Mx** denotes a nitro motif, **Mx'** – *endo*-nitrito motif and **Mx''** – *exo*-nitrito motif (x – motif number).</caption><thead><tr><th>Motif</th><th>E<sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>D···A</sub> / Å</th><th>θ<sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td>M1 ¶</td><td>-22.9</td><td>O1···H8-C8#1</td><td>3.595</td><td>129.06</td></tr><tr><td>M1' ¶</td><td>-24.2</td><td>O1···H8-C8#1</td><td>3.627</td><td>126.70</td></tr><tr><td>M1" ¶</td><td>-25.0</td><td>O1···H8-C8#1</td><td>3.636</td><td>135.15</td></tr><tr><td>M2 ¶</td><td>-88.4</td><td>O1···H12a-C12#2<br>O3···H12c-C12#2</td><td>3.601<br>3.364</td><td>169.94<br>102.70</td></tr><tr><td>M2' ¶</td><td>-72.5</td><td>O1···H12a-C12#2<br>O3···H12c-C12#2</td><td>3.580<br>3.232</td><td>139.58<br>106.25</td></tr><tr><td>M2" ¶</td><td>-83.1</td><td>O1···H12a-C12#2<br>O3···H12c-C12#2</td><td>3.396<br>3.408</td><td>124.79<br>117.49</td></tr><tr><td>M3 ¶</td><td>-96.7</td><td>O2···H1b-C1#3<br>O2···Ni1#3<br>O3···H12a-C12#3<br>C12···H11-C11#3</td><td>3.468<br>2.947<br>3.566<br>3.705</td><td>124.04<br>-<br>158.06<br>161.58</td></tr><tr><td>M3' ¶</td><td>-86.3</td><td>O2···H1b-C1#3<br>O2···Ni1#3<br>O3···H12a-C12#3<br>C12···H11-C11#3</td><td>3.348<br>3.176<br>3.398<br>3.624</td><td>130.77<br>-<br>157.99<br>157.32</td></tr><tr><td>M3" ¶</td><td>-115.1</td><td>N1···H12a-C12#3§<br>O2···Ni1#3<br>O3···H12a-C12#3<br>C12···H11-C11#3</td><td>3.561<br>3.802<br>3.473<br>3.810</td><td>140.59<br>-<br>142.07<br>158.02</td></tr><tr><td>M4</td><td>-64.8</td><td>C9···H3-C3#4<br>C11···H2a-C2#4<br>O3···H1a-C1#4</td><td>3.348<br>3.810<br>3.257</td><td>117.52<br>146.25<br>130.19</td></tr><tr><td>M4'</td><td>-64.0</td><td>C11···H2a-C2#4<br>O3···H1a-C1#4</td><td>3.790<br>3.325</td><td>153.94<br>139.08</td></tr><tr><td>M4"</td><td>-60.5</td><td>C11···H2a-C2#4<br>O3···H1a-C1#4</td><td>3.784<br>3.611</td><td>161.45<br>135.68</td></tr><tr><td>M5</td><td>-50.1</td><td>O4···H3-C3#5<br>O4···C2#5</td><td>3.428<br>3.271</td><td>123.70<br>-</td></tr><tr><td>M5'</td><td>-48.6</td><td>O4···H2b-C2#5</td><td>3.307</td><td>114.25</td></tr><tr><td>M5"</td><td>-44.8</td><td>O4···H3-C3#5<br>O4···C2#5</td><td>3.801<br>3.305</td><td>118.03<br>-</td></tr><tr><td>M6" ¶</td><td>-9.1</td><td>O2···H10-C10#6</td><td>3.685</td><td>151.97</td></tr><tr><td>M7"</td><td>-29.4</td><td>C11(π)···C11(π)#7</td><td>3.277</td><td>-</td></tr><tr><td>M8"</td><td>-32.8</td><td>N4···H7-C7#8</td><td>3.417</td><td>121.22</td></tr></tbody></table>

<footnote>¶ Dimeric motif involving nitrate ligand. § Contact occurs only in the ground state crystal structure and *CRYSTAL*-optimized crystal structure due to the *exo*-nitrito moiety rotational mobility. Symmetry operations: (#1) x-1, y, z+1; (#2) -x+1, -y, -z+1; (#3) -x+1, -y+1, -z+1; (#4) x+1, y, z; (#5) -x+1, -y+2, -z; (#6) -x+1, -y, -z+1; (#7) -x, -y+1, -z; (#8) -x, -y, -z.</footnote>
Apart from the shape and reaction cavity volume, structural motifs involving the nitrite moiety need to be profoundly analyzed, as far as the isomerization reaction is concerned. Major motifs engaging the ambidentate ligand are shown in Figure 2, whereas the corresponding geometrical details are gathered in Table 2. Dimeric motifs present in the crystal structure at 100 K are stabilized mainly by interactions between ancillary ligands. There are only some weak hydrogen-bond-like contacts of the C-H···O type, which involve the ambidentate moiety (motifs **M1**, **M2** and **M3**). They can be seen in the fingerprint plot as small spikes on both sides of the pattern. Based on the results obtained for the optimized crystal structure of **Ni1d'** containing solely the nitro form (Table 2), only the **M1** motif is stabilized majorly by the interaction involving the nitro ligand, namely by the C8-H8···O1 contact. The resulting interaction energy, -22.9 kJ·mol⁻¹, indicates that it is a rather weak interaction. In turn, although motifs **M2** and **M3** constitute the best stabilized dimers in the examined crystal structure, both of them comprise several interactions apart from the nitrite ligand contacts, including the edge-to-face (Supporting Information) and C12-H12c···O3 contacts, which contribute to the total interaction energy of the components. Taking into consideration literature reports on photoswitchable nitrite compounds,[6, 12, 19, 21, 55] intermolecular interactions in the case of **Ni-1d'** are moderate, thus may to some extent affect the photoreaction, however, they should not hamper notably the isomerization of the nitrite ligand.

**Computational comparison of linkage isomers of Ni-1d' in the crystal structure.**

In order to perform structural and energetic comparative analyses for all examined linkage isomers, the three respective hypothetical crystal structures, composed of one linkage isomer each, were optimized with the *CRYSTAL* package. The reaction cavity volumes calculated based on the obtained crystal structures are shown in Figure 3, while numerical details regarding optimal unit cell parameters for each linkage isomer are given in Table 3.
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Visualization of reaction cavity volumes calculated for the *CRYSTAL*-optimized crystal structures for (a) nitro, (b) *endo*-nitrito and (c) *exo*-nitrito linkage isomers.</figcaption></figure>

<table><caption>**Table 3.** Unit-cell parameters and reaction-cavity volumes ($V_{cav}$) for the *CRYSTAL*-optimized crystal structures for each considered linkage isomer.</caption><thead><tr><th>Parameter</th><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>a / Å</td><td>7.612</td><td>7.664</td><td>7.912</td></tr><tr><td>b / Å</td><td>9.051</td><td>9.161</td><td>8.331</td></tr><tr><td>c / Å</td><td>10.908</td><td>10.954</td><td>11.755</td></tr><tr><td>α / °</td><td>78.78</td><td>77.90</td><td>78.94</td></tr><tr><td>β / °</td><td>82.05</td><td>79.66</td><td>75.05</td></tr><tr><td>γ / °</td><td>87.60</td><td>88.63</td><td>92.02</td></tr><tr><td>V / Å<sup>3</sup></td><td>730.0</td><td>739.8</td><td>731.9</td></tr><tr><td>V<sub>cav</sub> / Å<sup>3</sup></td><td>37.3</td><td>46.0</td><td>39.1</td></tr></tbody></table>

The nitro isomer, which is dominant in the ground state crystal structure, is characterized by the smallest unit cell, 730.0 Å³, whereas the *endo*-nitrito optimal unit cell is the most volumetric one. Nevertheless, as far as the isomerization process is concerned, the nitro-to-*endo*-nitrito isomerization would cause quite isotropic (a ↑ by 0.7%, b ↑ by 1.2%, c ↑ by 0.4%) expansion of the unit cell to 739.8 Å³ (volume increase by 1.3% when optimal unit cells are considered). In turn, the nitro-to-*exo*-nitrito reaction requires more significant unit cell deformation, despite rather marginal overall volume increase (0.2%). Optimal packing of *exo*-nitrito molecules imposes notable elongation of the c unit cell dimension (c ↑ by 7.8%) and contraction of the b edge (b ↓ by 8.6%), with less significant alteration along the X-axis (a ↑ by 3.9%). In the latter case, the oblong-shaped *exo*-nitrito moiety is oriented parallel to the c unit cell dimension, which results in the increase of this lattice parameter. The adjacent complex molecules are also slightly mutually shifted along the Z-axis when compared to the nitro form. It results in the shortening of the sum
of the two distinguished adjacent Ni···Ni distances, which translates into the notably
shortened *b* dimension. In the case of the *endo*-nitrito isomer, molecules are mutually
oriented very much alike these of the nitro isomer but the nitrite ligand forms a hook-like
shape pointing towards the other molecule in the unit cell, resulting in the longer Ni···Ni
distance. In general, deviations along the *Y*-axis are reflected in changes of the distance
between the metal centers.

Similar trend, as for the unit cell volumes, is observed for the size of reaction cavities.
Indeed, the optimal structure derived for the nitro isomer is characterized by the smallest
reaction cavity volume, 37.3 Å³. The cavity is slightly bigger for the *exo*-nitrito form and
its volume amounts to 39.1 Å³, while it is by far the largest for the *endo*-nitrito moiety –
46.0 Å³. As expected, the characteristics of both, the reaction cavity and the unit cell of
each isomeric form, are dictated by the shape of a given nitrite binding mode. The slender
shape of the *exo*-nitrito moiety and a slight offset between molecules in the unit cell
induces a quite oblong reaction cavity which is correlated with the above-discussed
elongation of the unit cell along the *Z*-axis and the shorter unit cell *b* dimension. The bulky
shape of the *endo*-nitrito isomer results in the largest reaction cavity and in the largest
unit cell volume. The increase of the unit-cell volume that accompanies the nitro-to-
nitrito isomerization is a common phenomenon (this is commensurate with less efficient
crystal packing of the photo-induced linkage isomer (PLI)).[³, ²¹]

It should also be noted that each isomer of the nitrite ligand forms similar types of
interactions, as depicted in Figure 2, usually with comparable stabilization energy (Table
2). Both nitro and *endo*-nitrito isomers form exactly the same kind of hydrogen-bond-like
contacts via oxygen atoms with aliphatic carbon atoms, namely C1 and C12, and aromatic
carbon C8. The nitro isomer is energetically favored in the case of **M2** and **M3**. Especially
notable difference, *ca.* 15 kJ·mol⁻¹, occurs for **M2** that includes the hydrogen-bond-like
interaction between the nitrite fragment and the methyl group. This stems from the more
beneficial angle between the engaged atoms in the nitro species. The significantly
stronger total stabilizing effect is observed in the case of the *exo*-nitrito isomer. It forms
four hydrogen bonds instead of three encountered for the remaining linkage isomers.
Similarly to both, nitro and *endo*-nitrito moieties, the *exo*-nitrito isomer interacts with the
aromatic C8 carbon and C12 methyl group protons. However, because of the slender
shape and orientation of the *exo*-nitrito group roughly along the [101] direction, the
contact with C1 is not formed. Instead, such a ligand arrangement facilitates the
N1···H12a–C12 hydrogen-bond-like interaction through the nitrogen atom (M3"). Furthermore, the *exo*-nitrito isomer forms the unique O2···H10–C10 contact thanks to the protruding O2 oxygen atom (M6"). In consequence, for the *exo*-nitrito isomer the intermolecular interaction energy sums up to –232.3 kJ·mol⁻¹ when solely the dimer motifs involving the key interactions with the nitrite ligand are considered. The summed-up interactions of this kind are notably weaker for the nitro moiety (–208.0 kJ·mol⁻¹), and even less beneficial in the case of the *endo*-nitrito binding mode (–183.0 kJ·mol⁻¹).

As far as the relative stability of the molecules of the considered linkage isomers is concerned, the optimized isolated molecules of the nitro and *endo*-nitrito forms are characterized by comparable energy values (Table 4), differing by no more than 3 kJ·mol⁻¹ in favor of the nitro moiety. At the same time, the *exo*-nitrito molecule is less stable than the parent nitro species by only 10 kJ·mol⁻¹. Including the effect of the crystal environment on molecular geometries, the difference between molecular energies of the nitro and *endo*-nitrito linkage isomers vanishes, whereas the nitro vs. *exo*-nitrito energy difference slightly increases (by about 1 kJ·mol⁻¹). The deformation energy between the optimal isolated molecules and these adopted to the crystal structure amounts to about 18–23 kJ·mol⁻¹ (Table 4).

<table><caption>**Table 4.** Energy differences ($E_{rel}$) between the ground (nitro) and metastable (*exo*- and *endo*-nitrito) linkage isomers computed for the optimized isolated-molecule geometries and for the molecular geometries from the *CRYSTAL*-optimized crystal structures. The deformation energy $E_{def}$ is the energy difference between that of the optimized isolated molecule and the molecule optimized in the crystal environment; in this case: $E_{def} = E_{CRYSTAL} - E_{isolated}$.</caption><thead><tr><th rowspan="2">Linkage isomer</th><th colspan="2">E<sub>rel</sub> / kJ·mol<sup>-1</sup></th><th rowspan="2">E<sub>def</sub> / kJ·mol<sup>-1</sup></th></tr><tr><th>crystal<br>environment</th><th>isolated<br>molecule</th></tr></thead><tbody><tr><td>nitro</td><td>0.0</td><td>0.0</td><td>22.7</td></tr><tr><td>exo-nitrito</td><td>15.7</td><td>9.6</td><td>19.8</td></tr><tr><td>endo-nitrito</td><td>15.1</td><td>2.6</td><td>18.4</td></tr></tbody></table>

Nevertheless, the most meaningful here is the overall energy of the unit cells formed by each linkage isomer. Comparison of the total energies of the optimal unit cells obtained for each considered linkage isomer indicates that the nitro isomer is the most
energetically stable form in the crystal structure at 0 K, whereas both *exo-* and *endo-*nitrito isomers are characterized by *ca.* 30 kJ·mol⁻¹ less advantageous unit cell energy values (which gives 15 kJ·mol⁻¹ per molecule). These outcomes show that the energy difference between the nitro form, dominant at ambient conditions, and the two nitrito isomers is relatively small. In the case of the *exo*-nitrito form, the less favorable molecular energy is compensated by intermolecular interactions in the crystal structure, which efficiently stabilize this linkage isomer. This is in contrast to the energetically more advantageous molecule of the *endo*-nitrito form, which is worst stabilized by intermolecular interactions in the crystal. The results support the presence of similar quantities of the nitrito isomers in the experimental crystal structure of **Ni-1d'** and allow the possibility of photoswitching behavior of the examined system. Additionally, such outcomes indicate that the population of the *exo*-nitrito isomer depends the most on the strength of intermolecular interactions, which may vary along with temperature changes. It should also be noted that the entropic factor becomes more significant at higher temperatures, which is neglected in these considerations.

**Multi-temperature X-ray diffraction studies.** The multi-temperature X-ray studies in the 100–290 K temperature range showed no signs of phase transitions in the crystal structure during heating or cooling. DSC experiments indicated that the sample is stable at least up to 500 K. Regarding populations of different linkage isomers, it appears that the total population of both nitrito forms slightly increases along with the elevated temperature, reaching *ca.* 18% at 290 K, which is up to 9% for each nitrito isomer, compared to 4–5% at 100 K (Supporting Information, Table S8). The slight drop of the nitrito population while cooling is recovered while heating. The relative population of both nitrito isomers undergoes only minor changes in the 100 K–RT range which are within the method's accuracy. The unit cell expands *ca.* isotopically along with temperature. Similarly, the reaction cavity volume of 43.7 Å³ at 100 K reaches 51.3 Å³ at 290 K.

Another aspect worth noting after the analysis of multi-temperature X-ray diffraction data is the orientation of the *exo*-nitrito fragment in the crystal structure. Based on the electron density map, it can be concluded that the *exo*-nitrito isomer has some freedom to rotate around the Ni–O1 bond. Its favorable position is different at 290 K and at 100 K (Supporting Information). The effect is thermally reversible. This phenomenon can be rationalized by the relatively low energy barrier between these two configurations which
is supported by theoretical rotational scans (Supporting Information). Similar calculations were performed for the *endo*-nitrito and nitro isomeric forms. The energy barriers between molecular configurations corresponding to local energy minima are, though, much higher than for the *exo*-nitrito isomer, which is reflected in the preserved conformations adopted by the nitro and *endo*-nitrito forms in the applied temperature range.

**Infrared spectroscopy.** In order to verify sample's behavior when exposed to light of various wavelengths at different temperatures, IR experiments were performed. Based on literature and theoretical infrared spectra calculated at the DFT(B3LYP)/6-311++G** level of theory for the optimized geometry of an isolated molecule, four bands relevant in view of linkage isomerism were identified, namely 1449, 1411, 1403, 1361, 1343, 1092 and 1045 cm⁻¹ (Supporting Information). The 1361 and 1343 cm⁻¹ bands were assigned to symmetric and asymmetric stretching vibration modes of NO₂, $v_s$ and $v_{as}$, respectively. The remaining four bands stand for stretching vibrations characteristic exclusively for the nitrito ONO linkage isomers. As far as computational results are concerned, it can be assumed that the well-separated band reflecting vibrations of the *exo*-nitrito form is located at slightly lower energy values (1045 cm⁻¹) than that of the *endo*-nitrito form (1092 cm⁻¹). Since the 1449 cm⁻¹ band behaves in the similar way as the 1045 cm⁻¹ one, it can also be attributed to the vibrations of the *exo*-nitrito isomer. In turn, the 1411 and 1403 cm⁻¹ bands show characteristics corresponding to the 1092 cm⁻¹ band, thus they can be assigned to the *endo*-nitrito form's vibrations. Such assumptions are supported by theoretical results. Overall, it is expected that upon nitro-to-nitrito isomerization the intensity of the 1361 and 1343 cm⁻¹ bands will decrease while that of the 1449, 1411, 1403, 1092 and 1045 cm⁻¹ bands will rise.

Based on experimental spectra, the highest nitro-to-nitrito isomerization conversion is achieved when the sample is irradiated with LEDs of 530, 470 and 405 nm central wavelengths at 100 K (Figure 4). The lowest-energy effective LED light, i.e. 530 nm, was chosen for further experiments as it should cause the least sample degradation. Using this LED, the maximum conversion is reached after 5 minutes of thin-film sample illumination in the 100–150 K temperature range. The highest temperature at which the isomerization reaction products can be induced and detected is 200 K (Figure 4). Again, the largest population is achieved after 5 minutes of the sample exposure to the LED light, however, the reaction efficiency is slightly worse in this case (Figure 5, Supporting Information).
The relative population of the photo-induced linkage isomers, *i.e.* the *endo*-nitrito and *exo*-nitrito forms, judged over the intensities of the respective bands denoted in Figures 4–6, is temperature-dependent. The maximum photo-generated population of the *exo*-nitrito isomer seems to be at the very similar level when the sample is irradiated in the 100–150 K temperature range, while it is slightly lower when the excitation takes place at 200 K (Figures 4–6). In contrast, the *endo*-nitrito isomer's achievable population seems to diminish along with the increased temperature at which the sample gets excited with light. Both PLIs are stable below 200 K. It should be noted that, after irradiation at 100 K, the generated population of the *endo*-nitrito form visibly decreases when the sample is heated to 160 K, and then to 200 K, which is not the case for the *exo*-nitrito form. The population of the latter seems to slightly increase at the 160–180 K range vs. that at 100 K, and then it drops a little when the temperature reaches 200 K. When the sample is exposed to the 530 nm LED light at 200 K, it relaxes completely to the initial state after 1.5 hours. The relaxation kinetics increases along with further temperature elevation. This means that the reaction is fully thermally reversible. It should also be noted that the population of the ONO linkage isomers slightly increases when the sample is heated from 240 K to RT (Supporting Information). The effect is marginal, but reproducible. This observation is also in accordance with the multi-temperature X-ray diffraction results.
<figure><img src="image_9.png" /><figcaption>**Figure 4.** IR spectra collected before (black line), after 5 minutes of irradiation with 530 nm LED (green line) at 100 K and during temperature relaxation: at 160 K (pink line), at 180 K (blue line), at 200 K (violet line), at 220 K (light blue line) and at 240 K (brown line).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 5.** IR spectra collected at 150 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (green line).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 6.** IR spectra collected at 200 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (green line), 5 minutes after irradiation (blue line), 10 minutes after irradiation (pink line), 15 minutes after irradiation (violet line), 30 minutes after irradiation (brown line), 1 hour after irradiation (yellow line), 1.5 hours after irradiation (light pink line).</figcaption></figure>

**Photocrystallography.** *Photocrystallography at 100 K:* The first photocrystallographic experiment was conducted at 100 K. The ground state structure contained 92% of the nitro isomer and about 3.5–4.5% of each nitrito form (Figures 7 & 8a). In order to observe a gradual build-up of the metastable state, the sample was irradiated with the 530 nm LED light in 20 minutes exposure steps. After 20 min. of sample illumination, a notable increase, up to 36%, of the *endo*-nitrito isomer’s population is observed (Figure 8a). In the case of the *exo*-nitrito form, it reaches 20%. In turn, the most prominent build-up of the *exo*-nitrito linkage isomer is observed after the second 20 min. irradiation period. The achieved population is estimated to 52%. At the same time, population of the nitro form decreases notably from 44% to 15%, whereas the *endo*-nitrito isomer’s population gets reduced by a few percent. At further stages of irradiation, only subtle changes in the isomeric form populations are observed. After 100 min. of irradiation, the *exo*-nitrito isomer was the dominant form in the crystal, reaching maximum yield of 66%, whereas the nitro isomer became the least populated form (9%).
<figure><img src="image_12.png" /><figcaption>**Figure 7.** (a) Molecular structure derived from the X-ray diffraction data collected at 100 K representing all the isomeric forms in the crystal: color-coding: orange – nitro, teal – *endo*-nitrito, purple – *exo*-nitrito. Atomic thermal motion is represented as ellipsoids at the 50% probability level. (b) 3-Dimensional photodifference map ($F^{ON} - F^{OFF}$) after 4 hours of sample irradiation with the 530 nm LED at 100 K (solid isosurfaces: ±6.275 e·Å⁻³, semi-transparent: ±4.275 e·Å⁻³, blue: positive values, red: negative).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 8.** Populations estimated for each isomer (orange dotted line – nitro, purple dotted-dashed – *exo*-nitrito isomer, teal dashed – *endo*-nitrito, colors and styles are retained for all panels) at different time points during crystal irradiation with 530 nm LED light (*a*,*b*), and along temperature relaxation (*c*,*d*). For more information see Supporting Information.</figcaption></figure>

In order to observe the impact of temperature on the stability of the metastable state, the system was slowly heated up. At 150 K, a rapid growth of the *exo*-nitrito population to 82% was observed. This change was accompanied by simultaneous decline in population of the *endo*-nitrito isomer, which dropped down to 12%. Population of the nitro form was reduced by 2.5%. Along with further heating at the 180–190 K temperature range, there was observed some minor relaxation of the sample. Based on the analysis of the mutual occupancy of the isomers, it can be concluded that, in the first steps, *exo*-nitrito isomer transforms partially both to the *endo*-nitrito and to the nitro form. At 200 K, the system is almost entirely relaxed and contains 78% of the nitro isomer. Nevertheless, under these conditions, there is still some significant fraction of the *endo*-nitrito isomer reaching *ca.* 13%. At 210 K, only traces of the nitrito forms are detected. At higher temperatures, the
*exo*-nitrito isomer's population and orientation agrees well with the results of the multi-temperature X-ray diffraction studies.

During the sample excitation, as well as along with its temperature relaxation, changes in the X-ray diffraction pattern can be observed between each pair of subsequent measurements in the series. The experiment at 100 K conducted in complete darkness, results in a pattern with well-defined Bragg peaks (Supporting Information, Figure S22). When the crystal is illuminated with the LED light, these peaks are smeared or even slightly separated. The effect gets stronger along with the irradiation time, which is clearly visible on the *0kl* and *h0l* reciprocal-space layers which correspond roughly to the *YZ* and *XZ* crystal planes. When the system reverts back to its initial state, the diffraction patterns return to their original forms, although with a worse merging (or 'internal') *R*-factor (*R*<sub>mrg</sub>) parameter[56] (3.3 vs. 12.5%) which suggests that the isomerization process, LED light, X-ray diffraction beam or temperature-induced strain, affected the crystal quality. Such behavior of the sample reflects the fact that, in the ideal case (one type of linkage isomer in the crystal structure), each isomeric form is characterized by different unit cell parameters. Thus, photoreaction taking place in the crystal introduces tensions and leads to deformation of some unit cells along with the rise of one isomer over another, which also contributes to lower quality of the respective diffraction pattern (smeared and fewer reflections, Supporting Information).

*Photocrystallography at 150 K:* To gain a deeper insight into the temperature effect on photoisomerization reaction, photocrystallographic experiments were also conducted at 150 and 200 K, the two key temperatures based on the results of the previous experiment (Figure 8b). As described above, the highest population of the *exo*-nitrito isomer was observed at 150 K, after previous crystal irradiation at 100 K. This might be related to its better stabilization in the crystal structure than that of the *endo*-nitrito analogue (intermolecular interactions), and also to the deformation of the majority of unit cells in a crystal lattice towards the shape adapted to the *exo*-nitrito isomer (which stimulates the further *exo*-nitrito isomer formation over the *endo*-nitrito analogue). Therefore, the examined system was irradiated with the 530 nm LED light with 1 hour steps at 150 K (Figure 8c). The measurement of the ground state structure indicated that the nitro isomer is the dominating form. Its population was estimated to 81%. The *endo*-nitrito and *exo*-nitrito isomer occupancies were determined to be about 12% and 7%, respectively. After the 1st hour of irradiation, population of the *exo*-nitrito form reached 69%, which is
12% more than in the case of the experiment conducted at 100 K after the same exposure time. In contrast, the overall yield of the nitrito isomers is lower. Along with further irradiation of the system (around 4 hours in total), the occupancy of the *exo*-nitrito isomer reaches 77%, which is mainly due to the reduction of the *endo*-nitrito isomer population. The final population of the latter form returns to that observed in the ground state. The remaining 11% is represented by nitro isomer. In order to compare the stability of the isomers to the previous series, the temperature was lowered to 100 K. As a result, the population of the *exo*-nitrito isomer grew by 3.5% which might be observed in the respective photodifference map (Supporting Information). Simultaneously, the unit cell elongates along the *Z* axis which is characteristic for the optimal unit cell shape determined for the *exo*-nitrito isomer. This supports the presumption that, once the *exo*-isomer starts to dominate over the *endo*-nitrito form in the crystal structure, unit cells undergo deformation to such an extent that it stimulates further increase of the *exo*-nitrito population upon irradiation. The PLI relaxation process observed along with the temperature increase is consistent with that reported above for the previous experiment series and summarized in Figure 8d.

*Photocrystallography at 200 K:* The analysis of the relaxation process in the previous experimental series allowed to estimate the temperature at which the system reverts back to its initial ('dark') state. Since the sharpest drop of the nitrito population occurs at 200 K, this temperature designates the so-called metastable limit.[8] Therefore, the sample behavior at 200 K has been investigated in more details. The system was irradiated with the 530 nm LED light until the highest possible nitrito form population was achieved. Then the X-ray data collection upon constant irradiation of the crystal was conducted. The measurement revealed that the *exo*-nitrito is still a dominating isomeric form (63%) in the crystal structure but its ratio to the *endo*-nitrito (22%) and nitro (15%) isomers' populations is not as high as it was at lower temperatures. The next stages of this experimental series involved monitoring of the relaxation process of the system until its complete return to the ground state. The X-ray data collection during the first 1.5 hour after the sample's exposure to light confirms rapid decline in the *exo*-nitrito population to 7% (population averaged over time of experiment). However, the decay of the *exo*-nitrito isomer fraction does not lead entirely to the nitro product, as the 10% increase of the *endo*-nitrito population was also observed. The subsequent crystallographic measurement (lasting another 1.5 hour) indicated that the sample returned completely
to the ground state in less than 3 hours. It should be noted that, according to the computational results (Supporting Information), the *endo*-nitrito form is stronger stabilized by intermolecular interactions at 200 K, than at 150 and 100 K, which may affect the relative stability of the *endo-* and *exo*-nitrito species.

**Activation energy estimation.** The activation energy of the nitrito-to-nitro transition in the examined linkage isomerization reaction was calculated using the DSC data. The ONO isomers were populated by irradiating the sample with the 530 nm LED light at 123 K (the lowest achievable temperature for DSC measurement) for 30 minutes and the DSC curve was recorded with 9 K·min⁻¹ heating rate in the 123–298 K range. The experimental curve was then fitted using a mathematical model that accounts for factors affecting the thermal behavior of the studied material:

$$ \frac{dH}{dt} = H_{tot}Z(1 - \alpha) \exp\left(-\frac{E_a}{RT}\right) \quad (1) $$

where $\frac{d}{dt}H$ is the heat flow rate, $H_{tot}$ is the total heat effect, $Z$ is the frequency factor, $\alpha$ is the degree of transformation, and $E_a$ is the activation energy. During the measurement, despite the co-existence of two nitrito isomers in a metastable state, only one exothermic effect was observed, which is related to reversion of the system back to the nitro isomer (Figure 9). This is probably related to the rapid transition between the *endo*-nitrito and *exo*-nitrito binding modes, and the further, more energetically demanding and slower decay of one of these forms to the nitro linkage isomer. Thus, the exothermic effects of reisomerization to the ground state coincide resulting in one exothermic signal.
<figure><img src="image_14.png" /><figcaption>**Figure 9.** Experimental DSC curve (black dots) and fitted heat flow rate model (red solid line).</figcaption></figure>

In order to properly interpret the kinetics of the reaction, it is crucial to provide both, the calculated activation energy and frequency factor.[57] These two parameters are related to each other[58] and allow to calculate the relaxation time of metastable form at every temperature point. The DSC-derived activation energy of the examined process amounts to 71.9(9) kJ·mol⁻¹, with the log Z value equal to 14(1). The respective temperature of decay (T<sub>dec</sub>)[59] corresponding to the rate constant k = 10⁻³ s⁻¹ is 209(3) K. The obtained kinetic parameters are consistent with the decay time of the photo-induced ONO isomers at 200 K estimated based on the observed IR spectral changes.

**Isomerization mechanism.** To elucidate a possible mechanism of the observed photoswitching, potential energy profiles (PEPs) in the ground (S₀) and in the first excited (S₁) electronic states were obtained for an isolated **Ni-1d'** molecule. Figure 10 shows the S₀ profiles between the nitro, *exo*-nitrito, and *endo*-nitrito forms, calculated along the O-N-Ni angle (nitro → *exo*-nitrito), and O-N-O-Ni dihedral angle (*exo*-nitrito → *endo*-nitrito) coordinates. It can be noticed that the barrier for the nitro → *exo*-nitrito transformation, estimated to ca. 81 kJ·mol⁻¹, significantly exceeds the *exo*-nitrito → *endo*-nitrito reaction barrier (21 kJ·mol⁻¹). This supports the high thermal stability of the nitro form, as well as stays in line with the relatively easy *exo*-nitrito ⇌ *endo*-nitrito interconversion. Furthermore, the theoretically-determined activation energy of the *exo*-nitrito → nitro transition (63 kJ·mol⁻¹) agrees reasonably well with the experimental estimation based on the DSC data.
<figure><img src="image_15.png" /><figcaption>**Figure 10.** Potential energy profiles calculated for the S₀ state along the O-N-Ni valence angle (nitro → *exo*-nitrito, left panel), and O-N-O-Ni dihedral angle (*exo*-nitrito → *endo*-nitrito, right panel), respectively. The green diamonds mark positions of the optimized nitro, *endo*-nitrito, and *exo*-nitrito forms.</figcaption></figure>

Following the S₀ scan, the possibility of the nitro → *exo*-nitrito transformation in the S₁ state was investigated. Firstly, an absorption spectrum of the nitro isomer was determined at the TDDFT/TDA level, with the vibronic effects accounted for under the Wigner distribution (Figure 11). In good agreement with the experiment (Supporting Information), the nitro form shows a broad absorption peak in the visible range, with maximum around 535 nm. This peak corresponds mostly to the S₀ → S₁ transition, with smaller contributions from higher excitations (S₀ → S₂⁻₄) gradually taking over for wavelengths below 500 nm. The leading S₀ → S₁ excitation can be well described as a d<sub>z²</sub> → d<sub>x²-y²</sub> transition, as stems from the shape of the corresponding NTOs (inset to Figure 11). In result, the excitation-induced electronic density change around the Ni atom pushes away all the in-plain bonded atoms and, at the same time, creates positively charged ‘hole’ over and under the Ni atom, leading to attraction of the neighboring electron-rich oxygen atoms of the nitrite ligand.
<figure><img src="image_16.png" /><figcaption>**Figure 11.** UV-Vis absorption spectrum of the nitro isomer calculated at the TDDFT/TDA level under the Wigner energy distribution at 0 K. The full black curve shows the total predicted absorption, while dotted black/red/blue/green lines show individual contributions from transitions to S<sub>1</sub>–S<sub>4</sub> states, respectively. The vertical lines mark transition energies calculated at the same level of theory at the optimized nitro structure. The inset shows leading NTOs obtained for the S<sub>0</sub> → S<sub>1</sub> transition.</figcaption></figure>

This expected excited-state relaxation finds further support upon inspection of the S<sub>1</sub>
state PEP shown in Figure 12 (for the corresponding NTOs calculated along the path,
please check Figure S24 in the Supporting Information). After the excitation moves the
system to a manifold of closely lying singlet states, the S<sub>1</sub> state is expected to undergo
rapid vibrational cooling (S<sub>1</sub>(FC) → S<sub>1</sub>(rel) transition in Figure 12), followed by
effectively barrierless up-rotation of the ONO moiety, during which the O–N–Ni angle is
also gradually reduced. This shift and rotation of the ONO group drives the molecule to a
conical intersection, at which population bifurcation between the nitro and *exo*-nitrito
forms is expected. In this scenario, the photo-produced *exo*-nitrito isomer can
immediately lead to the *endo*-nitrito form population rise, due to large excess of the
kinetic energy in the system after the S<sub>1</sub> → S<sub>0</sub> deactivation. At the same time, we note that
no direct photoreaction path between the nitro and *endo*-nitrito isomers was established
in the course of the present study (see also Figure S26 in the Supporting Information).
<figure><img src="image_17.png" /><figcaption>**Figure 12.** Potential energy profiles along the O-N-Ni reaction coordinate optimized for the S₀ (black) and S₁ (blue) states, respectively. The optimized state in each set has been marked with circles, while the other states, calculated vertically on the S₀/S₁ geometries, are marked with dashed/solid lines, respectively. The green diamonds mark positions of: vertically excited S₁ state at the nitro Franck-Condon region (S₁(FC)), relaxed S₁-nitro geometry (S₁(rel)), the minimum-energy conical intersection (MECI) point, and the S₀ energies of the optimized *exo*-nitrito and nitro forms. The inset shows optimized geometry of the MECI structure.</figcaption></figure>

**Conclusions**

The synthesized efficient nitrite nickel(II) photoswitch, **Ni-1d'**, appeared to be a good model system to investigate the isomerization reaction mechanism in planar quadratic nickel(II) complexes. This is because it can exist in the form of three linkage isomers, *i.e.* nitro, *exo*- and *endo*-nitrito binding modes. All of them are experimentally observed in the ground-state crystal structure, in which the nitro isomer is the dominant form (from *ca.* 90 to 80% population at 100 K and RT, respectively). In turn, significant populations of the nitrito isomers (exceeding 90% in total) can be optically generated using the 405-530 LED light. This is rather unique since, up to now, all the three linkage isomers were
observed only for a few other reported systems of this kind, among which there are only
two cases of the square-planar nickel(II) complexes,[⁶,²²] in which the *exo*-nitrito form was
only observed after sample irradiation. Based on the performed photocrystallographic
experiments, the relative population of the photo-generated nitrito isomers depends on
temperature, however, the *exo*-nitrito isomer is in the end the preferred photoproduct.
The reaction is fully reversible. The photo-induced species are stable up to 200 K, which
is supported both by spectroscopic and photocrystallographic results. According to the
DSC experiments, the activation energy was estimated to 71.9(9) kJ·mol⁻¹. This is
concurrent with the rate of the signal decay in the respective IR spectra, and also with the
computational evaluation of the *exo*-nitrito → nitro transformation energy barrier (~63
kJ·mol⁻¹).

Regarding the isomerization mechanism, experimental data and computational
modelling were combined. Based on photocrystallographic outcomes, the *endo*-nitrito
isomer's population is higher than that of the *exo*-nitrito form during the first stage of
irradiation at 100 K. Then, the *endo*-nitrito isomer's population decreases along with
further sample exposure to light. Whereas, after sample exposure to light, followed by
gradual heating, the *endo*-nitrito isomer's population rises only around the metastable
limit (as defined by Hatcher *et al.*[⁸]). These could be signs of some direct photoreaction
pathway linking the nitro and *endo*-nitrito forms, which was not established
computationally and which might result from some crystal packing effects not taken into
account in the computations. On the other hand, such observations may solely indicate a
rather easy interconversion of the nitrito moieties (energy barrier amounts to about 21
kJ·mol⁻¹ for isolated molecules). Thus, in the beginning of irradiation, *exo*-nitrito species
convert rather efficiently to the *endo*-nitrito ones (which to some extent might be
explained by different absorption of the 530 nm LED light by the considered isomers,
Supporting Information), whereas, once the *exo*-nitrito population achieves a certain
level, this isomer takes over which results in its high population in the photostationary
point. The distortion of the unit cell may increase the *exo*-nitrito stability over the bulkier
*endo*-nitrito isomer, especially at low temperatures, as confirmed computationally. Once
the system is heated towards 200 K, the above-described factors (such as unit cell
expansion, weaker intermolecular interactions etc.) may lead to partial relaxation of the
least advantageous *exo*-nitrito isomer towards both *endo*-nitrito and nitro species. Upon
further heating, the energy barrier to form the nitro species can be easily overcome and
the systems relaxes fully through the accessible pathways until it achieves the thermodynamic equilibrium.

In view of the above, our contribution provides stronger support for the nitro → *exo*-nitrito → *endo*-nitrito reaction pathway, as suggested earlier by Warren *et al.*[22] for a different square-planar nitrite nickel(II) complex, which is in contrast to the results obtained for a triplet octahedral nitrite Ni¹¹ compound[26] (or singlet octahedral Co¹¹ photoswitchable complexes[25]). This could be rationalized by the photo-induced electronic density redistribution around the Ni atom which pushes away all the in-plain bonded atoms, and simultaneously creates a positively charged ‘cavities’ over and under the Ni atom, leading to attraction of electron-rich oxygens of the ONO group. In contrast, this space below and above the square core of the complex is not available in the case of octahedral species. It should be stressed again, though, that the crystal environment also affects the reaction mechanism, which was not addressed here. Thus, in order to fully confirm the results of the simplified theoretical considerations, transient-absorption, or time-resolved photocrystallographic measurements can be performed. The former is not expected to give an answer in the current case, as the UV-Vis absorption of all the isomeric forms is very similar. In turn, the latter method may give information of the initial and/or transient (semi)-products of the reaction. Such studies will be undertaken in the nearest future.

## Acknowledgements

K.A.D. and K.N.J. thank both the Polish National Science Centre (grant No. 2019/35/O/ST4/04197, PRELUDIUM BIS program) and Polish National Agency for Academic Exchange (grant No. PPN/STA/2021/1/00050/U/00001) for financial support. Furthermore, financial support by ANR (grant No. ANR-21-CE30-0045-01) is gratefully acknowledged. The Wrocław Centre for Networking and Supercomputing (grant No. 285) and Poland's high-performance computing infrastructure PLGrid (ACK Cyfronet AGH HPC Center, grant No. PLG/2023/016166) are gratefully acknowledged for providing computational facilities. The in-house X-ray diffraction experiments were carried out at the Department of Physics, University of Warsaw, on Rigaku Oxford Diffraction SuperNova diffractometer, which was co-financed by the European Union within the European Regional Development Fund (POIG.02.01.00-14.122/09). B. Civalleri and L. Doná (Torino, Italy) are thanked for their advice regarding computations in *CRYSTAL* and providing the solid-state-optimized functional and basis set which were used.
**Conflict of interests**

The authors declare no conflict of interests.

**Data availability statement**

The data that support the findings of this study are available in the Supporting Information of this article.

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For Table of Contents

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

An efficient square-planar nitrite nickel(II) photoswitch is reported and used as a model case to study the nitro-to-nitrito isomerization mechanism in the solid state. Based on combined experimental and theoretical studies the nitro → *exo*-nitrito → *endo*-nitrito reaction pathway is proposed.

**Twitter / X handles:** @rkaminski85, @UniWarszawski, @ChemistryUW
SUPPORTING INFORMATION

Interrelations between linkage isomers of an efficient square-planar
nickel(II) nitrite photoswitch in the solid state

Kinga Potempa,<sup>a,#</sup> Krystyna A. Deresz,<sup>a,#</sup> Joanna Jankowska,<sup>a</sup> Katarzyna N. Jarzembska,<sup>a*</sup>
Adam Krówczyński,<sup>a</sup> Artem Mikhailov,<sup>b</sup> Dominik Schaniel,<sup>b</sup> Radosław Kamiński<sup>a</sup>

<sup>a</sup> Department of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland

<sup>b</sup> Université de Lorraine, CNRS, CRM<sup>2</sup>, 54500 Nancy, France

* Corresponding author: Katarzyna N. Jarzembska (katarzyna.jarzembska@uw.edu.pl)

# Both authors contributed equally
<figure><img src="image_19.png" /><figcaption>**Scheme S1.** Reaction scheme of the obtained **Ni-1d'** compound.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure S1.** ¹H NMR spectrum of **Ni-1d'** compound (400 MHz, DMSO-d₆).</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S2.** ¹H NMR spectrum of **Ni-1d'** compound (400 MHz, CDCl₃).</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Figure S3.** ¹H NMR spectrum of **Ni-1d'** compound (400 MHz, acetone-d₆).</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure S4.** Solid-state UV-Vis absorption spectrum of Ni1-d'.</figcaption></figure>
<table><caption>**Table S1.** Summary of all collected X-ray diffraction datasets.</caption><thead><tr><th>Crystal No.</th><th>Data collection code</th><th>CCDC No.</th><th>Temp., T / K</th><th>Remarks</th></tr></thead><tbody><tr><td rowspan="20">1<sup>st</sup></td><td>Ni-1dprim-290K-dark-cooling-xtal-1</td><td>2287078</td><td>290</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-250K-dark-cooling-xtal-1</td><td>2287076</td><td>250</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-220K-dark-cooling-xtal-1</td><td>2287074</td><td>220</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-210K-dark-cooling-xtal-1</td><td>2287072</td><td>210</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-200K-dark-cooling-xtal-1</td><td>2287070</td><td>200</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-190K-dark-cooling-xtal-1</td><td>2287068</td><td>190</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-180K-dark-cooling-xtal-1</td><td>2287066</td><td>180</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-150K-dark-cooling-xtal-1</td><td>2287064</td><td>180</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-100K-dark-cooling-xtal-1</td><td>2287063</td><td>150</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-150K-dark-heating-xtal-1</td><td>2287065</td><td>100</td><td>cooling cycle</td></tr><tr><td>Ni-1dprim-180K-dark-heating-xtal-1</td><td>2287067</td><td>150</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-190K-dark-heating-xtal-1</td><td>2287069</td><td>180</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-200K-dark-heating-xtal-1</td><td>2287071</td><td>190</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-210K-dark-heating-xtal-1</td><td>2287073</td><td>200</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-220K-dark-heating-xtal-1</td><td>2287075</td><td>220</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-250K-dark-heating-xtal-1</td><td>2287077</td><td>250</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-290K-dark-heating-xtal-1</td><td>2287079</td><td>290</td><td>heating cycle</td></tr><tr><td rowspan="10">2<sup>nd</sup></td><td>Ni-1dprim-100K-dark-xtal-2</td><td>2287080</td><td>100</td><td>dark data collection</td></tr><tr><td>Ni-1dprim-100K-irr-1-100K-xtal-2</td><td>2287081</td><td>100</td><td>data collection after 1<sup>st</sup> 20 min. of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-100K-irr-2-100K-xtal-2</td><td>2287082</td><td>100</td><td>data collection after 2<sup>nd</sup> 20 min. of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-100K-irr-3-100K-xtal-2</td><td>2287083</td><td>100</td><td>data collection after 3<sup>rd</sup> 20 min. of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-100K-irr-4-100K-xtal-2</td><td>2287084</td><td>100</td><td>data collection after 4<sup>th</sup> 20 min. of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-100K-irr-5-100K-xtal-2</td><td>2287085</td><td>100</td><td>data collection after 5<sup>th</sup> 20 min. of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-150K-irr-5-100K-xtal-2</td><td>2287086</td><td>150</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-180K-irr-5-100K-xtal-2</td><td>2287087</td><td>180</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-190K-irr-5-100K-xtal-2</td><td>2287088</td><td>190</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-200K-irr-5-100K-xtal-2</td><td>2287089</td><td>200</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-210K-irr-5-100K-xtal-2</td><td>2287090</td><td>210</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-220K-irr-5-100K-xtal-2</td><td>2287091</td><td>220</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-250K-irr-5-100K-xtal-2</td><td>2287092</td><td>250</td><td>heating cycle</td></tr></tbody></table>
<table><caption>**Table S1 (continued).** Summary of all collected X-ray diffraction datasets.</caption><thead><tr><th>Crystal No.</th><th>Data collection code</th><th>CCDC No.</th><th>Temp., T / K</th><th>Remarks</th></tr></thead><tbody><tr><td rowspan="12">3<sup>rd</sup></td><td>Ni-1dprim-150K-dark-1-xtal-3</td><td>2287096</td><td>150</td><td>dark data collection</td></tr><tr><td>Ni-1dprim-150K-irr-1-150K-xtal-3</td><td>2287098</td><td>150</td><td>data collection after 1<sup>st</sup> 1 hour of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-150K-irr-2-150K-xtal-3</td><td>2287100</td><td>150</td><td>data collection after 2<sup>nd</sup> 1 hour of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-150K-irr-3-150K-xtal-3</td><td>2287101</td><td>150</td><td>data collection after 3<sup>rd</sup> 1 hour of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-150K-irr-4-150K-xtal-3</td><td>2287102</td><td>150</td><td>data collection after 4<sup>th</sup> 1 hour of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-100K-irr-4-150K-xtal-3</td><td>2287095</td><td>100</td><td>cooling</td></tr><tr><td>Ni-1dprim-100K-dark-1-xtal-3</td><td>2287093</td><td>100</td><td>dark data collection</td></tr><tr><td>Ni-1dprim-100K-irr-1-100K-xtal-3</td><td>2287094</td><td>100</td><td>data collection after 4 hours of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-150K-dark-2-xtal-3</td><td>2287097</td><td>150</td><td>dark data collection</td></tr><tr><td>Ni-1dprim-150K-irr-1a-150K-xtal-3</td><td>2287099</td><td>150</td><td>data collection after 4 hours of irradiation of the rotating crystal with the 530 nm LED</td></tr><tr><td>Ni-1dprim-160K-irr-1-150K-xtal-3</td><td>2287103</td><td>160</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-180K-irr-1-150K-xtal-3</td><td>2287053</td><td>180</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-190K-irr-1-150K-xtal-3</td><td>2287054</td><td>190</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-200K-irr-1-150K-xtal-3</td><td>2287055</td><td>200</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-210K-irr-1-150K-xtal-3</td><td>2287056</td><td>210</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-220K-irr-1-150K-xtal-3</td><td>2287057</td><td>220</td><td>heating cycle</td></tr><tr><td>Ni-1dprim-250K-irr-1-150K-xtal-3</td><td>2287058</td><td>250</td><td>heating cycle</td></tr><tr><td rowspan="5">4<sup>th</sup></td><td>Ni-1dprim-200K-dark-xtal-4</td><td>2287059</td><td>200</td><td>dark data collection at the metastable limit</td></tr><tr><td>Ni-1dprim-200K-irr-1-200K-xtal-4</td><td>2287062</td><td>200</td><td>data collection during constant irradiation (ca. 1.5 hour) with the 530 nm LED before the measurement this crystal was also irradiated for 4 hours with the 530 nm LED while being rotated</td></tr><tr><td>Ni-1dprim-200K-irr-1-200K-relax-1-xtal-4</td><td>2287060</td><td>200</td><td>data collection at the metastable limit (relaxation process)</td></tr><tr><td>Ni-1dprim-200K-irr-1-200K-relax-2-xtal-4</td><td>2287061</td><td>200</td><td>data collection at the metastable limit (relaxation process)</td></tr></tbody></table>
<table><caption>**Table S2.** Major dimeric motifs and respective interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory based on the crystal structure at 100 K for each considered linkage isomer. Corrections for dispersion and BSSE were applied. **Mx** denotes a nitro motif, **Mx'** – endo-nitrito motif and **Mx''** – exo-nitrito motif (x – motif number).</caption><thead><tr><th>Motif</th><th>E<sub>int</sub> /<br>kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>D···A</sub> / Å</th><th>θ<sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td>M1 ¶</td><td>-20.6</td><td>O1···H8-C8<sup>#1</sup></td><td>3.397</td><td>139.20</td></tr><tr><td>M1' ¶</td><td>-21.6</td><td>O1···H8-C8<sup>#1</sup></td><td>3.605</td><td>140.20</td></tr><tr><td>M1" ¶</td><td>-23.6</td><td>O1···H8-C8<sup>#1</sup></td><td>3.748</td><td>136.44</td></tr><tr><td>M2 ¶</td><td>-95.1</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.451<br>3.184</td><td>160.85<br>99.56</td></tr><tr><td>M2' ¶</td><td>-81.0</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.285<br>2.977</td><td>136.39<br>109.69</td></tr><tr><td>M2" ¶</td><td>-91.6</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.423<br>3.195</td><td>136.03<br>109.69</td></tr><tr><td>M3 ¶</td><td>-107.5</td><td>O2···H1b-C1<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.303<br>2.985<br>3.357<br>3.740</td><td>128.14<br>-<br>148.92<br>166.23</td></tr><tr><td>M3' ¶</td><td>-105.8</td><td>O2···H1b-C1<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.167<br>2.977<br>3.398<br>3.778</td><td>127.88<br>-<br>147.46<br>165.01</td></tr><tr><td>M3" ¶</td><td>-108.2</td><td>N1···H12a-C12<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.581<br>3.688<br>3.471<br>3.778</td><td>148.09<br>-<br>147.46<br>165.01</td></tr><tr><td>M4</td><td>-63.6</td><td>C9···H3-C3<sup>#4</sup><br>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup><br>N4···H1a-C1<sup>#4</sup></td><td>3.476<br>3.702<br>3.195<br>3.221</td><td>125.57<br>148.24<br>137.51<br>113.69</td></tr><tr><td>M4'</td><td>-64.0</td><td>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup></td><td>3.731<br>3.332</td><td>158.73<br>139.02</td></tr><tr><td>M4"</td><td>-60.5</td><td>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup></td><td>3.731<br>3.332</td><td>158.73<br>139.02</td></tr><tr><td>M5</td><td>-47.7</td><td>O4···H3-C3<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.357<br>3.213</td><td>125.31<br>-</td></tr><tr><td>M5'</td><td>-47.7</td><td>O4···H2b-C2<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.493<br>3.216</td><td>119.82<br>-</td></tr><tr><td>M5"</td><td>-48.0</td><td>O4···H3-C3<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.493<br>3.216</td><td>119.82<br>-</td></tr><tr><td>M6" ¶</td><td>-14.0</td><td>O2···H10-C10<sup>#6</sup></td><td>3.253</td><td>153.49</td></tr><tr><td>M7"</td><td>-26.2</td><td>C11(π)···C11(π)<sup>#7</sup></td><td>3.379</td><td>-</td></tr><tr><td>M8"</td><td>-29.2</td><td>N4···H7-C7<sup>#8</sup></td><td>3.448</td><td>129.28</td></tr></tbody></table>

<footnote>¶ Dimeric motif involving nitrate ligand. Symmetry operations: (#1) x-1, y, z+1; (#2) -x+1, -y, -z+1; (#3) -x+1, -y+1, -z+1; (#4) x+1, y, z; (#5) -x+1, -y+2, -z; (#6) -x+1, -y, -z+1; (#7) -x, -y+1, -z; (#8) -x, -y, -z.</footnote>
<table><caption>**Table S3.** Major dimeric motifs and respective interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory based on the *CRYSTAL*-optimized crystal structures for each considered linkage isomer. Corrections for dispersion and BSSE were applied. **Mx** denotes a nitro motif, **Mx'** – endo-nitrito motif and **Mx''** – exo-nitrito motif (x – motif number).</caption><thead><tr><th>Motif</th><th>E<sub>int</sub> /<br>kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>D···A</sub> / Å</th><th>θ<sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td>M1 ¶</td><td>-22.9</td><td>O1···H8-C8<sup>#1</sup></td><td>3.595</td><td>129.06</td></tr><tr><td>M1' ¶</td><td>-24.2</td><td>O1···H8-C8<sup>#1</sup></td><td>3.627</td><td>126.70</td></tr><tr><td>M1" ¶</td><td>-25.0</td><td>O1···H8-C8<sup>#1</sup></td><td>3.636</td><td>135.15</td></tr><tr><td rowspan="2">M2 ¶</td><td rowspan="2">-88.4</td><td>O1···H12a-C12<sup>#2</sup></td><td>3.601</td><td>169.94</td></tr><tr><td>O3···H12c-C12<sup>#2</sup></td><td>3.364</td><td>102.70</td></tr><tr><td rowspan="2">M2' ¶</td><td rowspan="2">-72.5</td><td>O1···H12a-C12<sup>#2</sup></td><td>3.580</td><td>139.58</td></tr><tr><td>O3···H12c-C12<sup>#2</sup></td><td>3.232</td><td>106.25</td></tr><tr><td rowspan="2">M2" ¶</td><td rowspan="2">-83.1</td><td>O1···H12a-C12<sup>#2</sup></td><td>3.396</td><td>124.79</td></tr><tr><td>O3···H12c-C12<sup>#2</sup></td><td>3.408</td><td>117.49</td></tr><tr><td rowspan="4">M3 ¶</td><td rowspan="4">-96.7</td><td>O2···H1b-C1<sup>#3</sup></td><td>3.468</td><td>124.04</td></tr><tr><td>O2···Ni1<sup>#3</sup></td><td>2.947</td><td>-</td></tr><tr><td>O3···H12a-C12<sup>3</sup></td><td>3.566</td><td>158.06</td></tr><tr><td>C12···H11-C11<sup>#3</sup></td><td>3.705</td><td>161.58</td></tr><tr><td rowspan="4">M3' ¶</td><td rowspan="4">-86.3</td><td>O2···H1b-C1<sup>#3</sup></td><td>3.348</td><td>130.77</td></tr><tr><td>O2···Ni1<sup>#3</sup></td><td>3.176</td><td>-</td></tr><tr><td>O3···H12a-C12<sup>#3</sup></td><td>3.398</td><td>157.99</td></tr><tr><td>C12···H11-C11<sup>#3</sup></td><td>3.624</td><td>157.32</td></tr><tr><td rowspan="4">M3" ¶</td><td rowspan="4">-115.1</td><td>N1···H12a-C12<sup>#3</sup></td><td>3.561</td><td>140.59</td></tr><tr><td>O2···Ni1<sup>#3</sup></td><td>3.802</td><td>-</td></tr><tr><td>O3···H12a-C12<sup>#3</sup></td><td>3.473</td><td>142.07</td></tr><tr><td>C12···H11-C11<sup>#3</sup></td><td>3.810</td><td>158.02</td></tr><tr><td rowspan="3">M4</td><td rowspan="3">-64.8</td><td>C9···H3-C3<sup>#4</sup></td><td>3.348</td><td>117.52</td></tr><tr><td>C11···H2a-C2<sup>#4</sup></td><td>3.810</td><td>146.25</td></tr><tr><td>O3···H1a-C1<sup>#4</sup></td><td>3.257</td><td>130.19</td></tr><tr><td rowspan="2">M4'</td><td rowspan="2">-64.0</td><td>C11···H2a-C2<sup>#4</sup></td><td>3.790</td><td>153.94</td></tr><tr><td>O3···H1a-C1<sup>#4</sup></td><td>3.325</td><td>139.08</td></tr><tr><td rowspan="2">M4"</td><td rowspan="2">-60.5</td><td>C11···H2a-C2<sup>#4</sup></td><td>3.784</td><td>161.45</td></tr><tr><td>O3···H1a-C1<sup>#4</sup></td><td>3.611</td><td>135.68</td></tr><tr><td rowspan="2">M5</td><td rowspan="2">-50.1</td><td>O4···H3-C3<sup>#5</sup></td><td>3.428</td><td>123.70</td></tr><tr><td>O4···C2<sup>#5</sup></td><td>3.271</td><td>-</td></tr><tr><td>M5'</td><td>-48.6</td><td>O4···H3-C3<sup>#5</sup></td><td>3.307</td><td>114.25</td></tr><tr><td rowspan="2">M5"</td><td rowspan="2">-44.8</td><td>O4···H3-C3<sup>#5</sup></td><td>3.801</td><td>118.03</td></tr><tr><td>O4···C2<sup>#5</sup></td><td>3.305</td><td>-</td></tr><tr><td>M6" ¶</td><td>-9.1</td><td>O2···H10-C10<sup>#6</sup></td><td>3.685</td><td>151.97</td></tr><tr><td>M7"</td><td>-29.4</td><td>C11(π)···C11(π)<sup>#7</sup></td><td>3.277</td><td>-</td></tr><tr><td>M8"</td><td>-32.8</td><td>N4···H7-C7<sup>#8</sup></td><td>3.417</td><td>121.22</td></tr></tbody></table>

¶ Dimeric motif involving nitrate ligand. Symmetry operations: (#1) x-1, y, z+1; (#2) -x+1, -y, -z+1; (#3) -x+1, -y+1, -z+1; (#4) x+1, y, z; (#5) -x+1, -y+2, -z; (#6) -x+1, -y, -z+1; (#7) -x, -y+1, -z; (#8) -x, -y, -z.
<table><caption>**Table S4.** Major dimeric motifs and respective interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory based on the crystal structure at 150 K for each considered linkage isomer. Corrections for dispersion and BSSE were applied. **Mx** denotes a nitro motif, **Mx'** – endo-nitrito motif and **Mx''** – exo-nitrito motif (x – motif number).</caption><thead><tr><th>Motif</th><th>E<sub>int</sub> /<br>kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>D···A</sub> / Å</th><th>θ<sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td>M1 ¶</td><td>-20.9</td><td>O1···H8-C8<sup>#1</sup></td><td>3.297</td><td>139.86</td></tr><tr><td>M1' ¶</td><td>-19.5</td><td>O1···H8-C8<sup>#1</sup></td><td>3.716</td><td>142.05</td></tr><tr><td>M1" ¶</td><td>-22.5</td><td>O1···H8-C8<sup>#1</sup></td><td>3.748</td><td>138.47</td></tr><tr><td>M2 ¶</td><td>-95.4</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.457<br>3.213</td><td>159.13<br>100.71</td></tr><tr><td>M2' ¶</td><td>-80.9</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.580<br>3.232</td><td>139.58<br>106.09</td></tr><tr><td>M2" ¶</td><td>-91.7</td><td>O1···H12a-C12<sup>#2</sup><br>O3···H12c-C12<sup>#2</sup></td><td>3.382<br>3.235</td><td>129.90<br>112.41</td></tr><tr><td>M3 ¶</td><td>-104.9</td><td>O2···H1b-C1<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.350<br>2.035<br>3.391<br>3.747</td><td>128.33<br>-<br>149.53<br>165.95</td></tr><tr><td>M3' ¶</td><td>-96.4</td><td>O2···H1b-C1<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.119<br>2.880<br>3.411<br>3.818</td><td>124.81<br>-<br>143.59<br>165.26</td></tr><tr><td>M3" ¶</td><td>-104.7</td><td>N1···H12a-C12<sup>#3</sup><br>O2···Ni1<sup>#3</sup><br>O3···H12a-C12<sup>#3</sup><br>C12···H11-C11<sup>#3</sup></td><td>3.598<br>3.695<br>3.407<br>3.831</td><td>151.60<br>-<br>142.80<br>165.62</td></tr><tr><td>M4</td><td>-63.4</td><td>C9···H3-C3<sup>#4</sup><br>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup><br>N4···H1a-C1<sup>#4</sup></td><td>3.348<br>3.810<br>3.257<br>3.246</td><td>117.52<br>146.25<br>130.19<br>113.54</td></tr><tr><td>M4'</td><td>-60.6</td><td>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup></td><td>3.728<br>3.326</td><td>160.16<br>138.94</td></tr><tr><td>M4"</td><td>-65.7</td><td>C11···H2a-C2<sup>#4</sup><br>O3···H1a-C1<sup>#4</sup></td><td>3.757<br>3.331</td><td>158.91<br>137.73</td></tr><tr><td>M5</td><td>-46.9</td><td>O4···H3-C3<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.380<br>3.234</td><td>125.16<br>-</td></tr><tr><td>M5'</td><td>-43.4</td><td>O4···H3-C3<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.462<br>3.219</td><td>121.67<br>-</td></tr><tr><td>M5"</td><td>-45.4</td><td>O4···H3-C3<sup>#5</sup><br>O4···C2<sup>#5</sup></td><td>3.479<br>3.219</td><td>123.03<br>-</td></tr><tr><td>M6" ¶</td><td>-13.7</td><td>O2···H10-C10<sup>#6</sup></td><td>3.333</td><td>153.28</td></tr><tr><td>M7"</td><td>-26.5</td><td>C11(π)···C11(π)<sup>#7</sup></td><td>3.644</td><td>-</td></tr><tr><td>M8"</td><td>-31.3</td><td>N4···H7-C7<sup>#8</sup></td><td>3.415</td><td>127.53</td></tr></tbody></table>

<footnote>¶ Dimeric motif involving nitrate ligand. Symmetry operations: (#1) x-1, y, z+1; (#2) -x+1, -y, -z+1; (#3) -x+1, -y+1, -z+1; (#4) x+1, y, z; (#5) -x+1, -y+2, -z; (#6) -x+1, -y, -z+1; (#7) -x, -y+1, -z; (#8) -x, -y, -z.</footnote>
<table><caption>**Table S5.** Major dimeric motifs and respective interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory based on the crystal structure at 200 K for each considered linkage isomer. Corrections for dispersion and BSSE were applied. Mx denotes a nitro motif, Mx' – endo-nitrito motif and Mx'' – exo-nitrito motif (x – motif number).</caption><thead><tr><th>Motif</th><th>E<sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>D...A</sub> / Å</th><th>θ<sub>D-H...A</sub> / °</th></tr></thead><tbody><tr><td>M1 ¶</td><td>-20.6</td><td>O1...H8-C8<sup>#1</sup></td><td>3.441</td><td>139.06</td></tr><tr><td>M1' ¶</td><td>-19.8</td><td>O1...H8-C8<sup>#1</sup></td><td>3.716</td><td>141.41</td></tr><tr><td>M1" ¶</td><td>-22.9</td><td>O1...H8-C8<sup>#1</sup></td><td>3.843</td><td>136.18</td></tr><tr><td rowspan="2">M2 ¶</td><td rowspan="2">-94.6</td><td>O1...H12a-C12<sup>#2</sup></td><td>3.468</td><td>161.23</td></tr><tr><td>O3...H12c-C12<sup>#2</sup></td><td>3.243</td><td>102.35</td></tr><tr><td rowspan="2">M2' ¶</td><td rowspan="2">-85.8</td><td>O1...H12a-C12<sup>#2</sup></td><td>3.297</td><td>132.33</td></tr><tr><td>O3...H12c-C12<sup>#2</sup></td><td>3.297</td><td>109.53</td></tr><tr><td rowspan="2">M2" ¶</td><td rowspan="2">-91.5</td><td>O1...H12a-C12<sup>#2</sup></td><td>3.507</td><td>132.87</td></tr><tr><td>O3...H12c-C12<sup>#2</sup></td><td>3.264</td><td>114.63</td></tr><tr><td rowspan="4">M3 ¶</td><td rowspan="4">-107.3</td><td>O2...H1b-C1<sup>#3</sup></td><td>3.346</td><td>128.96</td></tr><tr><td>O2...Ni1<sup>#3</sup></td><td>3.041</td><td>-</td></tr><tr><td>O3...H12a-C12<sup>#3</sup></td><td>3.398</td><td>150.71</td></tr><tr><td>C12...H11-C11<sup>#3</sup></td><td>3.752</td><td>166.12</td></tr><tr><td rowspan="4">M3' ¶</td><td rowspan="4">-108.6</td><td>O2...H1b-C1<sup>#3</sup></td><td>3.315</td><td>130.95</td></tr><tr><td>O2...Ni1<sup>#3</sup></td><td>3.164</td><td>-</td></tr><tr><td>O3...H12a-C12<sup>#3</sup></td><td>3.460</td><td>145.08</td></tr><tr><td>C12...H11-C11<sup>#3</sup></td><td>3.806</td><td>165.35</td></tr><tr><td rowspan="4">M3" ¶</td><td rowspan="4">-108.4</td><td>N1...H12a-C12<sup>#3</sup></td><td>3.315</td><td>130.95</td></tr><tr><td>O2...Ni1<sup>#3</sup></td><td>3.164</td><td>-</td></tr><tr><td>O3...H12a-C12<sup>#3</sup></td><td>3.460</td><td>145.08</td></tr><tr><td>C12...H11-C11<sup>#3</sup></td><td>3.806</td><td>165.35</td></tr><tr><td rowspan="4">M4</td><td rowspan="4">-63.7</td><td>C9...H3-C3<sup>#4</sup></td><td>3.514</td><td>125.96</td></tr><tr><td>C11...H2a-C2<sup>#4</sup></td><td>3.734</td><td>149.08</td></tr><tr><td>O3...H1a-C1<sup>#4</sup></td><td>3.200</td><td>137.15</td></tr><tr><td>N4...H1a-C1<sup>#4</sup></td><td>3.241</td><td>113.46</td></tr><tr><td rowspan="2">M4'</td><td rowspan="2">-64.4</td><td>C11...H2a-C2<sup>#4</sup></td><td>3.755</td><td>154.80</td></tr><tr><td>O3...H1a-C1<sup>#4</sup></td><td>3.275</td><td>138.44</td></tr><tr><td rowspan="2">M4"</td><td rowspan="2">-69.4</td><td>C11...H2a-C2<sup>#4</sup></td><td>3.755</td><td>154.80</td></tr><tr><td>O3...H1a-C1<sup>#4</sup></td><td>3.375</td><td>138.44</td></tr><tr><td rowspan="2">M5</td><td rowspan="2">-46.9</td><td>O4...H3-C3<sup>#5</sup></td><td>3.390</td><td>125.94</td></tr><tr><td>O4...C2<sup>#5</sup></td><td>3.256</td><td>-</td></tr><tr><td>M6"</td><td>-15.2</td><td>O2...H10-C10<sup>#6</sup></td><td>3.166</td><td>153.09</td></tr></tbody></table>

¶ Dimeric motif involving nitrate ligand. Symmetry operations: (#1) x-1, y, z+1; (#2) -x+1, -y, -z+1; (#3) -x+1, -y+1, -z+1; (#4) x+1, y, z; (#5) -x+1, -y+2, -z; (#6) -x+1, -y, -z+1.
<table><caption>**Table S6.** Overview of interaction energies calculated at the DFT(B3LYP)/6-311++G** level of theory for each generated dimeric motif. In order to compare strength of intermolecular interactions in the crystal structure in each examined case, calculated energies were summarized with emphasis on the contribution from the nitrate ligand marked with #.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Nitro motifs</th><th colspan="4">Endo-nitrito motifs</th><th colspan="4">Exo-nitrito motifs</th></tr><tr><th>CRYSTAL-optimized</th><th>100 K</th><th>150 K</th><th>200 K</th><th>CRYSTAL-optimized</th><th>100 K</th><th>150 K</th><th>200 K</th><th>CRYSTAL-optimized</th><th>100 K</th><th>150 K</th><th>200 K</th></tr></thead><tbody><tr><td><b>M1#</b></td><td>-22.9</td><td>-20.6</td><td>-20.9</td><td>-20.6</td><td><b>M1#</b></td><td>-24.2</td><td>-21.6</td><td>-19.5</td><td>-19.8</td><td><b>M1#</b></td><td>-25.0</td><td>-23.6</td><td>-22.5</td><td>-22.9</td></tr><tr><td><b>M2#</b></td><td>-88.4</td><td>-95.1</td><td>-95.4</td><td>-94.6</td><td><b>M2#</b></td><td>-72.5</td><td>-81.0</td><td>-80.9</td><td>-85.8</td><td><b>M2#</b></td><td>-83.1</td><td>-91.6</td><td>-91.7</td><td>-91.5</td></tr><tr><td><b>M3#</b></td><td>-96.7</td><td>-107.5</td><td>-104.9</td><td>-107.3</td><td><b>M3#</b></td><td>-86.3</td><td>-105.8</td><td>-96.4</td><td>-108.6</td><td><b>M3#</b></td><td>-115.1</td><td>-108.2</td><td>-104.7</td><td>-108.4</td></tr><tr><td><b>M4</b></td><td>-64.8</td><td>-63.6</td><td>-63.4</td><td>-63.7</td><td><b>M4'</b></td><td>-64.0</td><td>-64.0</td><td>-60.6</td><td>-64.4</td><td><b>M4''</b></td><td>-60.5</td><td>-60.5</td><td>-65.7</td><td>-69.4</td></tr><tr><td><b>M5</b></td><td>-50.1</td><td>-47.7</td><td>-46.9</td><td>-46.9</td><td><b>M5'</b></td><td>-48.6</td><td>-47.7</td><td>-43.4</td><td>-</td><td><b>M5''</b></td><td>-44.8</td><td>-48.0</td><td>-45.4</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><b>M6''#</b></td><td>-9.1</td><td>-14.0</td><td>-13.7</td><td>-15.2</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><b>M7''</b></td><td>-29.4</td><td>-26.2</td><td>-26.5</td><td>-</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><b>M8''</b></td><td>-32.8</td><td>-29.2</td><td>-31.3</td><td>-</td></tr><tr><td>Σ</td><td>-322.9</td><td>-334.5</td><td>-331.5</td><td>-333.1</td><td>Σ</td><td>-295.8</td><td>-320.1</td><td>-300.8</td><td>-278.6</td><td>Σ</td><td>-399.8</td><td>-401.3</td><td>-401.5</td><td>-307.4</td></tr><tr><td>Σ#</td><td>-208.0</td><td>-223.2</td><td>-221.2</td><td>-222.5</td><td>Σ#</td><td>-183.0</td><td>-208.4</td><td>-196.8</td><td>-214.2</td><td>Σ#</td><td>-232.3</td><td>-237.4</td><td>-232.6</td><td>-238.0</td></tr></tbody></table>

<figure><img src="image_24.png" /><figcaption>**Figure S5.** Hirshfeld surface generated for the **Ni-1d'** nitro isomer at 100 K mapped with the shape index property indicating involvement of the aromatic fragment into intermolecular interactions in the crystal structure. Presence of (a) π-stacking and (b) end-to-face contacts is visualized by characteristic hollows (orange) and bumps (blue) (in the vicinity of the phenyl ring) complementing each other on the border of the two molecular Hirshfeld surfaces.</figcaption></figure>
<table><caption>**Table S7.** Energy differences (ΔE<sub>rel</sub>) between hypothetical *CRYSTAL*-optimized crystal structures of pure linkage isomers and that containing two different linkage isomers in the unit cell optimal for a given pure linkage isomer. During the latter optimization unit cell parameters were kept fixed at the optimal values for the respective unit cell previously optimized, while atomic positions were varied. Unit cell symmetry was lowered so as to accommodate for two different linkage isomers in the crystal structure.</caption><thead><tr><th rowspan="2">Unit cell</th><th rowspan="2">Linkage isomers</th><th colspan="3">ΔE<sub>rel</sub> / kJ·mol<sup>-1</sup></th></tr><tr><th>optimized<br>endo-nitrito</th><th>optimized<br>exo-nitrito</th><th>optimized<br>nitro</th></tr></thead><tbody><tr><td>endo-nitrito</td><td>endo-nitrito + exo-nitrito</td><td>+9.2</td><td>+8.0</td><td>+39.3</td></tr><tr><td>exo-nitrito</td><td>endo-nitrito + exo-nitrito</td><td>+13.2</td><td>+12.0</td><td>+43.3</td></tr><tr><td>nitro</td><td>nitro + exo-nitrito</td><td>-1.5</td><td>-0.3</td><td>+29.8</td></tr><tr><td>nitro</td><td>nitro + endo-nitrito</td><td colspan="3">convergence issues, less energetically beneficial than<br>nitro (nitro + exo-nitrito)</td></tr></tbody><tfoot><tr><td colspan="5">Comment: endo-nitrito (endo-nitrito + exo-nitrito) – exo-nitrito (endo-nitrito + exo-nitrito) = 4 kJ·mol<sup>-1</sup>;<br>convergence issues when two endo-nitrito molecules placed in the exo-nitrito optimal unit cell.</td></tr></tfoot></table>

<table><caption>**Table S8.** Unit cell parameters and linkage isomers' populations for multi-temperature X-ray diffraction measurements.</caption><thead><tr><th rowspan="2">T / K</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>290</td><td>7.624(2)</td><td>8.850(2)</td><td>11.292(2)</td><td>78.11(3)</td><td>81.94(3)</td><td>89.32(3)</td><td>738.0(3)</td><td>81.9(5)</td><td>9.2(4)</td><td>8.9(3)</td></tr><tr><td>250</td><td>7.625(2)</td><td>8.792(2)</td><td>11.253(2)</td><td>78.37(3)</td><td>82.17(3)</td><td>89.39(3)</td><td>731.9(3)</td><td>84.3(6)</td><td>6.9(4)</td><td>8.8(4)</td></tr><tr><td>220</td><td>7.626(2)</td><td>8.757(2)</td><td>11.230(2)</td><td>78.53(3)</td><td>82.29(3)</td><td>89.45(3)</td><td>728.2(3)</td><td>86.9(5)</td><td>5.2(4)</td><td>7.9(3)</td></tr><tr><td>210</td><td>7.626(2)</td><td>8.757(2)</td><td>11.230(2)</td><td>78.53(3)</td><td>82.29(3)</td><td>89.45(3)</td><td>728.2(3)</td><td>87.8(5)</td><td>4.7(4)</td><td>7.5(3)</td></tr><tr><td>200</td><td>7.624(2)</td><td>8.735(2)</td><td>11.217(2)</td><td>78.61(3)</td><td>82.36(3)</td><td>89.49(3)</td><td>725.7(3)</td><td>87.8(5)</td><td>4.6(4)</td><td>7.6(3)</td></tr><tr><td>190</td><td>7.623(2)</td><td>8.726(2)</td><td>11.212(2)</td><td>78.64(3)</td><td>82.38(3)</td><td>89.51(3)</td><td>724.7(3)</td><td>88.3(5)</td><td>4.7(3)</td><td>7.0(3)</td></tr><tr><td>180</td><td>7.622(2)</td><td>8.717(2)</td><td>11.208(2)</td><td>78.67(3)</td><td>82.40(3)</td><td>89.53(3)</td><td>723.7(3)</td><td>88.3(5)</td><td>4.8(3)</td><td>6.9(3)</td></tr><tr><td>150</td><td>7.622(2)</td><td>8.691(2)</td><td>11.194(2)</td><td>78.75(3)</td><td>82.46(3)</td><td>89.60(3)</td><td>720.9(3)</td><td>89.7(4)</td><td>4.2(3)</td><td>6.1(3)</td></tr><tr><td>100</td><td>7.621(2)</td><td>8.651(2)</td><td>11.174(2)</td><td>78.88(3)</td><td>82.54(3)</td><td>89.66(3)</td><td>716.6(3)</td><td>91.2(4)</td><td>4.0(3)</td><td>4.8(3)</td></tr><tr><td>150</td><td>7.624(2)</td><td>8.689(2)</td><td>11.194(2)</td><td>78.77(3)</td><td>82.43(3)</td><td>89.56(3)</td><td>720.9(3)</td><td>89.5(4)</td><td>4.5(3)</td><td>6.0(3)</td></tr><tr><td>180</td><td>7.625(2)</td><td>8.716(2)</td><td>11.207(2)</td><td>78.70(3)</td><td>82.38(3)</td><td>89.52(3)</td><td>723.8(3)</td><td>87.9(5)</td><td>5.1(3)</td><td>7.0(3)</td></tr><tr><td>190</td><td>7.625(2)</td><td>8.726(2)</td><td>11.214(2)</td><td>78.67(3)</td><td>82.35(3)</td><td>89.49(3)</td><td>725.0(3)</td><td>87.8(5)</td><td>5.1(3)</td><td>7.1(3)</td></tr><tr><td>200</td><td>7.626(2)</td><td>8.736(2)</td><td>11.219(2)</td><td>78.63(3)</td><td>82.34(3)</td><td>89.48(3)</td><td>726.1(3)</td><td>87.5(5)</td><td>5.1(4)</td><td>7.4(3)</td></tr><tr><td>210</td><td>7.626(2)</td><td>8.746(2)</td><td>11.224(2)</td><td>78.59(3)</td><td>82.31(3)</td><td>89.46(3)</td><td>727.1(3)</td><td>87.7(5)</td><td>4.9(4)</td><td>7.4(3)</td></tr><tr><td>220</td><td>7.626(2)</td><td>8.757(2)</td><td>11.231(2)</td><td>78.55(3)</td><td>82.29(3)</td><td>89.44(3)</td><td>728.3(3)</td><td>87.1(5)</td><td>5.4(4)</td><td>7.5(3)</td></tr><tr><td>250</td><td>7.626(2)</td><td>8.792(2)</td><td>11.252(2)</td><td>78.40(3)</td><td>82.16(3)</td><td>89.38(3)</td><td>732.0(3)</td><td>84.1(6)</td><td>7.3(4)</td><td>8.6(4)</td></tr><tr><td>290</td><td>7.625(2)</td><td>8.849(2)</td><td>11.290(2)</td><td>78.10(3)</td><td>81.93(3)</td><td>89.32(3)</td><td>738.0(3)</td><td>82.2(5)</td><td>9.0(4)</td><td>8.8(3)</td></tr></tbody></table>
<table><caption>**Table S9.** Reaction cavity volumes ($V_{cav}$) during multi-temperature X-ray diffraction measurements. Note that the standard deviation on $V_{cav}$ is estimated to be ca. 0.5 Å³.</caption><thead><tr><th>T / K</th><th>V<sub>cav</sub> / Å<sup>3</sup></th></tr></thead><tbody><tr><td>290</td><td>51.3</td></tr><tr><td>250</td><td>47.9</td></tr><tr><td>220</td><td>46.6</td></tr><tr><td>210</td><td>46.3</td></tr><tr><td>200</td><td>46.2</td></tr><tr><td>190</td><td>45.9</td></tr><tr><td>180</td><td>45.6</td></tr><tr><td>150</td><td>45.3</td></tr><tr><td>100</td><td>43.7</td></tr><tr><td>150</td><td>45.3</td></tr><tr><td>180</td><td>45.6</td></tr><tr><td>190</td><td>45.9</td></tr><tr><td>200</td><td>46.2</td></tr><tr><td>210</td><td>46.3</td></tr><tr><td>220</td><td>46.6</td></tr><tr><td>250</td><td>47.9</td></tr><tr><td>290</td><td>51.3</td></tr></tbody></table>

<figure><img src="image_25.png" /><figcaption>**Figure S6.** τ-Constrained rotational scan for each isomeric form performed at DFT(B3LYP)/ 6-311++G** level of theory (orange circles – nitro, teal squares– endonitrito, purple triangles – exo-nitrito).</figcaption></figure>
**Table S10.** Approximate LED powers used in IR and DSC experiments presented in the paper.

<table><thead><tr><th>Wavelength, λ / nm</th><th>Power, P / mW·cm<sup>-2</sup></th></tr></thead><tbody><tr><td>660</td><td>56</td></tr><tr><td>590</td><td>20</td></tr><tr><td>530</td><td>23</td></tr><tr><td>470</td><td>53</td></tr><tr><td>405</td><td>102</td></tr></tbody></table>

**Table S11.** Key vibrational modes characteristic for the examined linkage isomers of the nitrite group with their theoretical counterparts. All values are given in cm⁻¹.

<table><thead><tr><th>Isomer</th><th>Vibration type</th><th>Theory</th><th>Experiment</th></tr></thead><tbody><tr><td>nitro-(η¹-NO₂)</td><td>ν<sub>s</sub></td><td>1399</td><td>1361</td></tr><tr><td>nitro-(η¹-NO₂)</td><td>ν<sub>as</sub></td><td>1387</td><td>1343</td></tr><tr><td rowspan="3">endo-nitrito-(η¹-ONO)</td><td>ν<sub>as</sub></td><td>1082</td><td>1092</td></tr><tr><td>ν<sub>as</sub></td><td>1499</td><td>1403</td></tr><tr><td>ν<sub>as</sub></td><td>1512</td><td>1411</td></tr><tr><td rowspan="2">exo-nitrito-(η¹-ONO)</td><td>ν<sub>as</sub></td><td>977</td><td>1045</td></tr><tr><td>ν<sub>as</sub></td><td>1594</td><td>1449</td></tr></tbody></table>
<figure><img src="image_26.png" /><figcaption>**Figure S7.** IR spectra collected at 100 K: with no irradiation (black line), after irradiation with 660 nm LED for 15 minutes (red line), after irradiation with 590 nm LED for 15 minutes (yellow line), after irradiation with 530 nm LED for 15 minutes (green line), after irradiation with 470 nm LED for 15 minutes (blue line), after irradiation with 405 nm LED for 15 minutes (violet line).</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Figure S8.** IR spectra collected at 100 K: with no irradiation (black line), after irradiation with 660 nm LED for 15 minutes (red line), after irradiation with 590 nm LED for 15 minutes (yellow line), after irradiation with 530 nm LED for 15 minutes (green line), after irradiation with 470 nm LED for 15 minutes (blue line), after irradiation with 405 nm LED for 15 minutes (violet line).</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S9.** IR spectra collected at 100 K: with no irradiation (black line), after irradiation with 660 nm LED for 15 minutes (red line), after irradiation with 590 nm LED for 15 minutes (yellow line), after irradiation with 530 nm LED for 15 minutes (green line), after irradiation with 470 nm LED for 15 minutes (blue line), after irradiation with 405 nm LED for 15 minutes (violet line).</figcaption></figure>
<figure><img src="image_29.png" /></figure>

**Figure S10.** IR spectra collected: with no irradiation at RT (black line), with no irradiation at 100 K (dark green line), after irradiation with 530 nm LED for 5 minutes (light green line), after irradiation at 120 K (dark pink line), after irradiation at 140 K (light pink line), after irradiation at 160 K (dark violet line), after irradiation at 180 K (dark blue line), after irradiation at 200 K (light violet line), after irradiation at 220 K (light blue line), after irradiation at 240 K (dark brown line), after irradiation at 260 K (light brown line), after irradiation at 280 K (yellow line).
<figure><img src="image_30.png" /></figure>

**Figure S11.** IR spectra collected: with no irradiation at RT (black line), with no irradiation at 100 K (dark green line), after irradiation with 530 nm LED for 5 minutes (light green line), after irradiation at 120 K (dark pink line), after irradiation at 140 K (light pink line), after irradiation at 160 K (dark violet line), after irradiation at 180 K (dark blue line), after irradiation at 200 K (light violet line), after irradiation at 220 K (light blue line), after irradiation at 240 K (dark brown line), after irradiation at 260 K (light brown line), after irradiation at 280 K (yellow line).
<figure><img src="image_31.png" /></figure>

**Figure S12.** IR spectra collected: with no irradiation at RT (black line), with no irradiation at 100 K (dark green line), after irradiation with 530 nm LED for 5 minutes (light green line), after irradiation at 120 K (dark pink line), after irradiation at 140 K (light pink line), after irradiation at 160 K (dark violet line), after irradiation at 180 K (dark blue line), after irradiation at 200 K (light violet line), after irradiation at 220 K (light blue line), after irradiation at 240 K (dark brown line), after irradiation at 260 K (light brown line), after irradiation at 280 K (yellow line).
<figure><img src="image_32.png" /><figcaption>**Figure S13.** IR spectra collected at 150 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 15 minutes after irradiation (dotted blue line), after 30 minutes after irradiation (dashed dotted pink line), after 1 hour after irradiation (short dashed violet line), after 1.5 hour after irradiation (short dotted brown line).</figcaption></figure>
<figure><img src="image_33.png" /><figcaption>**Figure S14.** IR spectra collected at 150 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 15 minutes after irradiation (dotted blue line), after 30 minutes after irradiation (dashed dotted pink line), after 1 hour after irradiation (short dashed violet line), after 1.5 hour after irradiation (short dotted brown line).</figcaption></figure>
<figure><img src="image_34.png" /><figcaption>**Figure S15.** IR spectra collected at 150 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 15 minutes after irradiation (dotted blue line), after 30 minutes after irradiation (dashed dotted pink line), after 1 hour after irradiation (short dashed violet line), after 1.5 hour after irradiation (short dotted brown line).</figcaption></figure>
<figure><img src="image_35.png" /><figcaption>**Figure S16.** IR spectra collected at 200 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 5 minutes after irradiation (dotted blue line), after 10 minutes after irradiation (dashed dotted pink line), after 15 minutes after irradiation (short dashed violet line), after 30 minutes after irradiation (short dotted brown line), after 1 hour after irradiation (short dashed yellow line), after 1.5 hour after irradiation (short dashed dotted light pink line).</figcaption></figure>
<figure><img src="image_36.png" /></figure>

**Figure S17.** IR spectra collected at 200 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 5 minutes after irradiation (dotted blue line), after 10 minutes after irradiation (dashed dotted pink line), after 15 minutes after irradiation (short dashed violet line), after 30 minutes after irradiation (short dotted brown line), after 1 hour after irradiation (short dashed yellow line), after 1.5 hour after irradiation (short dashed dotted light pink line).
<figure><img src="image_37.png" /><figcaption>**Figure S18.** IR spectra collected at 200 K: with no irradiation (black line), after irradiation with 530 nm LED for 5 minutes (dashed green line), after 5 minutes after irradiation (dotted blue line), after 10 minutes after irradiation (dashed dotted pink line), after 15 minutes after irradiation (short dashed violet line), after 30 minutes after irradiation (short dotted brown line), after 1 hour after irradiation (short dashed yellow line), after 1.5 hour after irradiation (short dashed dotted light pink line).</figcaption></figure>
<figure><img src="image_38.png" /><figcaption>**Figure S19.** IR spectra collected at: RT (black line), 280 K (dark teal line), 260 K (light teal line), 240 K (dark pink line), 220 K (light pink line), 200 K (dark violet line), 180 K (dark blue line), 160 K (light violet line), 140 K (light blue line), 120 K (aqua line), 100 K (dark brown line), 80 K (brown line), 60 K (orange line), 40 K (khaki line), 20 K (grey line), 10 K (yellow line).</figcaption></figure>
<figure><img src="image_39.png" /></figure>

**Figure S20.** IR spectra collected at: RT (black line), 280 K (dark teal line), 260 K (light teal line), 240 K (dark pink line), 220 K (light pink line), 200 K (dark violet line), 180 K (dark blue line), 160 K (light violet line), 140 K (light blue line), 120 K (aqua line), 100 K (dark brown line), 80 K (brown line), 60 K (orange line), 40 K (khaki line), 20 K (grey line), 10 K (yellow line).
<figure><img src="image_40.png" /><figcaption>**Figure S21.** IR spectra collected at: RT (black line), 280 K (dark teal line), 260 K (light teal line), 240 K (dark pink line), 220 K (light pink line), 200 K (dark violet line), 180 K (dark blue line), 160 K (light violet line), 140 K (light blue line), 120 K (aqua line), 100 K (dark brown line), 80 K (brown line), 60 K (orange line), 40 K (khaki line), 20 K (grey line), 10 K (yellow line).</figcaption></figure>
<table><caption>**Table S12.** Unit cell parameters and linkage isomers' populations for X-ray diffraction measurements after irradiation with 530 nm LED at 100 K.</caption><thead><tr><th rowspan="2">t<sub>irr</sub> / min</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>0</td><td>7.620(2)</td><td>8.651(2)</td><td>11.177(2)</td><td>78.93(3)</td><td>82.58(3)</td><td>89.68(3)</td><td>716.9(3)</td><td>91.9(4)</td><td>4.5(3)</td><td>3.6(3)</td></tr><tr><td>20</td><td>7.636(2)</td><td>8.687(2)</td><td>11.208(2)</td><td>78.64(3)</td><td>81.37(3)</td><td>89.78(3)</td><td>720.4(3)</td><td>43.8(18)</td><td>36.3(10)</td><td>19.9(16)</td></tr><tr><td>40</td><td>7.663(2)</td><td>8.583(2)</td><td>11.467(2)</td><td>78.03(3)</td><td>79.16(3)</td><td>90.71(3)</td><td>723.6(3)</td><td>15.0(10)</td><td>32.9(12)</td><td>52.1(16)</td></tr><tr><td>60</td><td>7.661(2)</td><td>8.562(2)</td><td>11.482(2)</td><td>78.14(3)</td><td>78.96(3)</td><td>90.77(3)</td><td>722.3(3)</td><td>10.2(9)</td><td>33.1(10)</td><td>56.7(13)</td></tr><tr><td>80</td><td>7.661(2)</td><td>8.562(2)</td><td>11.482(2)</td><td>78.14(3)</td><td>78.96(3)</td><td>90.77(3)</td><td>722.3(3)</td><td>7.5(8)</td><td>32.9(10)</td><td>59.6(13)</td></tr><tr><td>100</td><td>7.715(3)</td><td>8.542(2)</td><td>11.463(4)</td><td>78.05(2)</td><td>77.96(3)</td><td>90.34(3)</td><td>721.8(4)</td><td>9.1(18)</td><td>24.6(20)</td><td>66.3(30)</td></tr></tbody></table>

<table><caption>**Table S13.** Unit cell parameters and linkage isomers' populations for X-ray diffraction measurements during temperature relaxation.</caption><thead><tr><th rowspan="2">t<sub>irr</sub> / min</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>0</td><td>7.621(2)</td><td>8.658(2)</td><td>11.177(2)</td><td>78.85(3)</td><td>82.57(3)</td><td>89.71(3)</td><td>717.4(3)</td><td>88.4(6)</td><td>6.5(5)</td><td>5.1(4)</td></tr><tr><td>240</td><td>7.702(2)</td><td>8.456(2)</td><td>11.600(2)</td><td>78.12(3)</td><td>78.33(3)</td><td>90.62(3)</td><td>722.9(3)</td><td>7.4(5)</td><td>10.4(7)</td><td>82.2(9)</td></tr></tbody></table>

<table><thead><tr><th rowspan="2">T / K</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>100</td><td>7.715(3)</td><td>8.542(2)</td><td>11.463(4)</td><td>78.05(2)</td><td>77.96(3)</td><td>90.34(3)</td><td>721.8(4)</td><td>9.1(18)</td><td>24.6(20)</td><td>66.3(30)</td></tr><tr><td>150</td><td>7.698(2)</td><td>8.490(2)</td><td>11.622(2)</td><td>78.04(3)</td><td>78.24(3)</td><td>90.75(3)</td><td>726.3(3)</td><td>6.6(7)</td><td>11.7(9)</td><td>81.7(11)</td></tr><tr><td>180</td><td>7.695(2)</td><td>8.544(2)</td><td>11.596(2)</td><td>78.01(3)</td><td>78.42(3)</td><td>90.68(3)</td><td>729.4(3)</td><td>8.6(8)</td><td>14.8(10)</td><td>76.6(13)</td></tr><tr><td>190</td><td>7.680(2)</td><td>8.602(2)</td><td>11.534(2)</td><td>78.14(3)</td><td>79.02(3)</td><td>90.51(3)</td><td>731.1(3)</td><td>14.8(10)</td><td>21.9(13)</td><td>63.3(16)</td></tr><tr><td>200</td><td>7.639(2)</td><td>8.745(2)</td><td>11.247(2)</td><td>78.57(3)</td><td>81.73(3)</td><td>89.50(3)</td><td>728.6(3)</td><td>78.0(13)</td><td>13.1(11)</td><td>8.9(7)</td></tr><tr><td>210</td><td>7.620(2)</td><td>8.698(2)</td><td>11.194(3)</td><td>78.78(2)</td><td>82.57(3)</td><td>89.62(2)</td><td>721.5(3)</td><td>90.0(8)</td><td>5.2(6)</td><td>4.8(5)</td></tr><tr><td>220</td><td>7.638(2)</td><td>8.765(2)</td><td>11.247(2)</td><td>78.57(3)</td><td>82.19(3)</td><td>89.42(3)</td><td>731.0(3)</td><td>90.4(8)</td><td>5.0(7)</td><td>4.6(5)</td></tr><tr><td>250</td><td>7.628(2)</td><td>8.833(2)</td><td>11.293(2)</td><td>78.18(3)</td><td>81.99(3)</td><td>89.33(3)</td><td>737.3(3)</td><td>83.5(8)</td><td>9.8(6)</td><td>6.7(5)</td></tr></tbody></table>
<table><caption>**Table S14.** Unit cell parameters and linkage isomers' populations for X-ray diffraction measurements after irradiation with 530 nm LED at 150 K.</caption><thead><tr><th rowspan="2">t<sub>irr</sub> / min</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>0</td><td>7.626(2)</td><td>8.706(2)</td><td>11.204(2)</td><td>78.74(3)</td><td>82.20(3)</td><td>89.56(3)</td><td>722.6(3)</td><td>81.1(4)</td><td>11.7(3)</td><td>7.2(3)</td></tr><tr><td>60</td><td>7.699(2)</td><td>8.533(2)</td><td>11.565(2)</td><td>78.20(3)</td><td>78.76(3)</td><td>90.55(3)</td><td>728.5(3)</td><td>14.1(9)</td><td>17.0(13)</td><td>68.9(15)</td></tr><tr><td>120</td><td>7.703(2)</td><td>8.520(2)</td><td>11.578(2)</td><td>78.13(3)</td><td>78.58(3)</td><td>90.55(3)</td><td>727.8(3)</td><td>12.1(7)</td><td>15.3(10)</td><td>72.6(12)</td></tr><tr><td>180</td><td>7.695(2)</td><td>8.517(2)</td><td>11.584(2)</td><td>78.12(3)</td><td>78.54(3)</td><td>90.58(3)</td><td>727.1(3)</td><td>11.1(6)</td><td>13.9(8)</td><td>75.0(10)</td></tr><tr><td>240</td><td>7.698(2)</td><td>8.511(2)</td><td>11.589(2)</td><td>78.14(3)</td><td>78.49(3)</td><td>90.62(3)</td><td>727.1(3)</td><td>10.7(5)</td><td>12.2(6)</td><td>77.1(8)</td></tr></tbody></table>

<table><caption>**Table S15.** Unit cell parameters and linkage isomers' populations for X-ray diffraction measurements during temperature relaxation.</caption><thead><tr><th rowspan="2">T / K</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>150</td><td>7.699(2)</td><td>8.511(2)</td><td>11.588(2)</td><td>78.19(3)</td><td>78.52(3)</td><td>90.73(3)</td><td>727.3(3)</td><td>8.3(5)</td><td>13.5(6)</td><td>78.2(8)</td></tr><tr><td>160</td><td>7.691(2)</td><td>8.526(2)</td><td>11.588(2)</td><td>78.19(3)</td><td>78.57(3)</td><td>90.70(3)</td><td>727.9(3)</td><td>8.6(5)</td><td>14.4(6)</td><td>77.0(8)</td></tr><tr><td>180</td><td>7.694(2)</td><td>8.560(2)</td><td>11.580(2)</td><td>78.08(3)</td><td>78.63(3)</td><td>90.57(3)</td><td>730.5(3)</td><td>9.7(5)</td><td>17.1(6)</td><td>73.2(8)</td></tr><tr><td>190</td><td>7.685(2)</td><td>8.604(2)</td><td>11.541(2)</td><td>78.15(3)</td><td>78.95(3)</td><td>90.46(3)</td><td>732.1(3)</td><td>15.0(5)</td><td>20.6(6)</td><td>64.4(8)</td></tr><tr><td>200</td><td>7.646(2)</td><td>8.746(2)</td><td>11.298(2)</td><td>78.45(3)</td><td>81.25(3)</td><td>89.67(3)</td><td>731.3(3)</td><td>57.7(13)</td><td>22.1(11)</td><td>20.2(7)</td></tr><tr><td>210</td><td>7.631(2)</td><td>8.757(2)</td><td>11.237(2)</td><td>78.57(3)</td><td>82.26(3)</td><td>89.47(3)</td><td>729.2(3)</td><td>85.9(7)</td><td>7.3(5)</td><td>6.8(4)</td></tr><tr><td>220</td><td>7.631(2)</td><td>8.765(2)</td><td>11.238(2)</td><td>78.55(3)</td><td>82.29(3)</td><td>89.43(3)</td><td>729.8(3)</td><td>87.9(6)</td><td>5.9(5)</td><td>6.2(4)</td></tr><tr><td>250</td><td>7.632(2)</td><td>8.801(2)</td><td>11.264(2)</td><td>78.37(3)</td><td>82.18(3)</td><td>89.37(3)</td><td>734.1(3)</td><td>86.6(7)</td><td>6.9(5)</td><td>6.5(4)</td></tr></tbody></table>

<table><caption>**Table S16.** Unit cell parameters and linkage isomers' populations for X-ray diffraction measurements during continuous irradiation with 530 nm LED at 200 K and subsequent relaxation.</caption><thead><tr><th rowspan="2">t<sub>irr</sub> / min</th><th rowspan="2">a / Å</th><th rowspan="2">b / Å</th><th rowspan="2">c / Å</th><th rowspan="2">α / °</th><th rowspan="2">β / °</th><th rowspan="2">γ / °</th><th rowspan="2">V / Å<sup>3</sup></th><th colspan="3">Population %</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>0</td><td>7.620(2)</td><td>8.766(2)</td><td>11.213(2)</td><td>78.60(3)</td><td>82.23(3)</td><td>89.50(3)</td><td>727.4(3)</td><td>87.6(4)</td><td>6.4(3)</td><td>6.0(3)</td></tr><tr><td>360 °</td><td>7.669(2)</td><td>8.625(2)</td><td>11.533(2)</td><td>78.08(3)</td><td>78.80(3)</td><td>90.48(3)</td><td>731.3(3)</td><td>14.9(7)</td><td>21.7(6)</td><td>63.4(5)</td></tr><tr><td>-</td><td>7.620(5)</td><td>8.773(8)</td><td>11.256(9)</td><td>78.47(7)</td><td>81.82(6)</td><td>89.56(6)</td><td>729.7(1)</td><td>60.1(9)</td><td>33.1(4)</td><td>6.8(5)</td></tr><tr><td>-</td><td>7.624(2)</td><td>8.770(2)</td><td>11.220(2)</td><td>78.59(3)</td><td>82.10(3)</td><td>89.50(3)</td><td>728.3(3)</td><td>84.0(5)</td><td>9.4(4)</td><td>6.6(3)</td></tr></tbody></table>

<footnote>¶ 360 min. of irradiation prior to X-ray crystallographic measurement and continuous irradiation during experiment (ca. 1.5 hour duration)</footnote>
**Figure S22 (below).** Reconstructed reciprocal planes for the series of photocrystallographic measurements at 100K with 530 nm LED. Sequence of images present X-ray diffracted pattern (a) before irradiation, (b) after first 20 min. of exposure to light, (c) after second 20 min. of exposure to light, (d) after third 20 min. of exposure to light, (e) after fourth 20 min. of exposure to light, (f) after fifth 20 min. of exposure to light, (g) after elevating the temperature to 150 K, (h) after elevating the temperature to 180 K, (i) after elevating the temperature to 190 K, (j) after elevating the temperature to 200 K, (k) after elevating the temperature to 210 K, (l) after elevating the temperature to 220 K, and (m) after elevating the temperature to 250 K.

<figure><img src="image_41.png" /></figure>
<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /></figure>
<figure><img src="image_46.png" /><figcaption>i)</figcaption></figure>

<figure><img src="image_47.png" /><figcaption>j)</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>k)</figcaption></figure>

<figure><img src="image_49.png" /></figure>
<figure><img src="image_50.png" /><figcaption>**Figure 23.** 3-Dimensional photodifference maps ($F_o^{ON} - F_o^{OFF}$; purple isosurfaces – positive values; red – negative). (a) Map after 4 hours of sample irradiation at 100 K (semi-transparent isosurfaces, ±0.516 e·Å⁻³). The crystal was previously irradiated for 4 hours at 150 K reaching maximal potential *exo*-nitrito population at this conditions, and then was cooled to 100 K which increased its population. (b) Map after 4 hours of sample irradiation at 100 K (solid isosurfaces: ±6.275 e·Å⁻³, semi-transparent: ±4.275 e·Å⁻³). (c) Map after 4-hour sample irradiation at 200 K (solid isosurfaces: ±5.225 e·Å⁻³; semi-transparent: ±3.225 e·Å⁻³).</figcaption></figure>
<table><thead><tr><th>O-N-Ni: 120° (nitro-S<sub>1</sub>)</th><th>O-N-Ni: 110°</th></tr></thead><tbody><tr><td><img src="image_0_0.png" alt="O-N-Ni: 120° (nitro-S<sub>1</sub>)"></td><td><img src="image_0_1.png" alt="O-N-Ni: 110°"></td></tr><tr><td>O-N-Ni: 100°</td><td>O-N-Ni: 90°</td></tr><tr><td><img src="image_0_2.png" alt="O-N-Ni: 100°"></td><td><img src="image_0_3.png" alt="O-N-Ni: 90°"></td></tr><tr><td>O-N-Ni: 80°</td><td>O-N-Ni: 70°</td></tr><tr><td><img src="image_0_4.png" alt="O-N-Ni: 80°"></td><td><img src="image_0_5.png" alt="O-N-Ni: 70°"></td></tr></tbody></table>

**Figure S24.** Natural transition orbitals (NTOs) calculated along the S<sub>1</sub> reaction profile along the O-N-Ni angle.

<figure><img src="image_51.png" /></figure>

**Figure S25.** UV-Vis absorption spectrum of all considered linkage isomers calculated at the TDDFT/TDA level under the Wigner energy distribution at 0 K.
<figure><img src="image_52.png" /><figcaption>**Figure S26.** Relaxed scans along the reaction coordinate in which the O–N–Ni angles for both NO₂ oxygen atoms were simultaneously fixed at an equal value, mimicking the out-of-plane rotation of the nitro group. Both scans were followed by unrestricted (a) S₀ and (b) S₁ optimizations of the resulting temporary structures of the κ-nitrito isomer.</figcaption></figure>