Comparative analysis of photoswitching properties of
analogous nickel(II) and palladium(II) nitrite complexes
in the solid state

Kacper Paszczyk, Krystyna Deresz, Radoslaw Kamiński, Patryk Borowski, Dominik Schaniel, Adam Krówczyński, Katarzyna Jarzembska

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To cite this version:

Kacper Paszczyk, Krystyna Deresz, Radoslaw Kamiński, Patryk Borowski, Dominik Schaniel, et al. Comparative analysis of photoswitching properties of analogous nickel(II) and palladium(II) nitrite complexes in the solid state. Crystal Growth & Design, 2024, 24 (24), pp.10152-10170. 10.1021/acs.cgd.4c00956 . hal-04964842

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https://hal.science/hal-04964842v1

Submitted on 25 Feb 2025

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<figure><img src="image_2.png" /></figure>
Comparative analysis of photoswitching properties of analogous
nickel(II) and palladium(II) nitrite complexes in the solid state

Kacper Paszczyk,ª Krystyna A. Deresz,ª Radosław Kamiński,ª Patryk Borowski,ª
Dominik Schaniel,ᵇ Adam Krówczyński,ª Katarzyna N. Jarzembska a*

ª Department of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland

ᵇ Université de Lorraine, CNRS, CRM², 54500 Nancy, France

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

**Abstract:** Two analogous photoswitchable square-planar nitrite coordination compounds of Ni¹¹ and Pd¹¹ with the (N,N,O)-[1-phenyl-3-(2'-pikoliloamido)prop-2-en-1-one] ancillary ligand, **Ni-1a** and **Pd-1a**, are reported. The samples were thoroughly characterized (photo)crystallographically, spectroscopically and computationally. **Ni-1a** crystalizes in the *P*1̄ space group with one molecule comprising the asymmetric unit, whereas **Pd-1a** tends to form polymorphic (**Pd-1a** and **Pd-1a'**) and solvatomorphic (with dichloromethane: **Pd-1a-DCM** and with chloroform: **Pd-1a-CHCl3**) crystal structures. In the **Ni-1a** crystal structure the nitro binding mode is dominant, however, some traces of the *endo*-nitrito isomer are detected, whereas for the palladium derivative purely the nitro form is observed for all the crystal forms in the 100–290 K temperature range. **Ni-1a**, **Pd-1a** and **Pd-1a-DCM** are efficient photoswitches working between 100 and 200 K, with conversions as high as 80–100% upon 470–530 nm light-emitting diode irradiation. At 200 K and above the decrease in population of the photo-generated product is observed. Under continuous light irradiation some notable population of the *endo*-nitrito isomer can be generated and crystallographically detected also at higher temperatures (up to around 240 K). In the case of **Ni-1a** the *endo*-nitrito form is generated faster and its decay is slower at 200 K and at higher temperatures than for **Pd-1a**. It should also be noted that for **Pd-1a** only one symmetry-independent molecule is photoswitchable. In turn, the **Pd-1a'** and **Pd-1a-CHCl3** systems appeared non-photoactive. Such behavior can be explained by the energetic factors, nitrite group orientation and the shape of the reaction cavity. Interestingly, a strong linear correlation between the M···M distance in the crystal structures of the examined photoswitches and the population of the *endo*-nitrito form is noted.
**Keywords:** nickel(II) coordination compounds, palladium(II) coordination compounds, nitro-nitrito isomerization, solid-state chemistry, crystal engineering, DFT calculations
# 1. Introduction

One of the main goals of modern chemistry and physics is to develop new materials that are able to respond rapidly and reliably to changes in the local environment, providing us with the information on what is happening. Molecular switches are materials that undergo chemical transformations (isomerization, cyclization etc.) upon some external stimuli (light pulse, temperature, high pressure or static electric fields).¹⁻¹⁰ Photoswitchable solid-state materials may find various important real-life applications, including these in opto(bio)electronics, sensors, or in high-capacity storage devices.¹¹⁻²³ Among molecular switches, transition-metal complexes possessing ambidentate ligands (e.g. NO, NO₂, or SO₂) constitute a promising and readily modifiable group of compounds.², ²⁴⁻³⁵ The industrially-desirable photoswitchable materials should work at ambient conditions, be stable, relatively cheap, non-toxic, and easy to synthesize in high-yield. In turn, the photo-induced transformation should be selective, efficient, controllable and reproducible.

Thorough and systematic investigations of dynamics and nature of an ambidentate ligand isomerization reactions occurring in crystals of transition-metal coordination complexes are indispensable for understanding of these processes and identifying factors governing molecular switching in the solid state. Hence, such studies are of great importance regarding the conscious design of functional materials of this kind.

In view of the above in the current contribution we focus our attention on the nitrite group isomerization. In mononuclear complexes this ligand may adopt four different binding modes, i.e. it may exist as nitro, *exo*-nitrito, *endo*-nitrito, or *κ*-nitrito form (Scheme 1). Once the appropriate conditions are met, such compounds can be switched between various isomeric forms in the solid state by external stimulus.³², ³⁴, ³⁶⁻³⁹

<figure><img src="image_3.png" /></figure>

**Scheme 1.** Schematic representation of the nitrite group binding modes under consideration (M = metal center).
For the purpose of current investigations, a pair of model square-planar nitro-
complexes of Ni¹¹ and Pd¹¹ coordination compounds with the (N,N,O)-donor ancillary
ligands were chosen. The systems, referred to as **Ni-1a** and **Pd-1a**, are shown in Figure 1.
The study constitutes a part of our wider project.²⁴, ³⁸, ⁴⁰⁻⁴¹ The selected nickel compound
is a more basic, parent system compared to the previously investigated **Ni-2a** and **Ni-2b**
along with their copper analogues, **Cu-2a** and **Cu-2b**,⁴¹ or to **QTNiNO₂**²⁴ (Scheme S1.2,
Supporting Information). As the examined copper derivatives exhibit minor photoactivity
and at very low temperatures (below 80 K), here we supplemented the Ni compound with
its Pd equivalent. In this way we may explore the effect of a heavier metal center on
structural and spectroscopic properties of the resulting solid-state material, together
with that caused by changes in the molecular structure when compared to the already
studied systems of this kind. A series of some other Ni, Pd and Pt nitro coordination
compounds had been discussed in works by Warren *et al*.³⁷, ⁴² Based on their findings we
should expect a decrease in the nitrite group reactivity going from Ni, through Pd to Pt,
especially if the structures are isomorphous. Consequently, comprehensive multi-
temperature structural analyzes were performed for the chosen pair of Ni and Pd
analogues (Scheme 2) along with detailed (photo)-infrared (IR) spectroscopic
investigations, both supplemented with computational results. Given the gathered
information, targeted photocrystallographic measurements were conducted, so as to
obtain a possibly complete picture of the studied systems' properties sufficient for their
explanation.

<figure><img src="image_4.png" /><figcaption>**Scheme 2.** Schematic representation of the studied **M-1a** compounds (M = Ni, Pd).</figcaption></figure>
## 2. Experimental part

**2.1. Synthesis and crystallization.** All solvents and substrates were purchased from chemical companies and used without further purification. The synthesis is analogous to that described in literature<sup>24, 38, 41</sup> (Supporting Information). To a mixture of 1.0 mmol acetophenone in 2.0 mmol ethyl formate 1.0 mmol of sodium in 20 mL of Et<sub>2</sub>O was added, and the reaction mixture was stirred for 12 h. After solvent evaporation, the obtained hydroxymethyl ketone sodium salt was dissolved in 30 mL of EtOH. To the solution was then added 1.0 mmol 2-picolylamine in 10 mL of EtOH and the solution was acidified with AcOH until it reached a pH of 6. The obtained intermediate product was not isolated (Supporting Information); after the solution was brought to a boil, 1.2 mmol Ni(OAc)<sub>2</sub> or Pd(OAc)<sub>2</sub> in 10 mL of MeOH, respectively, was added. Then, the solution was stirred for 2 h without heating. The dark brown mixture was purified by filtration. To the mixture was then added *ca.* 2.0 mmol LiNO<sub>2</sub> in MeOH. The resulting mixture containing the final product (**M-1a**, M = Ni or Pd) was cooled using an ice bath, and the product was filtered. For **Ni-1a**, small needles were obtained, but too small for single-crystal X-ray diffraction experiments. In the case of **Pd-1a** yellow powder was obtained. Therefore, crystallizations from various solvents were carried out using the slow solvent evaporation method. As a result red needles, in the case of the nickel(II) compound, and yellow blocks, in the case of the palladium(II) analogue, suitable for single crystal X-ray diffraction experiments were obtained. The best crystals which were used for experiments were grown from DCM (dichloromethane), MeOH and EtOH.

Nuclear magnetic resonance (NMR) spectra (collected only for the nickel complexes) were recorded with an Agilent NMR 400 MHz Varian spectrometer. ¹H chemical shifts are given relative to TMS using residual solvent resonances. In the case of **Pd-1a** ¹H NMR spectra both in CDCl<sub>3</sub> and DMSO-d<sub>6</sub> show sharp narrow signals. It was also possible to collect good quality ¹³C{¹H} NMR spectrum in DMSO for this sample. For **Ni-1a** in DMSO-d<sub>6</sub> the ¹H NMR spectrum shows broadened signals, most probably due to strong dynamics of the nitrite group, or some ligand exchange with DMSO. In CDCl<sub>3</sub> this effect is much weaker, however, some signals are still broadened which reflects dynamics in the vicinity of the metal center. For the respective spectra see Supporting Information. Elemental analyses were carried out with an Elementar Vario EL III analyzer.

**Ni-1a:** Yield: 0.208 g (61%). ¹H NMR (400 MHz, CDCl<sub>3</sub>) $δ$ = 7.69 (m, 2H), 7.62 (br, 2H), 7.43-7.31 (m, 1H), 7.29 (d, *J* = 7.7 Hz, 2H), 7.15 (d, *J* = 8.2 Hz, 2H), 7.07 (br, 1H), 5.93 (br,
1H), 4.83 (s, 2H) ppm. Elemental analysis: C₁₅H₁₃N₃O₃Ni (341.97); calculated: C 52.68%, H 3.83%, N 12.29%; found: C 52.68%, H 3.83%, N 12.29%.

**Pd-1a:** Yield: 0.226 g (58%). ¹H NMR (400 MHz, CDCl₃) δ = 8.75 (ddd, *J* = 5.8, 1.5, 0.8 Hz, 1H), 7.90–7.81 (m, 2H), 7.77 (td, *J* = 7.7, 1.5 Hz, 1H), 7.45–7.23 (m, 5H), 7.12 (d, *J* = 6.5 Hz, 1H), 5.88 (d, *J* = 6.5 Hz, 1H), 5.20 (d, *J* = 1.3 Hz, 2H). ¹³C{¹H} NMR (101 MHz, DMSO-d₆) δ = 170.72, 163.34, 156.92, 149.38, 140.25, 137.27, 130.89, 128.90, 127.73, 124.71, 122.12, 93.74, 65.02 ppm. Elemental analysis: C₁₅H₁₃N₃O₃Pd (389.70); calculated: C 46.23%, H 3.36%, N 10.78%; found: C 46.25%, H 3.27%, N 10.60%.

**2.2. X-ray diffraction.** All X-ray diffraction experiments were carried out on a Rigaku Oxford Diffraction SuperNova single-crystal diffractometer equipped with a CCD Atlas detector, a copper microfocus X-ray source with multilayer optics, a low-temperature nitrogen gas flow Oxford Cryosystems device, and our home-made light-delivery device, which allows *in situ* photocrystallographic experiments.⁴³ The light from the fiber-coupled LED was delivered onto the sample through the multimode fiber with 400 µm core diameter, similarly as described in our previous contributions.²⁴, ⁴⁰⁻⁴¹, ⁴⁴⁻⁴⁵ The optimal data-collection strategy took into account the mounted light-delivery device and was prepared using the native diffractometer software. For photocrystallographic experiments, whenever possible the same strategy was used for all data collected for a given crystal. Note that due to the large number of data sets collected the crystal mounted onto the glass capillary was sometimes removed from the goniometer head. Re-mounting of the crystal was always preceded by a new determination of the optimal strategy. All experiments were performed in a complete darkness (diffractometer lights permanently switched off and external light was blocked); note: some experiments were done under continuous LED irradiation during entire experiment). For details concerning data collection codes and the measurement sequence for each sample, see the Supporting Information. Data processing (i.e., unit-cell determination, diffraction-frame integration, absorption correction and scaling etc.) was essentially the same for all data collected. All structures were solved using an intrinsic phasing method as implemented in the *SHELXT* program⁴⁶ and refined with the *JANA* package⁴⁷ within the independent atom model approximation. Note that sometimes the structure solution was not needed, since the model was transferred from the previous and/or similar data set. The CIF files can be retrieved from the Cambridge Structural Database⁴⁸⁻⁴⁹ (deposition numbers: CCDC 2369023–5, 2369303–48, 2369447–65, 2369467–81, 2369483–509, 2369516–28,
2369538-48, 2369561-601, 2393915-43 and 2393996-8; a total of 207 crystal structures) or from Supporting Information.

Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker AXS D8 Discover powder diffractometer equipped with a VÅNTEC detector and a copper X-ray tube. All data were collected in a parallel-beam geometry (locked-couple experiment mode) in up to $2\theta = 40^{\circ}$. Le Bail refinements⁵⁰ of the unit-cell and powder-profile parameters were accomplished with the JANA program. The results are presented in the Supporting Information.

**2.3. Spectroscopy.** All 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 grounded, mixed with spectroscopic-grade KBr, pressed into thin 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 405 to 660 nm).

**2.4. Computations.** Isolated-molecule and quantum-mechanics/molecular-mechanics (QM/MM) geometry optimizations, dimer interaction energies and normal-mode frequencies were calculated within the density functional theory (DFT) approach using the GAUSSIAN package (ver. 16, rev. C.01).⁵¹ For harmonic-mode computations, no imaginary frequencies were found. In the case of the QM/MM method, 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, where the C–H distances were set to the neutron-normalized values.⁴⁸,⁵² In the case of DFT calculations the B3LYP functional⁵³⁻⁵⁵ was applied together with the 6-311++G** basis set⁵⁶⁻⁶² for all atoms but Pd, whereas for the Pd atoms, the aug-cc-pVTZ-PP basis set⁶³ was utilized. These basis sets were selected based on initial tests, as shown in the Supporting Information. The above-mentioned level of theory was applied for the optimization of the central molecule, while for the molecular shell the Universal Force Field⁶⁴ (UFF) was applied using Hirshfeld atomic charges⁶⁵ derived at the same level of theory as described above. In the case of interaction energy calculations, the Grimme empirical dispersion correction,⁶⁶⁻⁶⁷ modified by the Becke-Johnson damping function,⁶⁸⁻⁶⁹ and correction for basis set superposition error⁷⁰⁻⁷¹ were applied. The semi-automatic generation of input files was
accomplished with the CLUSTERGEN program.⁷² Additionally, atomic positions for the **Ni-1a** crystal structure were optimized within periodic approach implemented in the CRYSTAL program (ver. 17)⁷³ using the PBESOL0-3C functional and the sol-def2-mSVP basis set for all atoms.⁷⁴

**3. Results and discussion**

**Ni-1a** and **Pd-1a** are coordination compounds in which the metal center is coordinated by the nitrite group and the (N,N,O)-[1-phenyl-3-(2'-pikoliloamido)prop-2-en-1-one] ancillary ligand (Figure 1). In both cases the nitrite moiety exists in the nitro binding mode, either solely or as the major component, and the metal center adopts a quasi-square planar configuration in the solid state. However, despite the analogous molecular structure, the two compounds form different molecular architectures when crystallized (Table 1, Figure 1, Table S4.1, Supporting Information). Whereas **Ni-1a** tends to form one crystalline form, in the case of its palladium(II) derivative various polymorphic and solvatomorphic forms can be obtained dependent on crystallization conditions (i.e. two polymorphs and two solvatomorphs, see Supporting Information). Based on powder diffraction the polycrystalline synthesis product constitutes mainly the **Pd-1a'** polymorph (Supporting Information) which can only be obtained as single crystals of suitable size and quality, with considerable difficulty. As far as recrystallization is concerned, the other non-solvent form constitutes the dominant product, namely **Pd-1a**, which easily grows as good quality crystals. During crystallizations of the Pd derivative from various solvents, individual crystals of a DCM solvatomorph (**Pd-1a-DCM**) can be obtained, as well as, needle-shape crystals of a chloroform-containing form, **Pd-1a-CHCl3**.

Consequently, all the received crystal forms were tested toward photoswitching properties. For the purpose of more detailed (photo)crystallographic studies **Ni-1a**, **Pd-1a** and **Pd-1a-DCM** were considered. The remaining crystal forms, i.e. **Pd-1a'** and **Pd-1a-CHCl3**, appeared not to be photoactive (see Supporting Information).
<table><caption>**Table 1.** Selected crystal structure and refinement parameters for the **Ni-1a**, **Pd-1a** and **Pd-1a-DCM** crystal structures measured at 100 K (for more details see the Supporting Information). Note the partial occupancy of the DCM molecule in **Pd-1a-DCM**.</caption><thead><tr><th>Complex</th><th>Ni-1a</th><th>Pd-1a</th><th>Pd-1a-DCM</th></tr></thead><tbody><tr><td>Formula</td><td>C<sub>15</sub>H<sub>13</sub>NiN<sub>3</sub>O<sub>3</sub></td><td>C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub></td><td>2 (C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub>) + 0.9 CH<sub>2</sub>Cl<sub>2</sub></td></tr><tr><td>Formula mass, M<sub>r</sub> / a.u.</td><td>341.98</td><td>389.69</td><td>856.02</td></tr><tr><td>Crystal system</td><td>triclinic</td><td>monoclinic</td><td>monoclinic</td></tr><tr><td>Space group</td><td>P<span style="text-decoration: overline">1</span> (No. 2)</td><td>P2<sub>1</sub>/n (No. 14)</td><td>I<sub>2</sub>/a (No. 15)</td></tr><tr><td>Z</td><td>2</td><td>8</td><td>4</td></tr><tr><td>F<sub>000</sub></td><td>352</td><td>1552</td><td>1703</td></tr><tr><td>Crystal color &amp; shape</td><td>red cut needle</td><td>yellow block</td><td>yellow cut block</td></tr><tr><td>a / Å</td><td>8.126(2)</td><td>13.965(3)</td><td>11.462(2)</td></tr><tr><td>b / Å</td><td>8.496(2)</td><td>9.219(2)</td><td>11.716(2)</td></tr><tr><td>c / Å</td><td>10.681(2)</td><td>21.774(4)</td><td>22.745(5)</td></tr><tr><td>α / °</td><td>93.95(3)</td><td>90</td><td>90</td></tr><tr><td>β / °</td><td>112.21(3)</td><td>96.82(3)</td><td>92.96(3)</td></tr><tr><td>γ / °</td><td>91.03(3)</td><td>90</td><td>90</td></tr><tr><td>V / Å<sup>3</sup></td><td>680.3(3)</td><td>2783.4(10)</td><td>3050.2(10)</td></tr><tr><td>d<sub>calc</sub> / g·cm<sup>-3</sup></td><td>1.670</td><td>1.860</td><td>1.864</td></tr><tr><td>R<sub>int</sub></td><td>2.61%</td><td>3.26%</td><td>2.63%</td></tr><tr><td>R[F] (I > 3σ(I))</td><td>2.61%</td><td>2.40%</td><td>2.06%</td></tr><tr><td>wR[F<sup>2</sup>] (all data)</td><td>7.32%</td><td>5.98%</td><td>5.79%</td></tr><tr><td>ρ<sub>res</sub><sup>min/max</sup> / e·Å<sup>-3</sup></td><td>-0.33 / +0.34</td><td>-0.47 / +0.49</td><td>-0.68 / +1.84</td></tr><tr><td>CCDC code</td><td>2369447</td><td>2369330</td><td>2393942</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Molecular structures of **Ni-1a** (upper panel), **Pd-1a** (left bottom panel) and **Pd-1a-DCM** (right bottom panel) derived from the X-ray diffraction data collected in darkness at 100 K. Atomic thermal motion is represented as ellipsoids at the 50% probability level (H-atoms are shown as spheres of arbitrary radii); the disorder is shown as gold ball-and-stick fragment (note all atoms are refined as anisotropic); the DCM molecule in **Pd-1a-DCM** is located on a 2-fold symmetry axis.</figcaption></figure>

**3.1. Crystal structures.** **Ni-1a** crystallizes in the *P*1̄ space group with one molecule in the asymmetric unit (ASU; Figure 1). The nitro isomer is dominant in the ground-state crystal structure of **Ni-1a** in the 100–290 K temperature range. Nevertheless, based on the residual density map some traces of the *endo*-nitrito form were also detected (ca. 2–7% depending on temperature; no specific correlation between temperature and population was observed; Table S4.2 series 1, Supporting Information). Main dimeric structural motifs formed in the crystal structure of **Ni-1a** are shown in Figure 2. The crystal structure exhibits layered architecture. Molecules form layers parallel to the (201) crystal plane, stabilized by numerous hydrogen bonds. The strongest interactions between molecules in such layers correspond to those stabilizing **A2** (interaction energy: *E*<sub>int</sub> = –36.6 kJ·mol<sup>–1</sup>), centrosymmetric **A3** (–32.1 kJ·mol<sup>–1</sup>) and **A1** (–15.9 kJ·mol<sup>–1</sup>)
structural motifs (Table 2). In the case of **A2**, the two hydrogen-bonds, *i.e.* C13-H1c13···O2 and C15-H1c15···O2, involve the same oxygen atom from the nitrite ligand. **A3** is stabilized by a pair of analogous interactions, C7-H1c7···O3, between the other nitrite oxygen, O3, and an aromatic proton of the adjacent moiety. In turn, the **A1** motif has approximately half of the interaction energy compared to **A2** and **A3**, since it is stabilized by a single weak hydrogen bond. Similarly as in **A3**, this interaction is formed with the O3 nitrite oxygen atom. As far as the interlayer interactions are concerned, molecular stacks play an important role, since the studied compound has a relatively flat coordination sphere which includes aromatic fragments. The π-stacking interactions effectively stabilize two motifs, namely **A4** and **A5**. These dimers are characterized by the most advantageous interaction energy values among all dimeric motifs in the discussed crystal structure, *i.e.* −144 kJ·mol⁻¹ and −117 kJ·mol⁻¹, respectively. The distance between aromatic-ring planes in both cases is equal to *ca.* 3.5 Å. It should also be noted here that the π-stacking contacts are accompanied by numerous hydrogen-bond-type interactions. The distances between nickel centers in such motifs amount to 3.569(3) Å and 6.274(2) Å, respectively. **A4** and **A5** propagate alternately in space along the [100] crystallographic direction (Figure 4).

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Major dimeric structural motifs present in the crystal structure of Ni-1a.</figcaption></figure>
<table><caption>**Table 2.** Main molecular dimers in the **Ni-1a** crystal structure along with respective short contacts, interaction energy values (*E*<sub>int</sub>), and selected geometrical parameters (*d* – distance, *θ* – angle).</caption><thead><tr><th>Motif</th><th><i>E</i><sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th><i>d</i><sub>H···A</sub> / Å</th><th><i>d</i><sub>D···A</sub> / Å</th><th><i>θ</i><sub>D–H···A</sub> / °</th></tr></thead><tbody><tr><td><b>A1</b></td><td>-15.9</td><td>C6–H6···O3<sup>#1</sup></td><td>2.53</td><td>3.482(3)</td><td>169.65</td></tr><tr><td rowspan="2"><b>A2</b></td><td rowspan="2">-36.6</td><td>C13–H1c13···O2<sup>#2</sup></td><td>2.49</td><td>3.302(3)</td><td>142.01</td></tr><tr><td>C15–H1c15···O2</td><td>2.69</td><td>3.373(3)</td><td>128.39</td></tr><tr><td><b>A3</b></td><td>-32.1</td><td>C7–H1c7···O3<sup>#3</sup></td><td>2.65</td><td>3.328(2)</td><td>127.65</td></tr><tr><td rowspan="2"><b>A4</b></td><td rowspan="2">-144.4</td><td>C15–H1c15···O2<sup>#4</sup></td><td>2.63</td><td>3.353(3)</td><td>132.38</td></tr><tr><td>C3(π)···N2(π)</td><td></td><td>3.433(3)</td><td></td></tr><tr><td rowspan="2"><b>A5</b></td><td rowspan="2">-117.9</td><td>C5–H1c5···O3<sup>#5</sup></td><td>2.66</td><td>3.448(3)</td><td>139.80</td></tr><tr><td>C3(π)···C3(π)</td><td></td><td>3.375(3)</td><td></td></tr></tbody></table>

Symmetry operations: (#1) x, y, z – 1; (#2) x, y + 1, z; (#3) –x + 1, –y, –z + 1; (#4) –x + 2, –y + 1, –z + 1; (#5) –x + 1, –y + 1, –z + 1.

In the case of the palladium analogue, we will focus our attention mainly on the **Pd-1a** form, dominant after synthesis product recrystallization, and also on its less common, but interesting regarding the current study, **Pd-1a-DCM** solvatomorph. The structure of **Pd-1a** exhibits the *P2*<sub>1</sub>/<n> space group symmetry. Two symmetrically inequivalent molecules (denoted as A and B; Figure 1), comprising ASU, are arranged nearly perpendicularly one to another in space. They differ mainly by the orientation of the nitrite ligand, which is well illustrated by the corresponding dihedral angles between the plane of the molecule core and that of the NO<sub>2</sub> ligand, which amount to 75.9 and 47.8° for molecules A and B, respectively (Table 4). The ambidentate ligand's conformation in molecule A is more like that observed for the nitro biding mode in **Ni-1a** (the corresponding dihedral angle is 86.1°). Based on the multi-temperature X-ray diffraction data, the nitrite group in the Pd<sup>II</sup> derivative adopts purely the nitro binding mode in the 100–290 K temperature range. In contrast to **Ni-1a**, no disorder was observed.
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Major dimeric structural motifs present in the crystal structure of Pd-1a.</figcaption></figure>

<table><caption>**Table 3.** Main molecular dimers in the Pd-1a crystal structure along with respective short contacts, interaction energy values ($E_{int}$), and selected geometrical parameters ($d$ - distance, $\theta$ - angle).</caption><thead><tr><th>Motif</th><th><i>E</i><sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th><i>d</i><sub>H···A</sub> / Å</th><th><i>d</i><sub>D···A</sub> / Å</th><th><i>θ</i><sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td>BB1</td><td>-37.0</td><td>C13'-H1c13'#1···O3'</td><td>2.58</td><td>3.460(4)</td><td>153.20</td></tr><tr><td>BB2</td><td>-15.6</td><td>C11'-H1c11'#2···O2'</td><td>2.75</td><td>3.308(4)</td><td>117.75</td></tr><tr><td rowspan="4">AB3</td><td rowspan="4">-26.0</td><td>C6'-H1c6'#3···O3#3</td><td>2.51</td><td>3.318(4)</td><td>141.64</td></tr><tr><td>C5'-H1c5'#3···C9</td><td>2.75</td><td>3.588(4)</td><td>146.54</td></tr><tr><td>C5'-H1c5'#3···C9</td><td>2.87</td><td>3.826(5)</td><td>173.54</td></tr><tr><td>C2'-H1c2'#3···C8</td><td>2.81</td><td>3.739(4)</td><td>161.99</td></tr><tr><td rowspan="3">AB4</td><td rowspan="3">-35.6</td><td>C6-H1c6···O2'#4</td><td>2.71</td><td>3.288(4)</td><td>118.94</td></tr><tr><td>C10'-H1c10'#4···C5</td><td>2.91</td><td>3.593(4)</td><td>129.23</td></tr><tr><td>C10'-H1c10'#4···C6</td><td>2.86</td><td>3.707(4)</td><td>147.93</td></tr><tr><td rowspan="3">AB5</td><td rowspan="3">-41.5</td><td>C1-H1c1#5···O3'#5</td><td>2.70</td><td>3.473(4)</td><td>138.58</td></tr><tr><td>C15-H2c15#5···O1'</td><td>2.65</td><td>3.508(4)</td><td>148.70</td></tr><tr><td>C15-H1c15#5···C9'</td><td>2.66</td><td>3.448(3)</td><td>139.80</td></tr><tr><td rowspan="3">AB6</td><td rowspan="3">-69.5</td><td>C11-H1c11···O2'</td><td>2.54</td><td>3.477(4)</td><td>165.22</td></tr><tr><td>C15'-H1c15'···O2</td><td>2.54</td><td>3.281(4)</td><td>133.90</td></tr><tr><td>C1'-H1c1'···O2</td><td>2.68</td><td>2.965(4)</td><td>97.94</td></tr><tr><td rowspan="3">AB7</td><td rowspan="3">-19.1</td><td>C8'-H1c8'···O3'#6</td><td>2.55</td><td>3.460(4)</td><td>157.73</td></tr><tr><td>C8'-H1c8'···O2'#6</td><td>2.68</td><td>3.311(4)</td><td>123.59</td></tr><tr><td>C9'-H1c9'···O2'#6</td><td>2.77</td><td>3.348(4)</td><td>119.80</td></tr><tr><td rowspan="2">BB8</td><td rowspan="2">-147.9</td><td>C1'-H1c1'···O3'#7</td><td rowspan="2">2.65</td><td>3.148(4)</td><td rowspan="2">112.72</td></tr><tr><td>C3'(π)···N2'(π)#7</td><td>3.494(4)</td></tr><tr><td>AA9</td><td>-94.8</td><td>C13(π)···N2(π)#8</td><td></td><td>3.562(4)</td><td></td></tr><tr><td rowspan="2">AA10</td><td rowspan="2">-143.4</td><td>C1-H1c1···O2#9</td><td rowspan="2">2.70</td><td>2.953(4)</td><td rowspan="2">95.89</td></tr><tr><td>C3'(π)···N2#9</td><td>3.401(4)</td></tr></tbody></table>

Symmetry operations: (#1) x, y + 1, z; (#2) -x + 1.5, y + 0.5, -z + 0.5; (#3) mol. A: -x + 1.5, y + 0.5, -z + 1.5; mol. B: x, y, z; (#4) mol. A: x + 0.5, -y + 0.5, z - 0.5; mol. B: x, y, z; (#5) mol. A: -x + 1, -y + 1, -z + 1; mol. B: x, y, z; (#6) mol. A: x, y + 1, z; mol. B: x, y, z; (#7) -x + 2, -y + 1, -z + 1; (#8) -x + 1, -y + 1, -z + 1; (#9) -x + 1, -y, -z + 1.
<table><caption>**Table 4.** Twist of the NO₂ ligand plane in respect to the rest of the molecule computed as dihedral angle (δ) between mean-square planes (NO₂ ligand: O2, N3, O3 atoms; complex: Ni1/Pd1, N1, N2, O1 atoms; for molecule B in **Pd-1a** respective primed atoms are used). Reaction cavity volumes (V~cav~) are included.</caption><thead><tr><th>Crystal structure</th><th>Dihedral angle, δ / °</th><th>Cavity volume, V<sub>cav</sub> / Å<sup>3</sup></th></tr></thead><tbody><tr><td><b>Ni-1a</b></td><td>86.1</td><td>28.5</td></tr><tr><td><b>Pd-1a</b> (mol. A)</td><td>75.9</td><td>25.8</td></tr><tr><td><b>Pd-1a</b> (mol. B)</td><td>47.8</td><td>33.3</td></tr><tr><td><b>Pd-1a-DCM</b></td><td>49.0</td><td>32.7</td></tr><tr><td><b>Pd-1a'</b></td><td>46.3</td><td>38.0</td></tr><tr><td><b>Pd-1a-CHCl<sub>3</sub></b></td><td>40.7</td><td>29.6</td></tr></tbody></table>

The similarities between **Ni-1a** and **Pd-1a** regarding crystal packing are shown in Figure 4, major structural motifs of **Pd-1a** are presented in Figure 3, whereas the most important short intermolecular contacts together with interaction energy values are gathered in Tables 2–3. Molecules A in the **Pd-1a** crystal structure alike that of **Ni-1a** are engaged in two types of π-stacking interactions, which stabilize the **AA10** ($E_{int}$ = -134.4 kJ·mol⁻¹) and **AA9** (-94.8 kJ·mol⁻¹) dimers, and lead to column-like motifs propagating along the [010] direction (Figure 4). In turn, molecules B are also involved in a dimeric motif comprising parallel molecules denoted as **BB8** (-147.9 kJ·mol⁻¹). However, in contrast to the case of molecule A, such dimers are further held by weak side interactions which results in parallel molecular tapes shown in the right panel of Figure 4. Molecules in these tapes are, among others, stabilized by hydrogen bonds present in the **BB1** motif (-36.99 kJ·mol⁻¹) formed between adjacent species from the upper and lower tapes, similarly as it is in the analogous dimers found in the **Ni-1a** crystal structure, i.e. **A1** and **A2**. Such formed tapes correspond to molecular layers viewed along the [010] direction in the **Ni-1a** crystal structure and marked in violet in Figure 4. The above-described stacks and tapes in the **Pd-1a** crystal structure are arranged alternately to create a chessboard-like pattern visible along the Y axis. Interactions between the stacked columns and tapes are described by the remaining six motifs (Figure 3). Among them, the **AB6** motif stands out with the most advantageous value of interaction energy between its components, i.e. -69.5 kJ·mol⁻¹. It is formed by two perpendicular-oriented molecules and is the only motif in which both nitro groups form hydrogen bonds. **AB5** constitutes another worth noting motif stabilized by a weak hydrogen-bond-type interaction involving the carbonyl oxygen atom from the Pd¹¹ coordination sphere (-41.5 kJ·mol⁻¹).
It is also possible to distinguish layers parallel to the (100) crystal plane. The adjacent layers are identical but rotated relatively one to another by an angle of *ca.* 100° as illustrated in Figure S6.1 in the view along the [021] and [021] crystallographic directions.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Comparison of crystal packing of the **Ni-1a** (dashed red frame; left panel) and **Pd-1a** (dashed olive frame; right panel) crystal structures.</figcaption></figure>

In turn, the crystal structure of the dichlomethane solvatomorph, **Pd-1a-DCM**, belongs to the *I2/*a* space group, whereas ASU comprises one complex molecule and a half of the DCM species of *ca.* 90% occupancy (dependent on the chosen crystal). The key structural motifs present in **Pd-1a-DCM** are depicted in Figure 5, while the respective interaction energy values and geometrical parameters are summarized in Table 5. Similarly to the previously described structures, the dimer characterized by the highest interaction energy (*E*<sub>int</sub> = -161.4 kJ·mol<sup>-1</sup>), **B8**, is stabilized by π-stacking interactions, between aromatic rings of different type, and hydrogen-bond-like interactions engaging the nitrite fragment. The **B8** stacked molecules form further **B7** motifs with the adjacent species, held mainly by notably weaker π···π interactions between the phenyl rings. The alternated **B8** and **B7** dimers propagate along the [111] direction (Figure S6.2*a*, Supporting Information). There can also be distinguished edge-to-face interactions
present in the **B3** motif. Apart from the **B8** motif, the nitrite group is involved in the
hydrogen-bond-type interactions stabilizing the **B1**, **B2**, **B4** and **B5** dimers. Whereas in
the case of **B1** and **B2** it forms two typical medium-strength contacts, **B4** exhibits less
advantageous interactions energy, and the remaining **B5** motif is the weakest interacting
one among this set. In this last dimer the nitrite ligand forms a hydrogen bond with a
solvent molecule. Each DCM species also weakly interacts with the aromatic hydrogen
atom leading to the **B6** motif. It should be noted that the weakly-held DCM species are
located in the structural tunnels which are well visible along the [100] direction, as shown
in Figure S6.2b (Supporting Information). Such arrangement of solvent species may
explain less accurate determination of their positions and fractional occupancy.

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Major dimeric structural motifs present in the **Pd-1a-DCM** crystal structure.</figcaption></figure>
<table><caption>**Table 5.** Main molecular dimers in the **Pd-1a-DCM** crystal structure along with respective short contacts, interaction energy values (*E*<sub>int</sub>), and selected geometrical parameters (*d* – distance, *θ* – angle).</caption><thead><tr><th>Motif</th><th><i>E</i><sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th><i>d</i><sub>H···A</sub> / Å</th><th><i>d</i><sub>D···A</sub> / Å</th><th><i>θ</i><sub>D–H···A</sub> / °</th></tr></thead><tbody><tr><td rowspan="3"><b>B1</b></td><td rowspan="3">-54.4</td><td>C11–H1C11···C2<sup>#1</sup></td><td>2.89</td><td>3.760(3)</td><td>151.33</td></tr><tr><td>C15–H2C15···O3<sup>#1</sup></td><td>2.66</td><td>3.622(3)</td><td>175.07</td></tr><tr><td>C13···O2<sup>#1</sup></td><td></td><td>3.188(3)</td><td></td></tr><tr><td><b>B2</b></td><td>-56.4</td><td>C11–H1C11···O3<sup>#2</sup></td><td>2.64</td><td>3.176(3)</td><td>115.99</td></tr><tr><td><b>B3</b></td><td>-30.0</td><td>C1<sup>#3</sup>–H1C1<sup>#3</sup>···C13</td><td>2.89</td><td>3.827(3)</td><td>165.84</td></tr><tr><td rowspan="2"><b>B4</b></td><td rowspan="2">-28.2</td><td>C8–H1C8···O2<sup>#4</sup></td><td>2.80</td><td>3.750(3)</td><td>170.86</td></tr><tr><td>C8–H1C8···O3<sup>#4</sup></td><td>2.81</td><td>3.538(3)</td><td>133.18</td></tr><tr><td><b>B5</b></td><td>-18.5</td><td>C1<sup>"5</sup>–H1C1<sup>"5</sup>···O2</td><td>2.46(2)</td><td>3.337(3)</td><td>152(2)</td></tr><tr><td><b>B6</b></td><td>-10.4</td><td>C12–H1C12···C11<sup>"6</sup></td><td>2.88</td><td>3.657(3)</td><td>138.67</td></tr><tr><td><b>B7</b></td><td>-19.8</td><td>C5···C7<sup>#7</sup></td><td></td><td>3.398(4)</td><td></td></tr><tr><td rowspan="3"><b>B8</b></td><td rowspan="3">-161.3</td><td>C15–H1C15···O2<sup>#8</sup></td><td>2.52</td><td>3.339(3)</td><td>143.70</td></tr><tr><td>C5···C11<sup>#8</sup></td><td></td><td>3.365(3)</td><td></td></tr><tr><td>C9···C13<sup>#8</sup></td><td></td><td>3.340(3)</td><td></td></tr></tbody></table>

Symmetry operations: (#1) $-x + 1, y + 0.5, -z + 1.5$; (#2) $-x + 1.5, -y + 1.5, -z + 1.5$; (#3) $x + 0.5, -y + 2, z$; (#4) $-x + 0.5, y, -z + 1$; (#5) $-x + 0.5, y, -z + 1$; (#6) $-x + 1.5, y, -z + 2$; (#7) $-x, -y + 2, -z + 1$; (#8) $-x + 0.5, -y + 1.5, -z + 1.5$.

**3.2. Isomerization potential.** In order to assess the photo-switching potential of the examined systems, several factors should be analyzed. These are the mutual stability of the considered linkage isomers, including molecular energies, as well as the above-mentioned intermolecular interactions involving the ambidentate ligand (also the photo-generated one), energy barriers accompanying the isomerization process,³⁷⁻³⁸, ⁷⁵ or the reaction cavity volume.⁷⁶⁻⁷⁷

Referring to the first factor, isolated molecules of the considered linkage isomers were optimized at the DFT(B3LYP)/6-311++G**/aug-cc-pVTZ-PP level of theory. Additionally, for the two main crystal forms of both compounds, **Ni-1a** and **Pd-1a**, geometry optimizations were performed within the QM/MM approach, which enables mimicking of the crystal environment. The respective calculated relative energies are gathered in Table 6.
<table><caption>**Table 6.** Energy differences ($E_{diff} = E_{MS} - E_{GS}$) between the ground (GS; nitro) and metastable (MS; *exo-* and *endo*-nitrito) linkage isomers for optimized isolated and MM-constrained molecules of both **Ni-1a** and **Pd-1a** complexes. Note the energies for QM/MM cases are computed separately as for isolated molecules after removing the UFF force-field part (QM/MM approach was used only in the computations of the geometry).</caption><thead><tr><th rowspan="2">Linkage isomer</th><th colspan="2">E<sub>diff</sub><sup>isol</sup> / kJ·mol<sup>-1</sup></th><th colspan="3">E<sub>diff</sub><sup>QM/MM</sup> / kJ·mol<sup>-1</sup></th></tr><tr><th>Ni-1a</th><th>Pd-1a</th><th>Ni-1a</th><th>Pd-1a (mol. A)</th><th>Pd-1a (mol. B)</th></tr></thead><tbody><tr><td>nitro</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>endo-nitrito</td><td>4.02</td><td>18.96</td><td>3.3</td><td>19.3</td><td>27.4</td></tr><tr><td>exo-nitrito</td><td>10.38</td><td>27.31</td><td>30.8</td><td>42.3</td><td>—<sup>a</sup></td></tr></tbody></table>

The results show that as far isolated molecules are concerned, the nitro isomer is the most stable form of the nitrite ligand whereas the *exo*-nitrito moiety is the least energetically favored one. This is typical for square-planar nickel(II) nitrite coordination compounds.<sup>24, 37, 41</sup> It is also well visible that the energetic differences between various isomers are not very pronounced, especially in the case of nickel compound, for which they hardly exceed 10 kJ·mol<sup>-1</sup> (i.e., <2.5 kcal·mol<sup>-1</sup>) in the most extreme case (nitro vs. *exo*-nitrito). The differences are more significant for palladium derivative, however, still comparable to these found for other photoswitchable molecules of this kind.

In the case of molecular energies calculated for the QM/MM-optimized geometries of **Ni-1a** and molecule A of **Pd-1a**, the energy differences between the nitro and *endo*-nitrito species are comparable to these calculated for the optimized isolated molecules. The nitro-*endo*-nitrito energy difference slightly rises for the molecule B when the simplified crystal environment is taken into account. It should also be noted that in the case of molecule A in the **Pd-1a** crystal structure it was possible to optimize the *exo*-nitrito isomer embedded by the nitro molecules, which, however, resembles more the transition state species between these two isomers. In turn, for the molecule B the starting arbitrary *exo*-nitrito isomer transfers to the *endo*-nitrito species upon optimization. This may suggest that the *exo*-nitrito isomer of molecule B is not sufficiently supported in the **Pd-1a** crystal structure.

It is also worth having a closer look at the orientation of the nitro group with respect to the plane of the molecule. It is greater for the nickel compound than for its palladium derivative and differs significantly between the optimized isolated molecule (dihedral $\delta$

<footnote><sup>a</sup> Geometry optimization always converges to the *endo*-nitrito isomer.</footnote>
angles for the Ni compound: 66.0° for the Pd compound: 35.9°; Tables S6.1–2, Supporting Information) and experimental geometry (Table 4). In the case of **Pd-1a** the optimal dihedral angle value is closer to the experimental value determined for the molecule B. Consequently, the nitro isomer of the molecule B is slightly more energetically stable than that of molecule A (Supporting Information). In **Pd-1a'** the orientation of the nitro group is almost the same as that in molecule B in the **Pd-1a** crystal structure, while the dihedral angle is even smaller, thus closer to the optimal conformation, for **Pd-1a-CHCl₃**. It seems that as far as the Pd¹¹ derivative is concerned, the nitro ligand forms a dihedral angle notably more towards the right angle with the plane of the complex molecule only in the case of the molecule A from **Pd-1a** (75.9°). The second largest value of the dihedral angle is observed for **Pd-1a-DCM** (49.0°), however, it is only slightly greater than that determined for **Pd-1a'**, or for the molecule B from **Pd-1a**. It should also be noted that for **Ni-1a** the nitro group is oriented almost perpendicularly to the plane of the ancillary ligand (86.1°). The changes in conformation are imposed by the crystal environment, as reflected by the *CRYSTAL* and QM/MM-optimizations (Tables S6.1–2, Supporting Information), and may also affect the photo-reactivity of the studied systems given the theoretically predicted mechanism of the isomerization process in square-planar coordination compounds.<sup>37-38</sup> In this view, the more perpendicular orientation of the nitro group with respect to the metal center square-planar coordination sphere should promote the isomerization reaction.

Regarding energetic stabilization of the nitrite ligands in the crystal structures, interaction energies were calculated for structural motifs encountered in **Ni-1a**, **Pd-1a** and **Pd-1a-DCM**. In general, the complex molecules form a few moderate hydrogen bonds in each case. Determination of interaction energy of every single hydrogen-bond is not straightforward. However, based on their number, geometry and interaction energy values derived for the respective dimers, they overall seem to stabilize the nitrite group comparably as it was reported for the previously studied photoswitchable systems. For instance, in the case of the nitro isomer of **Ni-1a** each nitrite oxygen atom forms three hydrogen-bond-type contacts, which are encountered in the **A1**, **A3** and **A5** motifs for O3 and in **A2** and **A4** for O2 (Table 2). Based on the corresponding interaction energy values determined for the mentioned dimers it can be assumed that the interaction energy of single contacts of this kind oscillates between –15 and –40 kJ·mol⁻¹. For the *endo*-nitrito isomer of **Ni-1a** also 6 hydrogen-bond-type contacts involve the nitrite moiety. Among
them, one contact is formed with O2', two with N3', and three engage O3' which is located
at the same position as the O3 atom. It should be noted, however, that two contacts
formed by O2' and N3', although relatively short, seem to be less advantageous than the
remaining ones due to their geometry and also less nucleophilic nitrogen atom (Table
S6.3, Supporting Information). Similarly to **Ni-1a**, the nitro isomer of molecule B in the
**Pd-1a** crystal structure is involved in 6 short contacts present in different structural
motifs, *i.e.* in **BB1**, **BB2**, **AB4**, **AB5**, **AB6** and **BB8**. Apart from the first two, numerous
intermolecular contacts contribute to the total interaction energy of these motifs.
Nevertheless, based on the computational results, it seems that again the hydrogen bond-
type interaction energies are in-between –15 and –40 kJ·mol⁻¹. In turn, for the nitro
isomer of molecule A, the O2 atom creates five contacts, while O3 only two. They are in
general shorter than that of **Ni-1a**, but at the same time, the angles are smaller, *i.e.* below
134°, some slightly above 90°. Judging based on the interaction energies of the respective
dimers (mainly **AB3**, **AB6** and **AB7**) it can be concluded that the nitro moiety in molecule
A seems slightly less stabilized than in B and that of **Ni-1a**. Finally, as it was already
mentioned, in **Pd-1a-DCM** the nitro group is involved in stabilization of 5 dimeric motifs,
including a weak hydrogen bond with the dichloromethane moiety. Again, these
interactions do not exceed –30 kJ·mol⁻¹ for a single hydrogen-bond-type contact.

Importantly, the potential isomerization product should also be rather well held in
the crystal structure so it is sufficiently stable. Based on the QM/MM results it seems that
the *endo*-nitrito isomer of **Ni-1a** should be only slightly less supported by the interactions
with the adjacent molecules in the crystal structure than the nitro form (Supporting
Information), which is in line with our expectations based on the structural motif
interaction energies. The *exo*-nitrito species becomes less beneficial when the
intermolecular interactions are considered. For **Pd-1a** the difference between the *endo*-
nitrito and nitro isomers is much more pronounced and additionally emphasized by the
crystal environment, which changes the trend for the benefit of molecule B for which the
*endo*-nitrito isomer would be slightly better stabilized in the crystal structure. On the
other hand, as already mentioned above, the *exo*-nitrito form does not fit neither the
reaction cavity, nor the interaction pattern. These results support the experimental
observation of the *endo*-nitrito isomer found in the ground state crystal structure of **Ni-1a**
in the examined temperature range (from 100 K to room temperature; RT), which is
not the case for **Pd-1a**.
The analysis of the reaction cavity provides information on the shape and volume of the space available for the nitrite ligand to rearrange and transform to the potential photo-product. The reaction cavity was considered for each molecule separately and computed for the 100 K ground-state crystal structures **Ni-1a**, **Pd-1a**, **Pd-1a'**, **Pd-1a-DCM** and **Pd-1a-CHCl₃** using the *MERCURY* program.⁷⁸ Visualizations of the obtained reaction cavities for the examined molecules are shown in Figure 6, whereas their volumes are gathered in Table 4. In the case of the **Ni-1a** molecule, the reaction cavity volume amounts to 29.5 Å³ which is comparable to typical values for the literature-available photoswitches of this kind. It has an elongated shape hosting well the nitrite ligand in the nitro binding, but, on the other hand, it is bulkier in the central part thanks to which the *endo*-nitrito isomer also fits in. Due to the asymmetry of the cavity, the conformation of the nitrito group is well-defined. As far as the **Pd-1a** crystal structure is concerned, the two symmetry-inequivalent molecules differ significantly in terms of reaction cavity volume and shape. In the case of the molecule A reaction cavity volume is moderate, 26 Å³, and it consists of two distinctly delineated spheres as shown in Figure 6. The one on the left in the presented projection is notably smaller than that on the right. This indicates that the *endo*-nitrito isomer should be well hosted if formed, since this binding mode is asymmetric unlike nitro and tilted more to one side. In turn, the molecule B has the second largest reaction space available, *i.e.* 33 Å³, among the discussed moieties. However, its shape does not fully overlap with the nitro group and is curved. As a result, despite the theoretically large overall space, its shape does not facilitate the isomerization process and may even hinder it. For **Pd-1a-DCM** the reaction cavity volume is similar to that of molecules B in **Pd-1a** and amounts to 32.7 Å³. However, in this case the shape of the cavity is quite regular and well surrounds the nitro isomer, which should enable its rearrangement. This is in contrast to the situation in the other obtained solvatomorph, **Pd-1a-CHCl₃**, in which one complex molecule falls on one solvent molecule which results in a very limited volume in the area of the ambidentate ligand as shown in Figure 6. Interestingly, the reaction cavity volume in **Pd-1a'** is the largest among studied systems but, similarly to that of molecule B in **Pd-1a**, it is elongated and tilted to one side. Thus, the free space is not optimally located as far as the isomerization is concerned.

In summary, the reaction cavity volumes computed per one complex molecule are within the respective literature-reported values derived for similar complexes undergoing photo-induced isomerization,²⁵, ³⁷, ⁴¹, ⁴⁴, ⁷⁹⁻⁸⁰ however, it is their shape which
may be crucial in determining such transformations. Considering all the above-discussed factors which may facilitate or hamper the isomerization process, it seems that **Pd-1a-CHCl<sub>3</sub>** should be least prone to undergo the photo-induced transformation. Also in the case of **Pd-1a'** and molecule B from **Pd-1a** the nitrite group isomerization is expected to be more difficult. In the case of **Pd-1a-DCM** the shape and size of the reaction cavity as well as the weak interaction with the solvent species should facilitate the nitro group transformation.

<figure><img src="image_10.png" /><figcaption>**Figure 6.** Visualization of the reaction cavities generated based on the 100 K crystal structures of the examined systems using the *MERCURY* program<sup>78</sup> (probe radius of 1.2 Å, grid spacing of 0.1 Å). See also Figure S6.3 (Supporting Information).</figcaption></figure>

**3.2. IR spectroscopy.** Having performed the above structural analysis, including multi-temperature X-ray diffraction experiments, and having got some feeling on the photoswitching potential of the studied systems, solid-state infrared spectroscopy measurements were conducted for **Ni-1a**, **Pd-1a** and **Pd-1a'** to monitor the samples' behavior upon irradiation. In the case of the remaining forms of the Pd compound, *i.e.* **Pd-1a-DCM** and **Pd-1a-CHCl<sub>3</sub>**, the amount of the sample was not sufficient for conducting such experiments. The purity of the **Ni-1a** and **Pd-1a'** powder samples used for thin-films preparation were confirmed using the PXRD technique (Supporting Information).
The conducted IR spectroscopic experiments for the **Pd-1a'** sample showed that there is almost no response to light in this case (Figures S7.16–18, Supporting Information). For **Ni-1a** and **Pd-1a** a series of IR experiments were performed for samples just after being exposed to different LEDs for defined periods of time at selected temperatures. Additionally, the effect of temperature on the samples' IR spectra was monitored in the RT – 100 K range (i.e. during cooling). In the latter case the temperature was gradually reduced and an IR spectrum was collected every 20 K. For both compounds no spectral changes were observed during these experiments which is consistent with the findings based on the multi-temperature X-ray structural analysis (Supporting Information). As far as linkage isomerization of the ambidentate ligands is concerned, it was traced via observation of the intensity changes of selected bands corresponding to vibrations of different nitrite binding modes. These bands were identified based on the theoretically-predicted IR spectra for the considered linkage isomers of **Ni-1a** and **Pd-1a** (Supporting Information) and on the literature reports for this kind of compounds.<sup>24, 38, 41, 81-84</sup>

The band associated with the asymmetric stretching vibration of the nitro-(η¹-N(O)₂) form of the ligand in the **Ni-1a** spectra is located around 1340 cm⁻¹, whereas the two bands at 1350 and 1369 cm⁻¹ can be attributed to the stretching vibrations of the N-O fragment. Similarly, for **Pd-1a** the population changes of the nitro isomer can be tracked using the following bands: 1399 cm⁻¹ linked to the nitro group asymmetrical stretching, as well as 1344 and 1375 cm⁻¹ associated with the N-O fragment stretching vibrations. Naturally the intensity of these bands diminishes while the other binding mode (or binding modes) is being formed. The increase in the population of the light-induced *endo*-nitrito-(η¹-O-N=O) isomer is evidenced in the **Ni-1a** spectra by the appearance of the following bands associated with N=O and N-O fragment stretching vibrations: 1070, 1110, 1157, 1404, 1410 and 1417 cm⁻¹. Additionally, the presence of the *exo*-nitrito-(η¹-O-N=O) form is represented by the appearance of the 1062 cm⁻¹ band attributed to the N-O stretching vibration. In the case of **Pd-1a** solely formation of the *endo*-nitrito binding mode is noted and indicated by the appearance of 1082 and 1425 cm⁻¹ bands, which reflect the N=O and N-O fragment stretching vibrations. According to theoretical calculations, the bands representing the *exo*-nitrito isomer should be located at lower wavenumbers compared to those of the *endo*-nitrito form and they should be visibly separated. The *exo*-nitrito form might also be indicated by one separated band at the
wavenumbers higher than 1425 cm⁻¹. No such signals were detected in the experimental IR spectra.

The ability of the sample to photoisomerization was first examined at 100 K using LEDs in the 405–660 nm wavelength range (Figures S7.7–9, Supporting Information). In the case of **Ni-1a** clear spectral changes, indicating that the isomerization reaction takes place, are noted once the sample is exposed to 405–530 nm LED light. Both *endo*-nitrito(η¹-ONO) and *exo*-nitrito isomers are formed. It is noteworthy that the bands associated with these two forms exhibit the same behavior during irradiation with different wavelength, however, the intensity of the *exo*-nitrito band is noticeably lower. In turn, the other compound, **Pd-1a** shows the best response when irradiated with higher energy wavelengths: 405–470 nm LED light. The only photo-generated form that is clearly distinguished in this case is the *endo*-nitrito isomer.

The optimal irradiation time was also established at 100 K. The **Ni-1a** sample required 30 min. of irradiation using 530 nm LED (71 mW·cm⁻²) to achieve maximum conversion to the *endo*-nitrito and *exo*-nitrito forms (Figure 7a,b). The increase in intensity of the crucial bands associated with both forms proceeded at comparable rate during the successive light exposure steps. Irradiation with a shorter wavelength, 470 nm (217 mW·cm⁻²), produced the maximum population of the *endo*-nitrito form within 8 min. In contrast, for **Pd-1a** when the 530 nm LED light was used, as long as 180 min. of irradiation were needed to achieve the maximum conversion of the nitro ground-state isomer to the *endo*-nitrito form. However, once the 470 nm LED light was applied (higher energy wavelength and greater LED power), the process took again only 8 min. until similar population of the photo-product was built up (Figure 7c,d). The very comparable and short 470 nm LED irradiation times for **Ni-1a** and **Pd-1a** show that these conditions assure fast and efficient isomerization, and under these conditions it is more difficult to catch the differences between both systems. It may also suggest that some other factors become crucial, such as the concentration of the compound in the pellet. Additionally, it is not sure whether in both cases complete conversion to the *endo*-nitrito form is indeed achieved within this time (see the further discussion of photocrystallographic results; it should be noted though that the behavior observed in powder would not necessarily ideally match the behavior in the single-crystal sample). It should be stressed here that a simple comparison of irradiation times is usually not sufficient. Indeed, comparison of the fluence (or energy density) values delivered to the sample is more informative.⁸⁵ For
static experiments this quantity is computed as $Q = P_{\text{avg}}t_{\text{irr}}/A$, where $P_{\text{avg}}$ is the average power, $t_{\text{irr}}$ is the irradiation time and $A$ denotes surface on which the power is delivered to the crystal. In this sense the **Pd-1a** sample needed much more energy to be delivered (ca. 765 J·cm⁻²) than **Ni-1a** (ca. 128 J·cm⁻²) as exemplary estimated for the 530 nm LED light, to achieve similar, near-saturation population of the photo-product. This reflects that the reaction cross section is smaller for **Pd-1a** than for **Ni-1a** under such experimental conditions. Consequently, efficient nitro-to-nitrito transformation is easier achieved for the **Ni-1a** solid-state thin film.

Thermal relaxation of the light-induced products was examined after generation of their maximum populations at 100 K. Then the temperature was gradually raised and the IR spectra were collected every 20 K. Both **Ni-1a** and **Pd-1a** samples show similar behavior, i.e. the photo-generated linkage isomers are stable up to 200 K, whereas visible population decrease is observed at 220 K and full relaxation to the nitro isomer is noted at 240 K (Figure 8).
<figure><img src="image_11.png" /><figcaption>**Figure 7.** IR spectra collected before (green line) and after (blue line) optimal irradiation time with: (a,c) 470 nm LED at 100 K and (b,d) 530 nm LED at 100 K for generating metastable forms in the (a,b) **Ni-1a** and (c,d) **Pd-1a** samples. Thin grey lines correspond to subsequent irradiation points.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 8.** IR spectra collected before (green line), after irradiation (blue line) at 100 K and during temperature relaxation (cyan and red lines) of metastable states in the **Ni-1a** (left panel) and **Pd-1a** (right panel) sample. Thin grey lines correspond to subsequent temperature points during sample heating.</figcaption></figure>

**3.3. Kinetics.** The stability of the *endo*-nitrito form both in **Ni-1a** and **Pd-1a** samples was assessed on the basis of IR spectroscopic data using the first-order kinetics and Arrhenius equation for the temperature dependence of reaction rates. For that purpose, a series of isothermal IR measurements were conducted during the relaxation of the *endo*-nitrito form. The following kinetic parameters were calculated: decay constant (*k*) for each temperature, activation energy (*E*<sub>a</sub>), frequency factor (*k*<sub>0</sub>) and the decay temperature (*T*<sub>d</sub>) which corresponds to the temperature at which the metastable form exists for 10<sup>3</sup> s (Table 7). In the case of the *exo*-nitrito isomer of **Ni-1a** the band corresponding to this form is weak which hampers similar analyzes.

<table><caption>**Table 7.** Kinetic parameters of the *endo*-nitrito form relaxation determined based on the IR spectroscopy results.</caption><thead><tr><th></th><th>Ni-1a</th><th>Pd-1a</th></tr></thead><tbody><tr><td><i>E</i><sub>a</sub> / kJ·mol<sup>-1</sup></td><td>51.4(3)</td><td>74.3(2)</td></tr><tr><td>log(<i>k</i><sub>0</sub>)</td><td>9.3(7)</td><td>14.6(4)</td></tr><tr><td><i>T</i><sub>d</sub> / K</td><td>218(12)</td><td>219(5)</td></tr></tbody></table>
It can be noticed that although $E_a$ and $\log(k_0)$ derived for the *endo*-nitrito form differ between **Pd-1a** and **Ni-1a**, the $T_d$ values, which take into account both activation energy and frequency factor, are very similar. Consequently, it can be concluded that the stability of the *endo*-nitrito form in both compounds is similar.

As far as the mutual stability of the photo-products generated for the nitrite Ni and Pd coordination compounds in the solid state is concerned, based on the available literature it seems there is no general rule. It should also be stressed, however, that there are not many comparative studies of analogous nitrite coordination compounds of these metals and only one treating about isostructural systems of this kind. In the case of the isostructural *trans*-[$Ni(PCy_3)_2(NO_2)_2$] and *trans*-[$Pd(PCy_3)_2(NO_2)_2$] compounds³⁷ the *endo*-nitrito isomer in the nickel complex exhibits visibly lower stability compared to its palladium equivalent, based on the photocrystallographic experimental results. Moreover, in both complexes the relaxation of metastable state occurs at lower temperatures, namely at 130 K for *trans*-[$Ni(PCy_3)_2(NO_2)_2$] and at 180 K for *trans*-[$Pd(PCy_3)_2(NO_2)_2$] (note that in the $Pd^{II}$ complex the *endo*-nitrito form converts back only partially to the nitro isomer) than it is in the current study. According to another work by Warren *et al*.,⁴² the stability of the related nitrito isomers of $Ni^{II}$ and $Pd^{II}$ compounds is quite similar, however, the relaxation of [$Ni(PEt_3)_2(NO_2)_2$] again occurs at lower temperature, 220 K, compared to [$Pd(PPh_3)_2(NO_2)_2$] and [$Pd(AsPh_3)_2(NO_2)_2$], for which the metastable form reverts to the initial state at 240 K. It should be stressed that in the above-mentioned papers kinetic parameters were not evaluated, thus we refer to the experimental observations that were made. In the studied case, although the molecules are analogous, the crystal structures differ significantly, thus the comparison of both systems is not that straightforward. Nevertheless, based on the IR spectroscopic results, the photo-induced linkage isomers in **Ni-1a** and **Pd-1a** are equally stable up to a rather high temperature, almost 240 K, compared to the known systems of this kind.²⁴, ³³, ³⁸, ⁴¹, ⁷⁹, ⁸⁶⁻⁸⁷ In turn, the derived, and more informative, $T_d$ values are close to 220 K.

**3.4. In situ photocrystallography.** In order to obtain a possibly complete picture of the photoswitching behavior of the studied compounds in the solid state, a series of photocrystallographic single-crystal X-ray diffraction experiments were performed for the $Ni^{II}$ and $Pd^{II}$ analogues using both 470 nm (ca. 3.1 mW) and 530 nm LED (ca. 1.4 mW) light (power was always measured at the sample position). For more information on the fluence values see Supporting Information. It was verified how much time and energy is
needed in each case to obtain the near-maximum conversion at 100 and 200 K, monitoring of the induced isomer's population after each light-exposure step, what is the maximum temperature at which the photo-generated isomer can be detected crystallographically, as well as the relaxation rate of the light-induced product at higher temperatures.

As far as single crystals of Ni-1a are concerned, close to 100% nitro-to-*endo*-nitrito conversion can be achieved at 100 K after over 3 h (fluence, *Q*, above 30 J·cm⁻²) of sample exposure to 470–530 nm LED light (Figure 10, Table S4.2). The population build-up is slightly slower when the 530 nm LED is used with respect to the 470 nm LED, however, more efficient once the fluence values compared as shown in Figure 10a. It should also be noted that the impact of shorter LED wavelengths, such as 405 nm or 365 nm, is significantly weaker (series 7 in Table S4.2, Supporting Information). Based on the temperature relaxation data presented in Figure 11, the photo-generated isomers' population remains stable up to over 180 K. At around 200 K some slow population decrease with time is observed (Table S4.5, Supporting Information). The difference between used LEDs is most visible in the beginning of sample irradiation. For instance, after 1 h of irradiation, the populations induced with the 470 and 530 nm LED light differ by nearly 18%, however, this difference diminishes along with further sample exposure to light (Figure 9). After 210 min. of irradiation, the induced populations of the *endo*-nitrito form were estimated to be approximately 97% and 89% and could still be increased if light exposure continued. The *endo*-nitrito population can be generated even faster using either the 470 or 530 nm LED light at 200 K. In such a case, after just 15 min. of irradiation (8 and 4 J·cm⁻² for 470 and 530 nm LED, respectively), the populations of the *endo*-nitrito isomer were estimated to about 60%, which is more than twice as much as during this time at 100 K (Figure 10). After a total of 90 min. (22 and 49 J·cm⁻² for 470 and 530 nm LED, respectively) of irradiation, the *endo*-nitrito isomer's population exceeded 95% and reached a plateau. Further irradiation led to incremental increase of conversion (within experimental error). The difficulties in achieving higher populations at this temperature are likely due to slow but noticeable thermal relaxation (*T*<sub>d</sub> ~220 K). Finally, continuous-irradiation single-crystal X-ray diffraction experiments were conducted at 220 and 230 K with the 470 nm LED. Before starting the measurement, the crystal was irradiated for 2 h and then continuously irradiated during the measurement. The estimated populations of the *endo*-nitrito isomer were still significant reaching about
83% and 55%, respectively (Table S4.2). Subsequently, the sample relaxation to the ground state structure was monitored.

<figure><img src="image_13.png" /></figure>

**Figure 9.** The *endo*-nitrito isomers' population build-up vs. irradiation time (left panel) and fluence (right panel) using 470 and 530 nm LEDs as radiation sources at 100 K and at 200 K. Note that for **Pd-1a** only data for molecule A are presented; molecule B is inert to light.
<figure><img src="image_14.png" /><figcaption>**Figure 10.** Evolution of the metastable linkage isomer's (endo-nitrito form) populations for **Ni-1a**, **Pd-1a** (results of two separate experiments are shown) and **Pd-1a-DCM** observed during the single-crystal sample heating from 100 K to 290 K.</figcaption></figure>

As far as **Pd-1a** is concerned, its behavior upon irradiation is generally similar to that of **Ni-1a**, however, with some notable differences. First of all, only molecule A undergoes isomerization, whereas molecule B remains inert to the light exposure. The *endo*-nitrito isomer of molecule A can be generated both with the 470 and 530 nm LED light. However, significantly more energy is required when using the 530 nm LED to achieve the same population levels as when the 470 nm LED is applied. This agrees with the IR results which indicated 405–470 nm LED light as most effective for the **Pd-1a** solid-state sample. Moreover, in contrast to the behavior of **Ni-1a**, there is almost no difference in the *endo*-nitrito isomer's population build-up rate when **Pd-1a** crystals are irradiated at either 100 or at 200 K. After 30 min. of irradiation with the 470 nm LED (18 J·cm⁻²) at 100 K around 50% of the *endo*-nitrito form is obtained, while after 285 min. (167 J·cm⁻²) the population of about 86% is observed at both temperatures (Figure 9). It should be noted that the quality of the diffraction pattern at 100 K decreases along with the duration of the sample exposure (for details see section 4.5 in Supporting Information). Overall, more energy needs to be delivered to the sample than in the case of **Ni-1a**, assuming similar crystal sizes, but a significant population of the metastable state is still reachable. Similarly to **Ni-1a** the *endo*-nitrito population seems to be stable up to ~200 K. Along with the elevated temperature, also the quality of the diffraction improves. For instance, when the **Pd-1a** single crystal was irradiated with the 530 nm LED at 100 K for 5 h it was problematic to determine isomers' populations, however, after temperature elevation to 180 K the
diffraction pattern's quality increased and the population was estimated to over 80%
(Supporting Information). At 200 K and above some relaxation to the ground-state
structure is observable. By irradiating of the sample continuously at 220 K one can obtain
the *endo*-nitrito population as high as 81%, whereas at 230 K it is about 40% (Table S4.6,
Supporting Information). Similarly to **Ni-1a**, using shorter LED wavelengths, such as 405
nm or 365 nm, significantly lower, or negligible, populations of the photo-induced linkage
isomer are achieved (series 8 in Table S4.3, Supporting Information). In turn, upon the
590 nm LED irradiation no photoswitching was detected. None of these wavelengths
affect the nitro group of the molecule B either.

In the case of **Pd-1a-DCM** similar conditions were tested. Irradiation of the sample
with 470 nm LED light, led to a significant disorder of the nitro group being the result of
the *endo*-nitrito binding mode formation. It was possible to achieve around 78%
population of the *endo*-nitrito isomer at 100 K after 270 min. of sample exposure. Simple
comparison of the conversion efficacy between the two photoswitchable forms of the
palladium compound indicates that isomerization of **Pd-1a-DCM** is less effective when
compared to the **Pd-1a** sample (*e.g.* with 470 nm LED 80% *endo*-nitrito isomer
population is reached with fluence of about 350 J·cm⁻² in the case of **Pd-1a-DCM** and 175
J·cm⁻² for **Pd-1a**). This difference can be, however, to some extent ascribed to the greater
size of the **Pd-1a-DCM** crystal, thus, a limited light penetration depth in this case. The
formation of the photo-induced linkage isomer resulted in breaking of the weak hydrogen
bond between the ground state nitro ligand and DCM species (dimer **B5**). Hence some
disorder in the solvent molecule was observed in the 120–220 K temperature range.
Similarly to **Ni-1a** and **Pd-1a**, the crystal fully relaxes to its initial state around 240 K,
while DCM molecules become well-ordered again. The solvent content does not change
during isomerization and further relaxation of the system.

According to studies by Warren *et al.*³⁷, ⁴² on Ni¹¹ and Pd¹¹ complexes, generation of the nitrito linkage isomers in palladium compounds in general requires longer irradiation times when compared to the nickel equivalents. Furthermore, the obtained populations of the nitrito forms are usually higher in the case of nickel complexes. In this view our findings are in agreement with the general behavior of **Ni-1a** vs. **Pd-1a** samples. However, literature contributions, despite some most recent cases,⁸⁵⁻⁸⁶ usually lack experimental details enabling more solid comparisons based on *Q*-values *etc*.
It should also be noted that as far as the unit-cell parameters are concerned, they do not change much upon isomerization, which is typical for successful single-crystal-to-single-crystal transformations. For instance, at 100 K the volume difference for **Pd-1a**, between 'before' and 'after' the nitro-to-nitrito transformation, reaches 1.1%, while for **Ni-1a** and **Pd-1a-DCM** the respective changes do not exceed *ca.* 3% (Figures S6.6–11, Supporting Information). This is comparable to the unit-cell volume thermal contraction between 290 and 100 K.

**3.5. Endo-nitrito population vs. metal···metal distances.** A closer inspection of structural changes in the analyzed crystal structures upon irradiation resulted in an interesting observation. It appears that the molecules forming stacks, described by the motifs **A4** and **A5** for **Ni-1a** and the analogues motifs **AA10** and **AA9** for **Pd-1a**, move closer together and further apart, respectively, along with the increase of the *endo*-nitrito isomer's population (Figures 11,12). This is well-reflected by the corresponding M···M distances, change of which is strongly correlated with the amount of the photo-generated linkage isomer. For instance, the Ni···Ni distance after irradiation with 470 nm at 100 K in **A4** gets reduced from 3.556(3) to 3.250(2) Å (shortening of 0.306(4) Å) along with the *endo*-nitrito form's population increase from about 6% to 97%. In turn, in the case of **A5** the opposite relationship is observed, and the metal-metal distance elongates from 6.273(3) to 6.648(3) Å (0.375(4) Å difference). Similarly, for **Pd-1a** the Pd···Pd distance gets reduced in **AA10** from 3.749(1) to 3.480(1)Å (shortening of *ca.* 0.27 Å) and elongates in **AA9** from 6.368(3) to 6.675(3) Å (*ca.* 0.31 Å difference) while the *endo*-nitrito form's population rises to *ca.* 85%.

Whereas the shorter M···M distances in **A4** and **AA10** are also affected by the temperature as shown in Figure 12, the longer M···M distances in **A5** and **AA9** are almost independent of temperature. Nevertheless, the effect of isomerization on the distances in all the cases is much stronger than that of temperature (**Ni-1a**, **A4** motif: 3.569(3) Å at 100 K vs. 3.699(3) Å at 290 K; *ca.* 0.130 Å difference; **Pd-1a**, **AA10** motif: 3.738(1) Å at 100 K vs. 3.803(1) Å at 290 K.; *ca.* 0.065 Å difference; the respective differences are of about 2.3 and 4.1 times smaller than the total shortening of these contacts upon the *endo*-nitrito isomer formation). The structural changes have though negligible effect on the respective dimer interaction energies.
<figure><img src="image_15.png" /><figcaption>**Figure 11.** Metal···metal distances in the studied crystal structures: (a) shorter (A4 motif) and longer (A5 motif) Ni···Ni distances within the unit cell of Ni-1a; (b) shorter (AA10 motif) and longer (AA9 motif) Pd···Pd distances within the unit cell of Pd-1a.</figcaption></figure>

Importantly, the M···M distance-population correlation is highly linear as shown in Figure 12. Interestingly, such a linear dependency is also observed for molecules B in motif **BB8** that does not undergo switching, however, the distance change is much less pronounced and amounts to *ca.* 0.08 Å. In the case of the **Pd-1a-DCM** structure the M···M distance linear correlation (coefficient of determination, *R*² > 0.99) with the population of the photo-induced *endo*-nitrito species is also observed (Section 6.2 in Supporting information). In consequence, based on the derived relationship a quick and simple estimation of the isomer's population at a given temperature is possible (Supporting Information). The experimental remarks are in agreement with structural changes predicted theoretically through the periodic computations performed for **Ni-1a** as a test case (Table S6.1, Supporting Information). Indeed, based on the *CRYSTAL* computations the shorter Ni···Ni distance shrinks from 3.570 to 3.234 Å, while the longer metal-metal contact elongates from 6.347 to 6.674 Å once the purely nitro isomer's crystal structure transforms to the *endo*-nitrito isomer.
<figure><img src="image_16.png" /><figcaption>**Figure 12.** M···M distance changes along with the *endo*-nitrito form's population built up in the **Ni-1a** (left panels) and **Pd-1a** (right panels) crystal structures. Red line on every plot denotes best linear fit; in all cases the determination coefficient *R*² > 0.99 (Supporting Information).</figcaption></figure>

**4. Summary and conclusions**

In the current contribution two new analogous square-planar nitrite coordination compounds of Ni<sup>II</sup> and Pd<sup>II</sup> are reported. The former one crystallizes as a single-crystal form, triclinic **Ni-1a**, whereas the palladium derivative tends to form polymorphic (**Pd-1a** and **Pd-1a'**) and solvatomorphic (**Pd-1a-DCM** and **Pd-1a-CHCl<sub>3</sub>**) crystal structures.
Despite different crystal structures, both of them are can efficient photoswitches working
in the 100–240 K temperature range. **Ni-1a**, **Pd-1a** and **Pd-1a-DCM** undergo nitro-to-
*endo*-nitrito isomerization with *ca*. 80–100% conversion upon 470–530 nm LED light
irradiation, whereas the photo-induced linkage isomers are fully stable up to 200 K, at
which their populations’ decline is observed. Nevertheless, significant populations of the
*endo*-nitrito form can be generated in the continuous-irradiation experiments also at 230
K for both **Ni-1a** and **Pd-1a** single crystal samples. The **Pd-1a'** and **Pd-1a-CHCl₃** solid-
state systems appeared to be non-photoactive.

The X-ray diffraction findings for **Ni-1a** and **Pd-1a** are in general consistent with the IR solid-state spectroscopy analysis results. Nevertheless, it should be noted that in the IR spectra in the case of **Ni-1a** some traces of the *exo*-nitrito isomer are observed, which is not detected photocrystallographically. The generated population of the *exo*-nitrito species may just constitute the semi-product of the isomerization reaction towards the *endo*-nitrito formation which would be in agreement with the theoretical predictions made for the square-planar nitrite Ni complexes.<sup>37-38</sup> In turn, for **Pd-1a** no clear signals of the potential *exo*-nitrito isomer were found. Otherwise, both methods indicate that in the case of **Ni-1a** the nearly 95–100% *endo*-nitrito isomers’ population can be achieved faster and it decays slightly slower at temperature 200 K and above when compared to **Pd-1a**. The estimated activation energy *E*<sub>a</sub> is lower for **Ni-1a** and also the energetic differences between various binding modes are notably smaller than for **Pd-1a**. In consequence, for **Ni-1a** traces of the *endo*-nitrito isomer are observed for the ground state structure in the 100 K – RT range, as well as the isomerization reaction cross sections seems greater than for **Pd-1a** in the examined experimental conditions. On the other hand, the kinetic model indicates that the stability of the photoproducts in both cases should be very comparable. Indeed, the metastable temperature for **Ni-1a** and **Pd-1a** in the solid state is around 220 K as indicated by *T*<sub>d</sub> value, which agrees well the experimental results.

There are also two interesting observations which deserve a comment. Firstly, the
M···M distance-population correlation is noted for the photoswitching crystal structures:
**Ni-1a**, **Pd-1a** and **Pd-1a-DCM**. As a result, based on the derived linear relationship a quick
and simple estimation of the isomer composition at 100–200 K temperature range is
possible. Secondly, it is the fact that in the **Pd-1a** crystal structure only one symmetry-
independent molecule undergoes photoisomerization, namely molecule A. Interestingly,
although a larger reaction cavity volume was estimated for molecule B, it is not
photoactive. This shows that the available space should be sufficient but there are other
important factors that need to be considered. Indeed, the shape of the reaction cavity
derived for molecule B is not optimal. Furthermore, the conformation of molecule B in
the crystal lattice is closer to the ideal geometry determined computationally for the
isolated molecule, i.e. the nitro group plane is closer to the plane of the ancillary ligand
than in the molecule A. This observation may suggest that the molecule B is more
energetically advantageous in the crystal structure than A, thus less eager to undergo a
transformation. Furthermore, the nitro group's orientation with respect to the ligand's
plane seems important regarding the first steps of the nitro-to-nitrito transformation
assuming that the mechanism of the process is analogous to that theoretically derived for
square-planar nickel(II) nitrite coordination compounds. In such case, the more
perpendicular orientation of the nitro group to the plane of the (N,N,O)-donor ligand's
plane is preferred, which is also true for **Ni-1a**. These factors may explain the inert
character of the nitrite ligand in the molecule B when compared to the reactive molecule
A. Indeed, also in the non-photoswitchable **Pd-1a'** and **Pd-1a-CHCl₃** crystals the
orientation of the nitro group is very similar to that in molecule B in **Pd-1a**. Obviously,
other factors, such as intermolecular interactions or the reaction-cavity volume and
shape, which is e.g. smallest in the case of **Pd-1a-CHCl₃**, while large but of improper shape
for **Pd-1a'** as opposite to **Pd-1a-DCM**, are also very important. In this view, the current
study contributes to our understanding of nitro-to-nitrito processes and factors
governing molecular switching in the solid state.

**Associated content**

The Supporting Information is available free of charge at [to be filled in by the
editorial office]. Additional data collection information, structural information, DFT
computations (molecular vibrations, interaction energies, molecular energies, basis set
dependence etc.), PXRD patterns, NMR spectra, additional IR absorption spectra under
various conditions before and after irradiation, results of crystallizations.
### Accession codes

CCDC 2369023-5, 2369303-48, 2369447-65, 2369467-81, 2369483-509, 2369516-28, 2369538-48 & 2369561-601, 2393915-43 & 2393996-8 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

### Notes

The authors declare no competing financial interest.

### Acknowledgements

Authors thank the National Science Centre in Poland (SONATA BIS grant No. 2020/38/E/ST4/00400 – K.P., P.B. & K.N.J.; PRELUDIUM BIS grant No. 2019/35/O/ST4/04197 – K.A.D. & K.N.J.) and Polish National Agency for Academic Exchange (grant No. PPN/STA/2021/1/00050/U/00001 – K.A.D & K.N.J.) for financial support. Financial support from the National Research Agency in France (grant No. ANR-21-CE30-0045-01) is gratefully acknowledged. The Wrocław Centre for Networking and Supercomputing (grant No. 285) is 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 (program No. POIG.02.01.00-14.122/09). We thank K. Durka (Warsaw, Poland) for help in NMR and elemental analyzes, and S.E. Kutniewska (Warsaw, Poland) for some preliminary studies.
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Comparative analysis of photoswitching properties of analogous nickel(II) and palladium(II) nitrite complexes in the solid state

Kacper Paszczyk, Krystyna A. Deresz, Radosław Kamiński, Patryk Borowski, Dominik Schaniel, Adam Krówczyński, Katarzyna N. Jarzembska

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

**Synopsis:** Two analogous square-planar nitrite NiII and PdII photoswitches are reported. The nickel compound forms one type of crystal structure, whereas the palladium derivative a set of polymorphs and solvatomorphs. Different photoreactivity of the obtained crystal forms can be explained by the energetic factors, orientation of the nitro ligand and the shape of the reaction cavity. Interestingly, some strong correlation between the M···M distance and the population of the *endo*-nitrito linkage isomer is noted for the photoswitchable crystalline forms of the studied compounds.
SUPPORTING INFORMATION

Comparative analysis of photoswitching properties of analogous
nickel(II) and palladium(II) nitrite complexes in the solid state

Kacper Paszczyk,ᵃ Krystyna A. Deresz,ᵃ Radosław Kamiński,ᵃ Patryk Borowski,ᵃ
Dominik Schaniel,ᵇ Adam Krówczyński,ᵃ Katarzyna N. Jarzembska a*

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

<footnote>ᵇ Université de Lorraine, CNRS, CRM², 54500 Nancy, France</footnote>

<footnote>* Corresponding author: Katarzyna N. Jarzembska (katarzyna.jarzembska@uw.edu.pl)</footnote>
Table of Contents

<table><tr><td>1. Synthesis</td><td>3</td></tr><tr><td>2. Nuclear magnetic resonance</td><td>4</td></tr><tr><td>2.1. NMR spectra</td><td>4</td></tr><tr><td>2.2. Dynamical effects</td><td>6</td></tr><tr><td>3. Crystallization</td><td>7</td></tr><tr><td>4. X-ray crystallography</td><td>10</td></tr><tr><td>4.1. Basic structures, polymorphs and solwatomorphs</td><td>10</td></tr><tr><td>4.2. Fluence calculations</td><td>11</td></tr><tr><td>4.3. Summary of all performed experiments</td><td>12</td></tr><tr><td>4.4. Short summary of population values vs. temperature</td><td>24</td></tr><tr><td>4.5. Symmetry breaking</td><td>24</td></tr><tr><td>5. Powder diffraction</td><td>25</td></tr><tr><td>6. Structural analysis</td><td>27</td></tr><tr><td>6.1. Packing, geometrical and energetic parameters</td><td>27</td></tr><tr><td>6.2. Metal-metal distances vs. endo-nitrito isomer's population</td><td>32</td></tr><tr><td>6.3. Unit-cell parameters vs. population and temperature</td><td>34</td></tr><tr><td>7. Solid-state IR spectroscopy</td><td>40</td></tr><tr><td>7.1. Tables with numerical data</td><td>40</td></tr><tr><td>7.2. Additional IR spectra</td><td>41</td></tr><tr><td>8. Computational part</td><td>51</td></tr><tr><td>9. Arrhenius plots</td><td>53</td></tr></table>
**1. Synthesis**

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

**Scheme S1.1.** Schematic representation of the synthetic route; M = Ni or Pd for **Ni-1a** and **Pd-1a**, respectively.

<figure><img src="image_19.png" /></figure>

**Scheme S1.2.** Schematic representation of the compounds studied in the previous contributions (**M-2a** and **M-2b** – ref. 41, **QTNiNO₂** – ref. 24; M = Ni or Cu).
## 2. Nuclear magnetic resonance

### 2.1. NMR spectra.

<figure><img src="image_20.png" /><figcaption>**Figure S2.1.** ¹H NMR spectrum of compound Pd-1a (400 MHz, DMSO-d₆).</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure S2.2.** ¹³C{¹H} NMR spectrum of compound Pd-1a (100.6 MHz, DMSO-d₆).</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S2.3.** ¹H NMR spectrum of compound Pd-1a (400 MHz, CDCl₃).</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Figure S2.4.** ¹H NMR spectrum of compound Ni-1a (400 MHz, DMSO-d₆).</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S2.5.** ¹H NMR spectrum of compound **Ni-1a** (400 MHz, CDCl₃).</figcaption></figure>

**2.2. Dynamical effects.** The spectrum in DMSO shows very strong dynamics which could be related either to the switching of the nitrite ligand or the exchange of ligand with DMSO. Signals are visible up to 11.5 ppm, in a wider range no more signals are detected. In CDCl₃ this effects is notably weaker, however, signals are still slightly broadened which may suggest that there is indeed some kind of dynamic behavior in the vicinity of the Ni center.
### 3. Crystallization

In the case of **Ni-1a** small needle crystals were obtained directly from synthesis, not suitable for single-crystal X-ray diffraction measurements using a home diffractometer. For **Pd-1a**, yellow powder was received from synthesis. Therefore, crystallizations from various solvents were carried out using the slow solvent evaporation method.

In each case, a small amount of the compound was placed in a glass vial. Then a given solvent was added and the mixture was gently heated until the contents were completely dissolved. Various solvents, both polar and non-polar, were used (Table S3.1). The vials were covered with parafilm and left on a shelf until the solvent evaporated.

In the case of the nickel complex, **Ni-1a**, the same crystalline form was always obtained despite the solvent used, but the crystals varied in quality and size. For **Pd-1a**, structures of two polymorphs (**Pd-1a** and **Pd-1a'** = **Pd-1a-prim** code in data the collection table and CIF file) and two solvatomorphs were evaluated (**Pd-1a-DCM**, **Pd-1a-CHCl3** = **Pd-1a-CHCl3** code in the data collection table and CIF file). The highest-quality and most reproducible crystallizations of the dominant forms were achieved using dichloromethane (DCM), methanol, and ethanol. A structure containing DCM within its framework was also obtained from the DCM solution, however, in smaller quantities (**Pd-1a-DCM** crystals can be distinguished by their block shape and significantly larger size compared to **Pd-1a** crystals). Additionally few crystals of a second polymorphic form (**Pd-1a'**) were found in this batch. The evaluated crystal structure matched the structure of the polycrystalline powder obtained straight after the synthesis as confirmed by PXRD analysis (see the PXRD results below). These crystals were of lower quality and appeared to be non-photoactive. In the case of chloroform solution, thin needle-like crystals of a solvatomorphic form were received (**Pd-1a-CHCl3**). The photos of the filtered and dried products of the synthesis are presented in Figure S3.1. The crystallization results summary is gathered in Table S3.1.
<figure><img src="image_25.png" /></figure>

**Figure S3.1.** Filtered and dried products of the synthesis; **Ni-1a** on the left and **Pd-1a** on the right.

**Table S3.1.** Results of crystallizations of both compounds carried out by slow solvent evaporation method.

<figure><img src="image_26.png" /></figure>
<figure><img src="image_27.png" /><figcaption>**Table S3.1 (continued).** Result of crystallization of both compounds carried out using a slow solvent evaporation method.</figcaption></figure>
# 4. X-ray crystallography

## 4.1. Basic structures, polymorphs and solwatomorphs.

<table><caption>**Table S4.1.** Selected X-ray data collection, processing and refinement parameters for the **Ni-1a** and **P1-1a** crystal structures, together with the 2nd polymorph (**Pd-1a-prim**) and two solwatomorphs (**Pd-1a-DCM**, **Pd-1a-CHCl3**) of the latter compound. For data collection codes see further tables and information therein.</caption><thead><tr><th>Data set</th><th>Ni-1a-dark-100K-10-xtal1</th><th>Pd-1a-dark-100K-17-xtal1</th><th>Pd-1a-prim</th><th>Pd-1a-DCM-dark-100K-08-xtal2</th><th>Pd-1a-CHCl3</th></tr></thead><tbody><tr><td>Remark</td><td>base structure used in this study</td><td>base structure used in this study</td><td>2nd polymorph of Pd-1a (Pd-1a')</td><td>Pd-1a solwatomorph with DCM (Pd-1a-DCM)</td><td>Pd-1a solwatomorph with CHCl<sub>3</sub> (Pd-1a-CHCl<sub>3</sub>)</td></tr><tr><td>Moiety formula</td><td>C<sub>15</sub>H<sub>13</sub>NiN<sub>3</sub>O<sub>3</sub></td><td>C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub></td><td>C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub></td><td>2 (C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub>) + ca. 0.9 CH<sub>2</sub>Cl<sub>2</sub></td><td>C<sub>15</sub>H<sub>13</sub>PdN<sub>3</sub>O<sub>3</sub> + CHCl<sub>3</sub></td></tr><tr><td>Formula mass, M<sub>r</sub> / a.u.</td><td>341.98</td><td>389.69</td><td>389.69</td><td>856.02</td><td>509.05</td></tr><tr><td>Crystal system</td><td>triclinic</td><td>monoclinic</td><td>monoclinic</td><td>monoclinic</td><td>monoclinic</td></tr><tr><td>Space group</td><td>P<span style="text-decoration: overline;">1</span>(No. 2)</td><td>P2<sub>1</sub>/n (No. 14)</td><td>P2<sub>1</sub>/c (No. 14)</td><td>I<sub>2</sub>/a (No. 15)</td><td>P2<sub>1</sub>/c (No. 14)</td></tr><tr><td>Z</td><td>2</td><td>8</td><td>4</td><td>4</td><td>4</td></tr><tr><td>F<sub>000</sub></td><td>352</td><td>1552</td><td>776</td><td>1703</td><td>1008</td></tr><tr><td>Crystal color and shape</td><td>red cut needle</td><td>brown block</td><td>yellow cut block</td><td>yellow cut block</td><td>yellow needle</td></tr><tr><td>Crystal size / mm<sup>3</sup></td><td>0.09×0.16×0.20</td><td>0.08×0.11×0.11</td><td>0.07×0.09×0.19</td><td>0.15×0.20×0.23</td><td>0.04×0.05×0.36</td></tr><tr><td>Temperature, T / K</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td></tr><tr><td>a / Å</td><td>8.126(2)</td><td>13.965(3)</td><td>11.551(2)</td><td>11.462(2)</td><td>13.722(3)</td></tr><tr><td>b / Å</td><td>8.496(2)</td><td>9.219(2)</td><td>11.095(2)</td><td>11.716(2)</td><td>6.795(2)</td></tr><tr><td>c / Å</td><td>10.681(2)</td><td>21.774(4)</td><td>11.633(2)</td><td>22.745(5)</td><td>19.766(4)</td></tr><tr><td>α / °</td><td>93.95(3)</td><td>90</td><td>90</td><td>90</td><td>90</td></tr><tr><td>β / °</td><td>112.21(3)</td><td>96.82(3)</td><td>105.73(3)</td><td>92.96(3)</td><td>93.37(3)</td></tr><tr><td>γ / °</td><td>91.03(3)</td><td>90</td><td>90</td><td>90</td><td>90</td></tr><tr><td>V / Å<sup>3</sup></td><td>680.3(3)</td><td>2783.4(10)</td><td>1435.1(5)</td><td>3050.2(10)</td><td>1839.8(8)</td></tr><tr><td>d<sub>calc</sub> / g·cm<sup>-3</sup></td><td>1.670</td><td>1.860</td><td>1.804</td><td>1.864</td><td>1.838</td></tr><tr><td>θ range</td><td>5.89–75.91°</td><td>3.58–73.34°</td><td>3.98–75.95°</td><td>3.89–76.07°</td><td>4.48–72.91°</td></tr><tr><td>Absorption coefficient, μ / mm<sup>-1</sup></td><td>2.219</td><td>10.918</td><td>10.588</td><td>11.447</td><td>12.349</td></tr><tr><td>No. of reflections collected / unique</td><td>10922 / 2799</td><td>21756 / 5505</td><td>15318 / 2977</td><td>15798 / 3174</td><td>15620 / 3615</td></tr><tr><td>R<sub>int</sub></td><td>2.61%</td><td>3.26%</td><td>6.11%</td><td>2.63%</td><td>5.75%</td></tr><tr><td>No. of reflections with I > 3σ(I)</td><td>2581</td><td>4735</td><td>2152</td><td>2937</td><td>2943</td></tr><tr><td>No. of parameters / restraints / constraints</td><td>206 / 0 / 78</td><td>397 / 0 / 104</td><td>199 / 0 / 52</td><td>217 / 1 / 55</td><td>235 / 0 / 56</td></tr><tr><td>R[F] (I > 3σ(I))</td><td>2.61%</td><td>2.40%</td><td>3.39%</td><td>2.06%</td><td>4.69%</td></tr><tr><td>wR[F<sup>2</sup>] (all data)</td><td>7.32%</td><td>5.98%</td><td>7.34%</td><td>5.79%</td><td>11.28%</td></tr><tr><td>ρ<sub>res</sub><sup>min/max</sup> / e·Å<sup>-3</sup></td><td>-0.33 / +0.34</td><td>-0.47 / +0.49</td><td>-0.69 / +0.73</td><td>-0.68 / +1.84</td><td>-1.18 / +1.95</td></tr><tr><td>CCDC code</td><td>2369447</td><td>2369330</td><td>2369023</td><td>2393942</td><td>2369025</td></tr></tbody></table>
**4.2. Fluence calculations.** Fluence, or energy density, is defined as:

$$Q = \frac{P \cdot t}{A},$$

where P is power, t is time and A is the surface, through the light is delivered (note that $E = P \cdot t$ is the total energy delivered). For our static IR or photocrystallographic experiments we consider average power measured via the powermeter at the sample position, $P_{avg}$, and the irradiation time, $t_{irr}$. The fluence of the LED will then be $Q_{beam}(t_{irr}) = P_{avg}t_{irr}/A_{beam}$. However, in the case of single-crystal photocrystallographic experiments, the spot of the beam is much larger than the crystals used. Furthermore, we used crystal of somewhat different sizes and examined two different compounds. Thus, for comparison purposes we estimated the fluence delivered onto the sample as follows below.

(i) Firstly, the beam size at the sample position was measured with the fluorescent screen and this gave a diameter $d_{beam} = 0.89$ Å and consequently a surface area as follows

$$A_{beam} = \frac{\pi d_{beam}^2}{4} = 0.63 \text{ mm}^2.$$

(ii) For each crystal a movie was collected and the faces were ascribed (including crystals with rather uneven sizes, e.g. cut ones). Based on that the CRYALISPRO software provided us with the volume of the crystal, $V_{cry}$. All our crystals were rather small (much smaller than the LED beam) and with quite comparable dimensions. It is then reasonable to approximate them as small spheres with the equivalent radius of

$$R_{cry} = \left( \frac{3V_{cry}}{4\pi} \right)^{1/3}.$$

Given this radius we computed the average area through the power is delivered: $A_{cry} = \pi R_{cry}^2$.

(iii) Finally, the fluence for a given crystal is computed as

$$Q_{cry} = Q_{beam} \left( \frac{A_{cry}}{A_{beam}} \right).$$

Some notes must follow here. We are fully aware this approach is only approximate, and does not take into account, for example, uneven primary LED-light spatial profile, uneven crystal area irradiated while the crystal is being rotated etc. However, this basic approach gives fluence values within reasonable ranges allowing for further comparisons, the results of which are concurrent with IR studies. The approach will be refined in the nearest future.
### 4.3. Summary of all performed experiments.

**Table S4.2.** Summary of all collected data sets for the **Ni-1a** crystal structure. The codes correspond to the CIF files' names and their data blocks; *T* – temperature of the measurement, *P* – population of the *endo*-nitrito isomer. Notation used: last two entries are always temperature of the measurement and data collection number, dark – data set without irradiation, irr – data set after irradiation (temperature of irradiation, time, LED wavelength), cirr – same as “irr” but the irradiation continued during the entire data collection. Note the order of data collections in the table does not always reflect the order in which they are discussed in the main text.

<table><thead><tr><th>Data collection code</th><th>T / K</th><th>Remarks</th><th>P%</th></tr></thead><tbody><tr><td colspan="4">Crystal 1, multi-temperature series (series 1)</td></tr><tr><td>Ni-1a-dark-290K-01-xtal1</td><td>290</td><td>Crystal mounted and centered at RT, T set to 290 K, lights switched off.</td><td>6.8(3)</td></tr><tr><td>Ni-1a-dark-cooling-240K-02-xtal1</td><td>240</td><td>Cooling data collection.</td><td>3.4(3)</td></tr><tr><td>Ni-1a-dark-cooling-230K-03-xtal1</td><td>230</td><td>Cooling data collection.</td><td>2.8(3)</td></tr><tr><td>Ni-1a-dark-cooling-220K-04-xtal1</td><td>220</td><td>Cooling data collection.</td><td>2.4(3)</td></tr><tr><td>Ni-1a-dark-cooling-210K-05-xtal1</td><td>210</td><td>Cooling data collection.</td><td>2.4(2)</td></tr><tr><td>Ni-1a-dark-cooling-200K-06-xtal1</td><td>200</td><td>Cooling data collection.</td><td>2.5(3)</td></tr><tr><td>Ni-1a-dark-cooling-190K-07-xtal1</td><td>190</td><td>Cooling data collection.</td><td>3.1(2)</td></tr><tr><td>Ni-1a-dark-cooling-180K-08-xtal1</td><td>180</td><td>Cooling data collection.</td><td>3.1(2)</td></tr><tr><td>Ni-1a-dark-cooling-150K-09-xtal1</td><td>150</td><td>Cooling data collection.</td><td>3.2(2)</td></tr><tr><td>Ni-1a-dark-100K-10-xtal1</td><td>100</td><td>Dark data collected at 100 K.</td><td>3.4(2)</td></tr><tr><td>Ni-1a-dark-heating-150K-11-xtal1</td><td>150</td><td>Heating data collection.</td><td>3.8(2)</td></tr><tr><td>Ni-1a-dark-heating-180K-12-xtal1</td><td>180</td><td>Heating data collection.</td><td>4.2(2)</td></tr><tr><td>Ni-1a-dark-heating-190K-13-xtal1</td><td>190</td><td>Heating data collection.</td><td>4.4(2)</td></tr><tr><td>Ni-1a-dark-heating-200K-14-xtal1</td><td>200</td><td>Heating data collection.</td><td>5.1(2)</td></tr><tr><td>Ni-1a-dark-heating-210K-15-xtal1</td><td>210</td><td>Heating data collection.</td><td>5.2(2)</td></tr><tr><td>Ni-1a-dark-heating-220K-16-xtal1</td><td>220</td><td>Heating data collection.</td><td>4.1(3)</td></tr><tr><td>Ni-1a-dark-heating-230K-17-xtal1</td><td>230</td><td>Heating data collection.</td><td>2.9(3)</td></tr><tr><td>Ni-1a-dark-heating-240K-18-xtal1</td><td>240</td><td>Heating data collection.</td><td>3.5(3)</td></tr><tr><td>Ni-1a-dark-290K-19-xtal1</td><td>290</td><td>Dark data collected at 290 K.</td><td>6.4(3)</td></tr><tr><td colspan="4">Crystal 2, photocrystallography at 100 K, 470 nm LED with 3.1 mW (series 2)</td></tr><tr><td>Ni-1a-dark-100K-01-xtal2</td><td>100</td><td>Crystal cooled down to 100 K.</td><td>5.9(6)</td></tr><tr><td>Ni-1a-irr-100K-15min-470nm-100K-02-xtal2</td><td>100</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>28.2(6)</td></tr><tr><td>Ni-1a-irr-100K-30min-470nm-100K-03-xtal2</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>47.4(8)</td></tr></tbody></table>
<table><tr><td>Ni-1a-irr-100K-45min-470nm-100K-04-xtal2</td><td>100</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>58.0(8)</td></tr><tr><td>Ni-1a-irr-100K-1h-470nm-100K-05-xtal2</td><td>100</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>69.7(6)</td></tr><tr><td>Ni-1a-irr-100K-90min-470nm-100K-06-xtal2</td><td>100</td><td>Data collection after 90 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>82.9(4)</td></tr><tr><td>Ni-1a-irr-100K-2h-470nm-100K-07-xtal2</td><td>100</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>89.1(3)</td></tr><tr><td>Ni-1a-irr-100K-150min-470nm-100K-08-xtal2</td><td>100</td><td>Data collection after 150 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>92.6(3)</td></tr><tr><td>Ni-1a-irr-100K-210min-470nm-100K-09-xtal2</td><td>100</td><td>Data collection after 210 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>96.5(2)</td></tr><tr><td>Ni-1a-irr-100K-390min-470nm-290K-11-xtal2</td><td>290</td><td>Data collection after 390 min. of irradiation of the rotating crystal with the 470 nm LED at 100 K and rising T to RT.</td><td>7.9(4)</td></tr></table>

Crystal 2, photocrystallography at 200 K, 470 nm LED with 3.1 mW (series 3)

<table><tr><td>Ni-1a-dark-200K-12-xtal2</td><td>200</td><td>T set to 200 K, crystal mounted and centered at 200 K, lights switched off.</td><td>5.1(3)</td></tr><tr><td>Ni-1a-irr-200K-15min-470nm-200K-13-xtal2</td><td>200</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>60.3(8)</td></tr><tr><td>Ni-1a-irr-200K-30min-470nm-200K-14-xtal2</td><td>200</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>80.0(5)</td></tr><tr><td>Ni-1a-irr-200K-45min-470nm-200K-15-xtal2</td><td>200</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>89.3(4)</td></tr><tr><td>Ni-1a-irr-200K-1h-470nm-200K-16-xtal2</td><td>200</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>92.8(4)</td></tr><tr><td>Ni-1a-irr-200K-90min-470nm-200K-17-xtal2</td><td>200</td><td>Data collection after 90 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>96.1(4)</td></tr><tr><td>Ni-1a-irr-200K-2h-470nm-200K-18-xtal2</td><td>200</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>96.9(4)</td></tr><tr><td>Ni-1a-irr-200K-3h-470nm-200K-19-xtal2</td><td>200</td><td>Data collection after 3 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>97.3(4)</td></tr></table>
Crystal 2, photocrystallography at 200 K, 530 nm LED with 1.4 mW (series 4)
(this series contains experiments where crystal was continuously irradiated during entire experiment)

<table><tr><td>Ni-1a-dark-200K-20-xtal2</td><td>200</td><td>T set to 200 K, crystal mounted and centered at 200 K, lights switched off. Slightly higher population than expected based on multi-temperature series results due to centering performed at 200 K, which required using the diffractometer light and induced already some isomerization.</td><td>9.5(3)</td></tr><tr><td>Ni-1a-irr-200K-15min-530nm-200K-21-xtal2</td><td>200</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>60.5(7)</td></tr><tr><td>Ni-1a-irr-200K-30min-530nm-200K-22-xtal2</td><td>200</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>74.7(4)</td></tr><tr><td>Ni-1a-irr-200K-45min-530nm-200K-23-xtal2</td><td>200</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>84.9(4)</td></tr><tr><td>Ni-1a-irr-200K-1h-530nm-200K-24-xtal2</td><td>200</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>89.6(3)</td></tr><tr><td>Ni-1a-irr-200K-90min-530nm-200K-25-xtal2</td><td>200</td><td>Data collection after 90 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>94.1(4)</td></tr><tr><td>Ni-1a-irr-200K-2h-530nm-200K-26-xtal2</td><td>200</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>95.3(3)</td></tr><tr><td>Ni-1a-irr-200K-3h-530nm-200K-27-xtal2</td><td>200</td><td>Data collection after 3 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>96.6(3)</td></tr><tr><td>Ni-1a-cirr-200K-4h-530nm-200K-28-xtal2</td><td>200</td><td>Data collection after 4 h of irradiation of the rotating crystal with the 530 nm LED and continuous irradiation during the measurement.</td><td>97.6(4)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-200K-29-xtal2</td><td>200</td><td>Time relaxation data collection.</td><td>96.4(3)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-200K-30-xtal2</td><td>200</td><td>Time relaxation data collection.</td><td>93.5(3)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-200K-31-xtal2</td><td>200</td><td>Time relaxation data collection.</td><td>91.5(3)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-200K-32-xtal2</td><td>200</td><td>Time relaxation data collection.</td><td>89.3(3)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-215K-33-xtal2</td><td>215</td><td>Temperature relaxation data collection.</td><td>58.4(7)</td></tr><tr><td>Ni-1a-irr-200K-400min-530nm-215K-34-xtal2</td><td>215</td><td>Time relaxation data collection.</td><td>34.5(5)</td></tr></table>
Crystal 2, photocrystallography at 100 K, 530 nm LED with 1.4 mW (series 5)

<table><tr><td>Ni-1a-dark-100K-36-xtal2</td><td>100</td><td>Crystal mounted and centered at RT, lights switched off, T set to 100 K.</td><td>4.2(3)</td></tr><tr><td>Ni-1a-irr-100K-15min-530nm-100K-37-xtal2</td><td>100</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>19.1(5)</td></tr><tr><td>Ni-1a-irr-100K-30min-530nm-100K-38-xtal2</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>33.7(6)</td></tr><tr><td>Ni-1a-irr-100K-45min-530nm-100K-39-xtal2</td><td>100</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>43.4(7)</td></tr><tr><td>Ni-1a-irr-100K-1h-530nm-100K-40-xtal2</td><td>100</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>51.9(7)</td></tr><tr><td>Ni-1a-irr-100K-90min-530nm-100K-41-xtal2</td><td>100</td><td>Data collection after 90 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>66.0(6)</td></tr><tr><td>Ni-1a-irr-100K-2h-530nm-100K-42-xtal2</td><td>100</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>75.4(6)</td></tr><tr><td>Ni-1a-irr-100K-150min-530nm-100K-43-xtal2</td><td>100</td><td>Data collection after 150 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>81.1(6)</td></tr><tr><td>Ni-1a-irr-100K-210min-530nm-100K-44-xtal2</td><td>100</td><td>Data collection after 210 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>88.2(4)</td></tr><tr><td>Ni-1a-irr-100K-390min-530nm-100K-45-xtal2</td><td>100</td><td>Data collection after 390 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>96.3(3)</td></tr><tr><td>Ni-1a-irr-100K-390min-530nm-150K-46-xtal2</td><td>150</td><td>Temperature relaxation data collection.</td><td>96.2(4)</td></tr><tr><td>Ni-1a-irr-100K-390min-530nm-180K-47-xtal2</td><td>180</td><td>Temperature relaxation data collection.</td><td>96.2(4)</td></tr></table>

Crystal 2, photocrystallography at 180, 220, 230 K, 470 nm LED with 3.1 mW (series 6)
(this series contains experiments where crystal was continuously irradiated during entire experiment)

<table><tr><td>Ni-1a-dark-220K-49-xtal2</td><td>220</td><td>T set to 220 K, crystal mounted and centered at 220 K, lights switched off.</td><td>3.9(3)</td></tr><tr><td>Ni-1a-cirr-220K-2h-470nm-220K-50-xtal2</td><td>220</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED and continuous irradiation during the measurement.</td><td>82.6(5)</td></tr></table>
<table><tr><td>Ni-1a-irr-220K-290min-470nm-220K-51-xtal2</td><td>220</td><td>Data collection after 290 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>46.5(6)</td></tr><tr><td>Ni-1a-irr-220K-290min-470nm-220K-52-xtal2</td><td>220</td><td>Time relaxation data collection.</td><td>15.9(3)</td></tr><tr><td>Ni-1a-irr-220K-290min-470nm-220K-53-xtal2</td><td>220</td><td>Time relaxation data collection.</td><td>7.4(4)</td></tr><tr><td>Ni-1a-dark-230K-54-xtal2</td><td>230</td><td>Crystal heated to 280 K and cooled down to 230 K.</td><td>4.4(4)</td></tr><tr><td>Ni-1a-cirr-230K-2h-470nm-230K-55-xtal2</td><td>230</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED and continuous irradiation during measurement.</td><td>55.1(7)</td></tr><tr><td>Ni-1a-irr-230K-290min-470nm-230K-56-xtal2</td><td>230</td><td>Time relaxation data collection.</td><td>10.6(5)</td></tr><tr><td>Ni-1a-irr-230K-290min-470nm-230K-57-xtal2</td><td>230</td><td>Time relaxation data collection.</td><td>4.2(3)</td></tr><tr><td>Ni-1a-dark-180K-58-xtal2</td><td>180</td><td>Crystal heated to 280 K and cooled down to 180 K.</td><td>4.5(3)</td></tr><tr><td>Ni-1a-irr-180K-3h-470nm-180K-59-xtal2</td><td>180</td><td>Data collection after 3 h of irradiation of the rotating crystal with the 470 nm LED.</td><td>97.6(4)</td></tr><tr><td>Ni-1a-cirr-180K-4h-470nm-180K-60-xtal2</td><td>180</td><td>Data collection after 4 h of irradiation of the rotating crystal with the 470 nm LED and continuous irradiation during the measurement.</td><td>98.1(4)</td></tr><tr><td>Ni-1a-irr-180K-410min-470nm-180K-61-xtal2</td><td>180</td><td>Time relaxation data collection.</td><td>98.2(4)</td></tr><tr><td>Ni-1a-irr-180K-410min-470nm-180K-62-xtal2</td><td>180</td><td>Time relaxation data collection.</td><td>98.3(4)</td></tr><tr><td>Ni-1a-irr-180K-410min-470nm-180K-63-xtal2</td><td>180</td><td>Time relaxation data collection.</td><td>97.5(4)</td></tr></table>

Crystal 2, photocrystallography at 100 K, 405 and 365 nm LED (power not measured, maximal current, test series (series 7))

<table><tr><td>Ni-1a-dark-100K-64-xtal2</td><td>100</td><td>T set to 100 K, Crystal mounted and centered at 290 K, lights switched off.</td><td>5.0(3)</td></tr><tr><td>Ni-1a-irr-100K-30min-405nm-100K-65-xtal2</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 405 nm LED.</td><td>20.1(4)</td></tr><tr><td>Ni-1a-irr-100K-30min-365nm-100K-66-xtal2</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 365 nm LED. Measurement performed just after the previous one.</td><td>22.6(4)</td></tr></table>

Crystal 3, photocrystallography at 100 K, 530 nm LED with 1.4 mW (series 8)

<table><tr><td>Ni-1a-dark-100K-01-xtal3</td><td>100</td><td>Crystal mounted and centered at 290 K, T set to 100 K, lights switched off.</td><td>4.0(4)</td></tr><tr><td>Ni-1a-irr-100K-1h-530nm-100K-02-xtal3</td><td>100</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>51.3(8)</td></tr><tr><td>Ni-1a-irr-100K-150min-530nm-100K-03-xtal3</td><td>100</td><td>Data collection after 150 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>81.3(4)</td></tr></table>
<table><caption>**Table S4.3.** Summary of all collected data sets for the **Pd-1a** crystal structure. The codes correspond to the CIF files' names and their data blocks; *T* – temperature of the measurement, *P* – population of the *endo*-nitrito isomer (molecule A only). Notation used: last two entries are always temperature of the measurement and data collection number, dark – data set without irradiation, irr – data set after irradiation (temperature of irradiation, time, LED wavelength), *cirr* – same as “irr” but the irradiation continued during the entire data collection. Superscript [a] denotes zero population values, as there was no *endo*-nitrito isomer detected and it was not included in the final refinement model. Superscript [b] denotes exactly 100% population of the *endo*-nitrito isomer. Note the order of data collections in the table does not always reflect the order in which they are discussed in the main text.</caption><tr><td>Ni-1a-irr-100K-210min-530nm-100K-04-xtal3</td><td>100</td><td>Data collection after 210 min. of irradiation of the rotating crystal with the 530 nm LED.</td><td>89.5(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-100K-05-xtal3</td><td>100</td><td>Data collection after 5 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>95.2(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-150K-06-xtal3</td><td>150</td><td>Temperature relaxation data collection.</td><td>95.0(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-180K-07-xtal3</td><td>180</td><td>Temperature relaxation data collection.</td><td>95.1(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-190K-08-xtal3</td><td>190</td><td>Temperature relaxation data collection.</td><td>94.5(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-200K-09-xtal3</td><td>200</td><td>Temperature relaxation data collection.</td><td>93.5(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-210K-10-xtal3</td><td>210</td><td>Temperature relaxation data collection.</td><td>84.1(5)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-230K-12-xtal3</td><td>230</td><td>Temperature relaxation data collection.</td><td>4.4(3)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-240K-13-xtal3</td><td>240</td><td>Temperature relaxation data collection.</td><td>3.0(4)</td></tr><tr><td>Ni-1a-irr-100K-5h-530nm-290K-14-xtal3</td><td>290</td><td>Temperature relaxation data collection.</td><td>6.4(3)</td></tr></table>

<table><thead><tr><th>Data collection code</th><th>T / K</th><th>Remarks</th><th>P%</th></tr></thead><tbody><tr><td colspan="4">Crystal 1, photocrystallography at 100 K, 530 nm LED with 1.4 mW (series 1)</td></tr><tr><td>Pd-1a-dark-290K-01-xtal1</td><td>290</td><td>T set to 290 K, crystal mounted and centered at 290 K, lights switched off.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-200K-02-xtal1</td><td>200</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-100K-03-xtal1</td><td>100</td><td>Cooling data collection.</td><td>0 [a]</td></tr></tbody></table>
<table><tr><td>Pd-1a-irr-100K-530nm-1h-100K-04-xtal1</td><td>100</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>48.1(5)</td></tr><tr><td>Pd-1a-irr-100K-530nm-2h-100K-05-xtal1</td><td>100</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>68(1)</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-150K-10-xtal1</td><td>150</td><td>T set to 150 K. Data collection after 5 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>88(1)</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-180K-11-xtal1</td><td>180</td><td>Temperature relaxation data collection.</td><td>92.2(7)</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-200K-12-xtal1</td><td>200</td><td>Temperature relaxation data collection.</td><td>89.7(7)</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-220K-13-xtal1</td><td>220</td><td>Temperature relaxation data collection.</td><td>32(1)</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-240K-14-xtal1</td><td>240</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-260K-15-xtal1</td><td>260</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-530nm-5h-290K-16-xtal1</td><td>290</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr></table>

Crystal 1, photocrystallography at 100 K, 470 nm LED with 3.1 mW (series 2)

<table><tr><td>Pd-1a-dark-100K-17-xtal1</td><td>100</td><td>T set to 100 K, crystal mounted and centered at 100 K, lights switched off.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-15min-470nm-100K-18-xtal1</td><td>100</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>35(1)</td></tr><tr><td>Pd-1a-irr-100K-30min-470nm-100K-19-xtal1</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>49(1)</td></tr><tr><td>Pd-1a-irr-100K-45min-470nm-100K-20-xtal1</td><td>100</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>57(2)</td></tr><tr><td>Pd-1a-irr-100K-75min-470nm-100K-21-xtal1</td><td>100</td><td>Data collection after 75 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>66(2)</td></tr><tr><td>Pd-1a-irr-100K-105min-470nm-100K-22-xtal1</td><td>100</td><td>Data collection after 105 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>71.6(8)</td></tr><tr><td>Pd-1a-irr-100K-165min-470nm-100K-23-xtal1</td><td>100</td><td>Data collection after 165 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>79.3(9)</td></tr><tr><td>Pd-1a-irr-100K-225min-470nm-100K-24-xtal1</td><td>100</td><td>Data collection after 225 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>84(1)</td></tr></table>
<table><tr><td>Pd-1a-irr-100K-285min-470nm-100K-25-xtal1</td><td>100</td><td>Data collection after 285 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>85(1)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-100K-26-xtal1</td><td>100</td><td>Time relaxation data collection.</td><td>86(1)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-100K-27-xtal1</td><td>100</td><td>Time relaxation data collection.</td><td>86(1)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-150K-28-xtal1</td><td>150</td><td>Temperature relaxation data collection.</td><td>86.9(6)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-180K-29-xtal1</td><td>180</td><td>Temperature relaxation data collection.</td><td>86.8(6)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-200K-30-xtal1</td><td>200</td><td>Temperature relaxation data collection.</td><td>84.1(6)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-210K-31-xtal1</td><td>210</td><td>Temperature relaxation data collection.</td><td>64.8(5)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-220K-32-xtal1</td><td>220</td><td>Temperature relaxation data collection.</td><td>17.9(6)</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-230K-33-xtal1</td><td>230</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-240K-34-xtal1</td><td>240</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-285min-470nm-290K-35-xtal1</td><td>290</td><td>Temperature relaxation data collection.</td><td>0 [a]</td></tr></table>

Crystal 1, photocrystallography at 200 K, 470 nm LED with 3.1 mW (series 3)

<table><tr><td>Pd-1a-dark-200K-36-xtal1</td><td>200</td><td>T set to 200 K.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-200K-15min-470nm-200K-37-xtal1</td><td>200</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>36.5(5)</td></tr><tr><td>Pd-1a-irr-200K-30min-470nm-200K-38-xtal1</td><td>200</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>49(1)</td></tr><tr><td>Pd-1a-irr-200K-45min-470nm-200K-39-xtal1</td><td>200</td><td>Data collection after 45 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>56(1)</td></tr><tr><td>Pd-1a-irr-200K-75min-470nm-200K-40-xtal1</td><td>200</td><td>Data collection after 75 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>64(1)</td></tr><tr><td>Pd-1a-irr-200K-105min-470nm-200K-41-xtal1</td><td>200</td><td>Data collection after 105 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>71.1(6)</td></tr><tr><td>Pd-1a-irr-200K-165min-470nm-200K-42-xtal1</td><td>200</td><td>Data collection after 165 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>77.7(6)</td></tr><tr><td>Pd-1a-irr-200K-225min-470nm-200K-43-xtal1</td><td>200</td><td>Data collection after 225 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>80.1(6)</td></tr><tr><td>Pd-1a-irr-200K-285min-470nm-200K-44-xtal1</td><td>200</td><td>Data collection after 285 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>81.1(6)</td></tr></table>
<table><tr><td>Pd-1a-irr-200K-285min-470nm-200K-45-xtal1</td><td>200</td><td>Time relaxation data collection.</td><td>75(1)</td></tr><tr><td>Pd-1a-irr-200K-285min-470nm-200K-46-xtal1</td><td>200</td><td>Time relaxation data collection.</td><td>72.9(6)</td></tr></table>

Crystal 1, photocrystallography at 200 K, 530 nm LED with 1.4 mW (series 4)

<table><tr><td>Pd-1a-dark-200K-47-xtal1</td><td>200</td><td>T set roughly to RT to switch the complex back, data collection proceeded and later T set to 200 K for subsequent measurements.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-200K-1h-530nm-200K-48-xtal1</td><td>200</td><td>Data collection after 1 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>16.7(6)</td></tr><tr><td>Pd-1a-irr-200K-2h-530nm-200K-49-xtal1</td><td>200</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>26.4(6)</td></tr><tr><td>Pd-1a-irr-200K-3h-530nm-200K-50-xtal1</td><td>200</td><td>Data collection after 3 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>32.3(6)</td></tr><tr><td>Pd-1a-irr-200K-4h-530nm-200K-51-xtal1</td><td>200</td><td>Data collection after 4 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>36.4(6)</td></tr><tr><td>Pd-1a-irr-200K-5h-530nm-200K-52-xtal1</td><td>200</td><td>Data collection after 5 h of irradiation of the rotating crystal with the 530 nm LED.</td><td>40.9(5)</td></tr></table>

Crystal 1, multi-temperature series (series 5)

<table><tr><td>Pd-1a-dark-290K-53-xtal1</td><td>290</td><td>Crystal mounted and centered at RT, T set to 290 K, lights switched off.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-260K-54-xtal1</td><td>260</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-230K-55-xtal1</td><td>230</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-220K-56-xtal1</td><td>220</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-210K-57-xtal1</td><td>210</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-200K-58-xtal1</td><td>200</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-180K-59-xtal1</td><td>180</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-150K-60-xtal1</td><td>150</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-cooling-120K-61-xtal1</td><td>120</td><td>Cooling data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-100K-62-xtal1</td><td>100</td><td>Dark data collected at 100 K.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-heating-150K-63-xtal1</td><td>150</td><td>Heating data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-heating-180K-64-xtal1</td><td>180</td><td>Heating data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-heating-200K-65-xtal1</td><td>200</td><td>Heating data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-heating-220K-66-xtal1</td><td>220</td><td>Heating data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-heating-260K-67-xtal1</td><td>260</td><td>Heating data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-290K-68-xtal1</td><td>290</td><td>Dark data collected at 290 K.</td><td>0 [a]</td></tr></table>
Crystal 2, photocrystallography at 100 K, 590 nm LED (power not measured, maximal current, test series) (series 6)

<table><tr><td>Pd-1a-dark-100K-01-xtal2</td><td>100</td><td>T set to 100 K, crystal mounted and centered at 100 K, lights switched off.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-590nm-2h-100K-02-xtal2</td><td>100</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 590 nm LED.</td><td>0 [a]</td></tr></table>

Crystal 3, photocrystallography at 180, 220, 230 K, 470 nm LED with 3.1 mW (series 7)
(this series contains experiments where crystal was continuously irradiated during entire experiment)

<table><tr><td>Pd-1a-dark-220K-01-xtal3</td><td>220</td><td>T set to 220 K, Crystal mounted and centered at 220 K, lights switched off.</td><td>0 [a]</td></tr><tr><td>Pd-1a-cirr-220K-2h-470nm-220K-02-xtal3</td><td>220</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED and continuous irradiation during the measurement.</td><td>81(1)</td></tr><tr><td>Pd-1a-irr-220K-250min-470nm-220K-03-xtal3</td><td>220</td><td>Data collection after 250 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>27.0(8)</td></tr><tr><td>Pd-1a-irr-220K-250min-470nm-220K-04-xtal3</td><td>220</td><td>Time relaxation data collection.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-220K-250min-470nm-220K-05-xtal3</td><td>220</td><td>Time relaxation cycle.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-230K-06-xtal3</td><td>230</td><td>Crystal heated to 280 K and cooled down to 230 K.</td><td>0 [a]</td></tr><tr><td>Pd-1a-cirr-230K-2h-470nm-230K-07-xtal3</td><td>230</td><td>Data collection after 2 h of irradiation of the rotating crystal with the 470 nm LED and continuous irradiation during the measurement.</td><td>40(1)</td></tr><tr><td>Pd-1a-irr-230K-280min-470nm-230K-08-xtal3</td><td>230</td><td>Time relaxation cycle.</td><td>0 [a]</td></tr><tr><td>Pd-1a-dark-100K-09-xtal3</td><td>100</td><td>Crystal heated to 280 K and cooled down to 100 K.</td><td>0 [a]</td></tr><tr><td>Pd-1a-irr-100K-5h-470nm-180K-11-xtal3</td><td>180</td><td>Crystal heated to 180 K, data collection after 5 h of irradiation of the rotating crystal with the 470 nm LED at 100 K before.</td><td>94.8(4)</td></tr><tr><td>Pd-1a-cirr-180K-1h-470nm-180K-12-xtal3</td><td>180</td><td>Data collection after additional 1 h of irradiation of the rotating crystal with the 470 nm LED at 180 K and continuous irradiation during the measurement.</td><td>100 [b]</td></tr><tr><td>Pd-1a-irr-180K-205min-470nm-180K-13-xtal3</td><td>180</td><td>Time relaxation data collection.</td><td>97.4(7)</td></tr><tr><td>Pd-1a-irr-180K-205min-470nm-180K-14-xtal3</td><td>180</td><td>Time relaxation data collection.</td><td>96.3(7)</td></tr><tr><td>Pd-1a-irr-180K-205min-470nm-180K-15-xtal3</td><td>180</td><td>Time relaxation data collection.</td><td>96.8(7)</td></tr></table>

Crystal 3, photocrystallography at 100 K, 405 and 365 nm LED (power not measured, maximal current, test series) (series 8)

<table><tr><td>Pd-1a-dark-100K-19-xtal3</td><td>100</td><td>T set to 100 K, Crystal mounted and centered at 290 K, lights switched off.</td><td>0 [a]</td></tr></table>
<table><caption>**Table S4.4.** Summary of all collected data sets for the **Pd-1a-DCM** crystal structure. The codes correspond to the CIF files' names and their data blocks; *T* – temperature of the measurement, *P* – population of the *endo*-nitrito isomer, *P*<sub>DCM</sub> – refined DCM molecule occupancy in the crystal structure. Notation used: last two entries are always temperature of the measurement and data collection number, dark – data set without irradiation, irr – data set after irradiation (temperature of irradiation, time, LED wavelength).</caption><tr><td>Pd-1a-irr-100K-30min-405nm-100K-20-xtal3</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 405 nm LED.</td><td>30.3(1)</td></tr><tr><td>Pd-1a-irr-100K-30min-365nm-100K-21-xtal3</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 365 nm LED. Measurement performed just after the previous one.</td><td>32.4(1)</td></tr></table>

<table><thead><tr><th>Data collection code</th><th>T / K</th><th>Remarks</th><th>P%</th><th>P<sub>DCM</sub>%</th></tr></thead><tbody><tr><td colspan="5"><strong>Crystal 1, crystal structure determination (series 1)</strong></td></tr><tr><td>Pd-1a-DCM-dark-100K-01-xtal1</td><td>100</td><td>Classical crystal structure determination. This data set was initially included just as a solvatomorph, but later found to be switchable, thus included here.</td><td>0 [a]</td><td>79.1(3)</td></tr><tr><td colspan="5"><strong>Crystal 2, multi-temperature series (series 2)</strong></td></tr><tr><td>Pd-1a-DCM-dark-290K-01-xtal2</td><td>290</td><td>Crystal mounted and centered at RT, T set to 290 K, lights switched off.</td><td>0 [a]</td><td>90.4(2)</td></tr><tr><td>Pd-1a-DCM-cooling-260K-02-xtal2</td><td>260</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.2(2)</td></tr><tr><td>Pd-1a-DCM-cooling-220K-03-xtal2</td><td>220</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.2(2)</td></tr><tr><td>Pd-1a-DCM-cooling-200K-04-xtal2</td><td>200</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.3(2)</td></tr><tr><td>Pd-1a-DCM-cooling-180K-05-xtal2</td><td>180</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.4(3)</td></tr><tr><td>Pd-1a-DCM-cooling-150K-06-xtal2</td><td>150</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.4(2)</td></tr><tr><td>Pd-1a-DCM-cooling-120K-07-xtal2</td><td>120</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.3(2)</td></tr><tr><td>Pd-1a-DCM-dark-100K-08-xtal2</td><td>100</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.2(2)</td></tr><tr><td>Pd-1a-DCM-heating-120K-09-xtal2</td><td>120</td><td>Cooling data collection.</td><td>0 [a]</td><td>90.3(2)</td></tr><tr><td>Pd-1a-DCM-heating-150K-10-xtal2</td><td>150</td><td>Dark data collected at 100 K.</td><td>0 [a]</td><td>90.2(2)</td></tr><tr><td>Pd-1a-DCM-heating-180K-11-xtal2</td><td>180</td><td>Heating data collection.</td><td>0 [a]</td><td>90.3(3)</td></tr><tr><td>Pd-1a-DCM-heating-200K-12-xtal2</td><td>200</td><td>Heating data collection.</td><td>0 [a]</td><td>90.2(3)</td></tr><tr><td>Pd-1a-DCM-heating-220K-13-xtal2</td><td>220</td><td>Heating data collection.</td><td>0 [a]</td><td>90.0(2)</td></tr><tr><td>Pd-1a-DCM-heating-260K-14-xtal2</td><td>260</td><td>Heating data collection.</td><td>0 [a]</td><td>90.2(2)</td></tr><tr><td>Pd-1a-DCM-dark-290K-15-xtal2</td><td>290</td><td>Dark data collected at 290 K.</td><td>0 [a]</td><td>90.1(2)</td></tr></tbody></table>
Crystal 2, photocrystallography at 200 K, 470 nm LED with 3.1 mW (series 3)

<table><tr><td>Pd-1a-dark-100K-16-xtal2</td><td>100</td><td>Crystal mounted and centered at RT, T set to 100 K, lights switched off.</td><td>0 [a]</td><td>88.4(5)</td></tr><tr><td>Pd-1a-irr-100K-15min-470nm-100K-17-xtal2</td><td>100</td><td>Data collection after 15 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>28.6(14)</td><td>88.2(5)</td></tr><tr><td>Pd-1a-irr-100K-30min-470nm-100K-18-xtal2</td><td>100</td><td>Data collection after 30 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>40.7(14)</td><td>87.9(5)</td></tr><tr><td>Pd-1a-irr-100K-60min-470nm-100K-19-xtal2</td><td>100</td><td>Data collection after 60 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>50.8(18)</td><td>87.4(6)</td></tr><tr><td>Pd-1a-irr-100K-90min-470nm-100K-20-xtal2</td><td>100</td><td>Data collection after 90 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>59.4(14)</td><td>87.4(6)</td></tr><tr><td>Pd-1a-irr-100K-210min-470nm-100K-21-xtal2</td><td>100</td><td>Data collection after 210 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>69.1(6)</td><td>87.3(6)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-100K-22-xtal2</td><td>100</td><td>Data collection after 270 min. of irradiation of the rotating crystal with the 470 nm LED.</td><td>75.0(7)</td><td>87.0(6)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-120K-23-xtal2</td><td>120</td><td>Temperature relaxation data collection.</td><td>75.0(7)</td><td>87.1(6)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-150K-24-xtal2</td><td>150</td><td>Temperature relaxation data collection.</td><td>74.3(7)</td><td>85.3(6)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-180K-25-xtal2</td><td>180</td><td>Temperature relaxation data collection.</td><td>73.8(7)</td><td>83.7(7)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-200K-26-xtal2</td><td>200</td><td>Temperature relaxation data collection.</td><td>72.0(8)</td><td>83.5(7)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-220K-27-xtal2</td><td>220</td><td>Temperature relaxation data collection.</td><td>60.9(11)</td><td>80.2(9)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-240K-28-xtal2</td><td>240</td><td>Temperature relaxation data collection.</td><td>0 [a]</td><td>88.3(5)</td></tr><tr><td>Pd-1a-irr-100K-270min-470nm-260K-29-xtal2</td><td>260</td><td>Temperature relaxation data collection.</td><td>0 [a]</td><td>88.4(5)</td></tr></table>

Note, that for **Pd-1a-DCM** the same crystal was used for both the multi-temperature and photocrystallographic series of measurements. The first measurement series was conducted and approximately one month later the second series was performed. In the initial measurement, the solvent molecule's occupancy was estimated to 0.904(2) and remained relatively unchanged. For the final measurement, the occupancy was 0.884(5). Therefore, it seems the DCM molecules are relatively strongly kept in the crystal structure tunnels. Also, the refined population clearly depends on the data quality, thus, as expected, the estimation of the population of DCM in a crystal is not very precise.
### 4.4. Short summary of population values vs. temperature.

**Table S4.5.** The *endo*-nitrito population (*P*) decay in time at 100 K and 200 K after sample irradiation at this temperature. No. provides the data collection number after the last irradiation step.

<table><thead><tr><th rowspan="2">No.</th><th>P at 100 K</th><th>P at 200 K</th><th>P at 100 K</th></tr><tr><th><b>Pd-1a</b> (mol. A)</th><th><b>Ni-1a</b></th><th><b>Pd-1a</b> (mol. A)</th></tr></thead><tbody><tr><td>1</td><td>85(1)%</td><td>96.4(3)%</td><td>81.1(6)%</td></tr><tr><td>2</td><td>86(1)%</td><td>93.5(3)%</td><td>75(1)%</td></tr><tr><td>3</td><td>86(1)%</td><td>91.5(3)%</td><td>72.9(6)%</td></tr><tr><td>4</td><td>-</td><td>89.3(3)%</td><td>-</td></tr></tbody></table>

**Table S4.6.** Population (*P*) values of the *endo*-nitrito isomer achieved for **Ni-1a** and **Pd-1a** after 2 h of irradiation and then continuously irradiated samples during data collection with the 470 LED light at 220 K and 230 K.

<table><thead><tr><th rowspan="2">Compound</th><th colspan="2">Temperature, T / K</th></tr><tr><th>220</th><th>230</th></tr></thead><tbody><tr><td><b>Ni-1a</b></td><td>82.6(5)%</td><td>55.1(7)%</td></tr><tr><td><b>Pd-1a</b> (mol. A)</td><td>81(1)%</td><td>40(1)%</td></tr></tbody></table>

### 4.5. Symmetry breaking.
After longer irradiation periods with the 470 or 530 nm LEDs the single-crystal samples of **Pd-1a** tend to exhibit additional Bragg peaks. Careful analysis show they begin to be seen at the positions of the $P2_1/n$ space-group symmetry forbidden reflections, thus exclusively at the *h0l* reciprocal layer (Figure S4.1, middle panel). However, they are of very weak intensity. This observation means the crystal experiences minor symmetry breaking during the irradiation. This is most likely related to the formation of the disordered structure at the $NO_2$ group position only.

In addition to this effect, the splitting of reflections is observed in the *hk0* reciprocal plane (Figure S4.1, middle panel). We ascribe this to some kind of 'twin'-like structure formation during the extended irradiation. We note that in some cases this prevented us from processing the data reasonably well, and further data descriptors were unacceptable and refinements unstable. Note that only high-angle reflections slightly split, whereas a vast majority highly overlap. Bothe effects, i.e. the extra symmetry-forbidden reflections and splitting, do disappear at temperatures higher than 180 K (Figure S4.1, right panel). All these effects were not taken into account in the present analysis, and will be presented elsewhere.
<figure><img src="image_28.png" /><figcaption>**Figure S4.1.** Reciprocal-space layers generated for different measurements of the **Pd-1a** crystal structure; data series No. 7. Note both symmetry-forbidden and split reflections in the middle panel; sample after longer 470 nm LED irradiation.</figcaption></figure>

**5. Powder diffraction**

Powder X-ray diffraction measurements performed for the **Ni-1a** sample obtained after the synthesis confirmed its purity. The diffractogram matched the single-crystal crystal structure of **Ni-1a**. Therefore, the thin-film sample prepared using the reaction product was suitable for further IR measurements.

In the case of **Pd-1a** the situation was more complicated. It appeared that the reaction product is pure, however, its structure matched that of the **Pd-1a'** polymorph (Table S4.1). Since **Pd-1a'** crystals were of lower quality (and appeared non-photoactive), the **Pd-1a** polymorph was selected for further studies. Therefore, for the purpose of the solid-state spectroscopic studies the thin-film sample was prepared after sample recrystallization from the methanol solution to assure formation of the **Pd-1a** form and to avoid impurities, such as the **Pd-1a-DCM** crystals.
<figure><img src="image_29.png" /><figcaption>**Figure S5.1.** PXRD pattern obtained for the **Ni-1a** bulk sample. Vertical axis – intensity, black curve – experimental pattern, red – Le Bail-fitted model, green bars – reflection positions (taking into account both $K_{\alpha1}-K_{\alpha2}$ splitting), blue – residual.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure S5.2.** PXRD pattern obtained for the freshly-synthesized palladium(II) sample, matching the **Pd-1a'** crystalline polymorph. Vertical axis – intensity, black curve – experimental pattern, red – Le Bail-fitted model, green bars – reflection positions (taking into account both $K_{\alpha1}-K_{\alpha2}$ splitting), blue – residual.</figcaption></figure>
# 6. Structural analysis

## 6.1. Packing, geometrical and energetic parameters.

<figure><img src="image_31.png" /></figure>

**Figure S6.1.** The layered structure of Pd-1a viewed along the (a) [021] and (b) [021] directions.

<figure><img src="image_32.png" /></figure>

**Figure S6.2.** (a) Alternated B7 and B8 motifs propagating along the [11̄1] direction. (b) Tunnels along the [100] direction filled with the DCM molecules.
<table><caption>**Table S6.1.** Twist of the NO₂ ligand plane in respect to the rest of the **Ni-1a** crystal structure computed as dihedral angle (δ) between mean-square planes (NO₂ ligand: O2, N3, O3 atoms; complex: Ni1, N1, N2, O1 atoms. XRD – X-ray diffraction results (2nd column), isolated-molecule optimization (B3LYP/6311++G** level of theory; 3rd column), QM/MM-optimization (4th column), CRYSTAL optimization (6th column).</caption><thead><tr><th>Isomer</th><th>XRD</th><th>isolated mol.</th><th>QM/MM</th><th>CRYSTAL</th></tr></thead><tbody><tr><td>nitro</td><td>86.1</td><td>66.00</td><td>85.89</td><td>88.57</td></tr><tr><td>endo-nitrito</td><td>88.2</td><td>79.01</td><td>85.05</td><td>86.57</td></tr></tbody></table>

<table><caption>**Table S6.2.** Twist of the NO₂ ligand plane in respect to the rest of the **Pd-1a** crystal structure computed as dihedral angle (δ) between mean-square planes (NO₂ ligand: O2, N3, O3 atoms; complex: Ni1, N1, N2, O1 atoms. XRD – X-ray diffraction results (2nd column), isolated-molecule optimization (B3LYP/6311++G** level of theory; 3rd column), QM/MM-optimization (4th column), CRYSTAL optimization (6th column).</caption><thead><tr><th>Isomer</th><th>XRD (mol. A)</th><th>XRD (mol. B)</th><th>isolated mol.</th><th>QM/MM (mol. A)</th><th>QM/MM (mol. B)</th></tr></thead><tbody><tr><td>nitro</td><td>75.9</td><td>47.8</td><td>35.92</td><td>82.65</td><td>43.95</td></tr></tbody></table>

<table><caption>**Table S6.3.** Main molecular dimers in the **Ni-1a** crystal structure along with the respective short contacts, interaction energy values ($E_{int}$), and selected geometrical parameters (d – distance, θ – angle) for both isomers.</caption><thead><tr><th>Motif</th><th>Isomer</th><th>E<sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>H···A</sub> / Å</th><th>d<sub>D···A</sub> / Å</th><th>θ<sub>D–H···A</sub> / °</th></tr></thead><tbody><tr><td rowspan="2">A1</td><td>nitro</td><td>-15.9</td><td>C6–H6···O3<sup>#1</sup></td><td>2.53</td><td>3.482(3)</td><td>169.65</td></tr><tr><td>endo-nitrito</td><td>-15.2</td><td>C6–H6···O3<sup>#1</sup></td><td>2.69</td><td>3.629(3)</td><td>164.69</td></tr><tr><td rowspan="4">A2</td><td>nitro</td><td>-36.6</td><td>C13–H1c13···O2<sup>#2</sup><br>C15–H1c15···O2<sup>#2</sup></td><td>2.49<br>2.69</td><td>3.302(3)<br>3.373(3)</td><td>142.01<br>128.39</td></tr><tr><td>endo-nitrito</td><td>-32.8</td><td>C13–H1c13···N3<sup>#2</sup><br>C15–H2c15···N3<sup>#2</sup></td><td>2.84<br>2.89</td><td>3.570(3)<br>3.706(3)</td><td>134.00<br>142.92</td></tr><tr><td>nitro</td><td>-32.1</td><td>C7–H1c7···O3<sup>#3</sup></td><td>2.65</td><td>3.328(2)</td><td>127.65</td></tr><tr><td>endo-nitrito</td><td>-26.6</td><td>C7–H1c7···N3<sup>#3</sup></td><td>2.69</td><td>3.328(3)</td><td>124.15</td></tr><tr><td rowspan="3">A4</td><td>nitro</td><td>-144.4</td><td>C15–H2c15···O2<sup>#4</sup><br>C3(π)···N2(π)<sup>#4</sup></td><td>2.63</td><td>3.353(3)<br>3.433(3)</td><td>132.38</td></tr><tr><td>endo-nitrito</td><td>-132.9</td><td>C15–H2c15···O2<sup>#4</sup><br>C3(π)···N2(π)<sup>#4</sup></td><td>2.58</td><td>3.251(3)<br>3.382(3)</td><td>127.43</td></tr><tr><td>nitro</td><td>-117.9</td><td>C5–H1c5···O3<sup>#5</sup><br>C3(π)···C3(π)<sup>#5</sup><br>C2–H1c2···O3<sup>#5</sup></td><td>2.66</td><td>3.448(3)<br>3.375(3)</td><td>139.80<br>156.45</td></tr></tbody></table>
<table><caption>**Table S6.4.** Main molecular dimers in the **Pd-1a** crystal structure along with the respective short contacts, interaction energy values (*E*<sub>int</sub>), and selected geometrical parameters (*d* – distance, θ – angle) for both isomers.</caption><tr><td>endo-nitrito</td><td>-113.1</td><td>C5-H1c5···O3'<sup>#5</sup></td><td>2.55</td><td>3.332(3)</td><td>138.89</td></tr><tr><td></td><td></td><td>C3(π)···C3(π)<sup>#5</sup></td><td></td><td>3.478(3)</td><td></td></tr><tr><td colspan="6">Symmetry operations: (#1) x, y, z - 1; (#2) x, y + 1, z; (#3) -x + 1, -y, -z + 1; (#4) -x + 2, -y + 1, -z + 1; (#5) -x + 1, -y + 1, -z + 1.</td></tr></table>

<table><thead><tr><th>Motif</th><th>Isomer</th><th><i>E</i><sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th><i>d</i><sub>H···A</sub> / Å</th><th><i>d</i><sub>D···A</sub> / Å</th><th><i>θ</i><sub>D-H···A</sub> / °</th></tr></thead><tbody><tr><td rowspan="2">BB1</td><td>nitro</td><td>-37.0</td><td>C13'-H1c13'#1···O3'</td><td>2.58</td><td>3.460(4)</td><td>153.20</td></tr><tr><td>endo-nitrito</td><td>-36.5</td><td>C13'-H1c13'#1···O3'</td><td>2.64</td><td>3.552(7)</td><td>153.08</td></tr><tr><td rowspan="2">BB2</td><td>nitro</td><td>-15.6</td><td>C11'-H1c11'#2···O2'</td><td>2.75</td><td>3.308(4)</td><td>117.75</td></tr><tr><td>endo-nitrito</td><td>-16.2</td><td>C11'-H1c11'#2···O2'</td><td>2.76</td><td>3.303(6)</td><td>116.85</td></tr><tr><td rowspan="4">AB3</td><td rowspan="4">nitro</td><td rowspan="4">-26.0</td><td>C6'-H1c6'#3···O3</td><td>2.51</td><td>3.318(4)</td><td>141.64</td></tr><tr><td>C5'-H1c5'#3···C9</td><td>2.75</td><td>3.588(4)</td><td>146.54</td></tr><tr><td>C5'-H1c5'#3···C8</td><td>2.87</td><td>3.826(5)</td><td>173.54</td></tr><tr><td>C2'-H1c2'#3···C8</td><td>2.81</td><td>3.739(4)</td><td>161.99</td></tr><tr><td rowspan="4">endo-nitrito</td><td rowspan="4">-26.5</td><td>C6'-H1c6'#3···O3''</td><td>2.44</td><td>3.17(2)</td><td>133.33</td></tr><tr><td>C5'-H1c5'#3···C9</td><td>2.75</td><td>3.570(8)</td><td>143.91</td></tr><tr><td>C5'-H1c5'#3···C8</td><td>2.84</td><td>3.789(8)</td><td>168.72</td></tr><tr><td>C2'-H1c2'#3'···C8</td><td>2.88</td><td>3.803(8)</td><td>160.74</td></tr><tr><td rowspan="3">AB4</td><td rowspan="3">nitro</td><td rowspan="3">-35.6</td><td>C6-H1c6···O2'#4</td><td>2.71</td><td>3.288(4)</td><td>118.94</td></tr><tr><td>C10'-H1c10'#4···C5</td><td>2.91</td><td>3.593(4)</td><td>129.23</td></tr><tr><td>C10'-H1c10'#4···C6</td><td>2.86</td><td>3.707(4)</td><td>147.93</td></tr><tr><td rowspan="3">endo-nitrito</td><td rowspan="3">-36.1</td><td>C6-H1c6···O2'#4</td><td>2.71</td><td>3.314(7)</td><td>121.06</td></tr><tr><td>C10'-H1c10'#4···C5</td><td>2.92</td><td>3.605(7)</td><td>129.08</td></tr><tr><td>C10'-H1c10'#4···C6</td><td>2.90</td><td>3.740(7)</td><td>146.25</td></tr><tr><td rowspan="3">AB5</td><td rowspan="3">nitro</td><td rowspan="3">-41.5</td><td>C1-H1c1'#5···O3'</td><td>2.70</td><td>3.473(4)</td><td>138.58</td></tr><tr><td>C15-H2c15'#5···O1'</td><td>2.65</td><td>3.508(4)</td><td>148.70</td></tr><tr><td>C15-H1c15'#5···C9'</td><td>2.66</td><td>3.448(3)</td><td>139.80</td></tr><tr><td rowspan="3">endo-nitrito</td><td rowspan="3">-40.0</td><td>C1-H1c1'#5···O3'</td><td>2.64</td><td>3.438(6)</td><td>140.86</td></tr><tr><td>C15-H2c15'#5···O1'</td><td>2.64</td><td>3.503(6)</td><td>149.75</td></tr><tr><td>C15-H1c15'#5···C9'</td><td>2.80</td><td>3.430(7)</td><td>123.75</td></tr><tr><td rowspan="3">AB6</td><td rowspan="3">nitro</td><td rowspan="3">-69.5</td><td>C11-H1c11···O2'</td><td>2.54</td><td>3.477(4)</td><td>165.22</td></tr><tr><td>C15'-H1c15'···O2</td><td>2.54</td><td>3.281(4)</td><td>133.90</td></tr><tr><td>C1'-H1c1'···O2</td><td>2.68</td><td>2.965(4)</td><td>97.94</td></tr><tr><td rowspan="3">AB7</td><td rowspan="3">nitro</td><td rowspan="3">-19.1</td><td>C11-H1c11···O2'</td><td>2.55</td><td>3.488(7)</td><td>164.09</td></tr><tr><td>C1'···N3''</td><td></td><td>2.966(12)</td><td></td></tr><tr><td>C2'···N3''</td><td></td><td>3.207(11)</td><td></td></tr><tr><td rowspan="3">endo-nitrito</td><td rowspan="3">-16.5</td><td>C8'-H1c8'#6···O3</td><td>2.55</td><td>3.460(4)</td><td>157.73</td></tr><tr><td>C8'-H1c8'#6···O2</td><td>2.68</td><td>3.311(4)</td><td>123.59</td></tr><tr><td>C9'-H1c9'#6···O2</td><td>2.77</td><td>3.348(4)</td><td>119.80</td></tr><tr><td></td><td></td><td>C8'-H1c8'#6···N3''</td><td>2.38</td><td>3.260(11)</td><td>152.68</td></tr></tbody></table>
<table><tbody><tr><td rowspan="2">BB8</td><td>nitro</td><td>-147.9</td><td>C1'-H1c1'...O3'<sup>#7</sup><br/>C3'(π)...N2'(π)<sup>#7</sup></td><td>2.65</td><td>3.148(4)<br/>3.494(4)</td><td>112.72</td></tr><tr><td>endo-nitrito</td><td>-146.1</td><td>C1'-H1c1'...O3'<sup>#7</sup><br/>C3'(π)...N2'(π)<sup>#7</sup></td><td>2.61</td><td>3.116(6)<br/>3.513(6)</td><td>113.19</td></tr><tr><td rowspan="2">AA9</td><td>nitro</td><td>-94.8</td><td>C13(π)...N2(π)<sup>#8</sup></td><td></td><td>3.562(4)</td><td></td></tr><tr><td>endo-nitrito</td><td>-99.5</td><td>C13(π)...N2(π)<sup>#8</sup><br/>13H1c13...O3''<sup>#8</sup></td><td>2.49</td><td>3.556(2)<br/>3.115(19)</td><td>122.21</td></tr><tr><td rowspan="2">AA10</td><td>nitro</td><td>-143.4</td><td>C1-H1c1...O2<sup>#9</sup><br/>C3(π)...N2(π)<sup>#9</sup></td><td>2.70</td><td>2.953(4)<br/>3.401(4)</td><td>95.89</td></tr><tr><td>endo-nitrito</td><td>-138.7</td><td>C1...O2''<sup>#9</sup><br/>C3(π)...N2(π)<sup>#9</sup></td><td></td><td>3.034(12)<br/>3.329(6)</td><td></td></tr></tbody></table>

Symmetry operations: (#1) x, y + 1, z; (#2) -x + 1.5, y + 0.5, -z + 0.5; (#3) mol. A: -x + 1.5, y + 0.5, -z + 1.5; mol. B: x, y, z; (#4) mol. A: x + 0.5, -y + 0.5, z - 0.5; mol. B: x, y, z; (#5) mol. A: -x + 1, -y + 1, -z + 1; mol. B: x, y, z; (#6) mol. A: x, y + 1, z; mol. B: x, y, z; (#7) -x + 2, -y + 1, -z + 1; (#8) -x + 1, -y + 1, -z + 1; (#9) -x + 1, -y, -z + 1.

<table><caption>**Table S6.5.** Main molecular dimers in the Pd-1a-DCM crystal structure along with the respective short contacts, interaction energy values ($E_{\text{int}}$), and selected geometrical parameters (d - distance, θ - angle) for both isomers.</caption><thead><tr><th>Motif</th><th>Isomer</th><th>E<sub>int</sub> / kJ·mol<sup>-1</sup></th><th>Selected interactions</th><th>d<sub>H...A</sub> / Å</th><th>d<sub>D...A</sub> / Å</th><th>θ<sub>D-H...A</sub> / °</th></tr></thead><tbody><tr><td rowspan="3">B1</td><td rowspan="3">nitro</td><td rowspan="3">-54.4</td><td>C11-H1C11...C2<sup>#1</sup></td><td>2.89</td><td>3.760(3)</td><td>151.33</td></tr><tr><td>C15-H2C15...O3<sup>#1</sup></td><td>2.66</td><td>3.622(3)</td><td>175.07</td></tr><tr><td>C13...O2<sup>#1</sup></td><td></td><td>3.188(3K)</td><td></td></tr><tr><td rowspan="2">B2</td><td rowspan="2">nitro</td><td rowspan="2">-56.4</td><td>C11-H1C11...C2<sup>#1</sup></td><td>2.85</td><td>3.732(5)</td><td>152.66</td></tr><tr><td>C15-H2C15...N3<sup>#1</sup></td><td>2.55</td><td>3.452(11)</td><td>156.76</td></tr><tr><td rowspan="2">B3</td><td rowspan="2">nitro</td><td rowspan="2">-30.0</td><td>C1<sup>#3</sup>-H1C1<sup>#3</sup>...C13</td><td>2.89</td><td>3.827(3)</td><td>165.84</td></tr><tr><td>C1<sup>#3</sup>-H1C1<sup>#3</sup>...C13</td><td>2.88</td><td>3.805(5)</td><td>163.09</td></tr><tr><td rowspan="2">B4</td><td rowspan="2">nitro</td><td rowspan="2">-28.2</td><td>C8-H1C8...O2<sup>#4</sup></td><td>2.80</td><td>3.750(3)</td><td>170.86</td></tr><tr><td>C8-H1C8...O3<sup>#4</sup></td><td>2.81</td><td>3.538(3)</td><td>133.18</td></tr><tr><td rowspan="2">B5</td><td rowspan="2">nitro</td><td rowspan="2">-18.5</td><td>C8-H1C8...N3<sup>#4</sup></td><td>2.63</td><td>3.497(11)</td><td>149.73</td></tr><tr><td>C8-H1C8...O3<sup>#4</sup></td><td>2.64</td><td>3.285(11)</td><td>124.52</td></tr><tr><td rowspan="2">B6</td><td rowspan="2">nitro</td><td rowspan="2">-10.4</td><td>C1<sup>#5</sup>-H1C1<sup>#5</sup>...O2</td><td>2.46(2)</td><td>3.337(3)</td><td>152(2)</td></tr><tr><td>C1<sup>#5</sup>-H1C1<sup>#5</sup>...N3'</td><td>3.39(4)</td><td>4.315(11)</td><td>162(3)</td></tr><tr><td rowspan="2">B7</td><td rowspan="2">nitro</td><td rowspan="2">-19.8</td><td>C12-H1C12...C11<sup>#6</sup></td><td>2.88</td><td>3.657(3)</td><td>138.67</td></tr><tr><td>C12-H1C12...C11<sup>#6</sup></td><td>2.90</td><td>3.688(3)</td><td>140.60</td></tr><tr><td rowspan="2">B8</td><td rowspan="2">nitro</td><td rowspan="2">-161.3</td><td>C5...C7<sup>#7</sup></td><td></td><td>3.398(4)</td><td></td></tr><tr><td>C5...C7<sup>#7</sup></td><td></td><td>3.427(5)</td><td></td></tr><tr><td rowspan="3">B8</td><td rowspan="3">nitro</td><td rowspan="3">-149.5</td><td>C15-H1C15...O2<sup>#8</sup></td><td>2.52</td><td>3.339(3)</td><td>143.70</td></tr><tr><td>C5...C11<sup>#8</sup></td><td></td><td>3.365(3)</td><td></td></tr><tr><td>C9...C13<sup>#8</sup></td><td></td><td>3.340(3)</td><td></td></tr><tr><td rowspan="2">B8</td><td rowspan="2">nitro</td><td rowspan="2">-149.5</td><td>C15-H1C15...O2<sup>#8</sup></td><td>2.72</td><td>3.397(9)</td><td>128.33</td></tr><tr><td>C5...C11<sup>#8</sup></td><td></td><td>3.342(5)</td><td></td></tr></tbody></table>
<table><tr><td>C9···C13<sup>#8</sup></td><td>3.372(5)</td></tr><tr><td>O2'···C1<sup>#8</sup></td><td>2.974(10)</td></tr></table>

Symmetry operations: (#1) $-x + 1, y + 0.5, -z + 1.5$; (#2) $-x + 1.5, -y + 1.5, -z + 1.5$; (#3) $x + 0.5, -y + 2, z$; (#4) $-x + 0.5, y, -z + 1$; (#5) $-x + 0.5, y, -z + 1$; (#6) $-x + 1.5, y, -z + 2$; (#7) $-x, -y + 2, -z + 1$; (#8) $-x + 0.5, -y + 1.5, -z + 1.5$.

<figure><img src="image_33.png" /><figcaption>**Figure S6.3.** Visualization of the reaction cavities generated based on the 100 K crystal structures of the examined systems using the *MERCURY* program (probe radius of 1.2 Å, grid spacing of 0.1 Å). Note this figure contains both isomers for molecule A of **Pd-1a** and **Pd-1a-DCM**, in comparison to what is shown in Figure 6 in the main manuscript.</figcaption></figure>
**6.2. Metal-metal distances vs. endo-nitrito isomer's population.** A shown in Figure 12 there are strong linear relationships between M···M distances ($d_{M...M}$) and endo-nitrito form population ($P$) in crystal. In all cases the coefficient of determination is very high ($R^2 > 99\%$). Below the derived fits are presented ($N$ – number of data points).

<table><thead><tr><th>Crystal structure</th><th>Motif</th><th>Temperature, T / K</th><th>Equation</th><th>N</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>Ni-1a</td><td>A4</td><td>100</td><td>d<sub>Ni...Ni</sub> / Å = -0.00348(74) · P + 3.5841(49)</td><td>19</td><td>0.9919</td></tr><tr><td>Pd-a</td><td>AA10</td><td>100</td><td>d<sub>Pd...Pd</sub> / Å = -0.00313(49) · P + 3.7537(32)</td><td>9</td><td>0.9980</td></tr><tr><td>Pd-1a-DCM</td><td>B8</td><td>100</td><td>d<sub>Pd...Pd</sub> / Å = -0.0049(19) · P + 3.8268(99)</td><td>7</td><td>0.9908</td></tr><tr><td>Ni-1a</td><td>A4</td><td>200</td><td>d<sub>Ni...Ni</sub> / Å = -0.00377(43) · P + 3.6469(36)</td><td>19</td><td>0.9974</td></tr><tr><td>Pd-a</td><td>AA10</td><td>200</td><td>d<sub>Pd...Pd</sub> / Å = -0.00314(28) · P + 3.7769(14)</td><td>15</td><td>0.9989</td></tr><tr><td>Ni-1a</td><td>A5</td><td>- [a]</td><td>d<sub>Ni...Ni</sub> / Å = 0.00438(66) · P + 6.2387(50)</td><td>44</td><td>0.9913</td></tr><tr><td>Pd-1a</td><td>AA9</td><td>- [a]</td><td>d<sub>Pd...Pd</sub> / Å = 0.00361(40) · P + 6.3709(23)</td><td>24</td><td>0.9972</td></tr></tbody></table>

It must be noted here the **A4** and **AA10** structural motifs in the **Ni-1a** and **Pd-1a** structures, respectively, at 100 and 200 K must be treated separately. This is due to the fact that the M···M distances in these cases are temperature-dependent. Similar case is observed for the **BB8** motif in the **Pd-1a** structure (Figure S6.3). For the remaining cases of this was not observed and the linear relation was fitted to multi-temperature data (these fits are denoted in table with [a]).

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

**Figure S6.4.** Pd···Pd distance changes along with the endo-nitrito form's population built up in the **Pd-1a** crystal structure shown for the **BB8** motif.
<figure><img src="image_35.png" /><figcaption>**Figure S6.5.** Pd···Pd distance changes along with the *endo*-nitrito form's population built up in the **Pd-1a-DCM** crystal structure for several structural motifs. Thin red line on bottom plot denotes best linear fit ($R^2 > 0.99$).</figcaption></figure>
6.3. Unit-cell parameters vs. population and temperature

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

**Figure S6.6.** The unit-cell parameters changes along with the *endo*-nitrito form's population built up in the **Ni-1a** crystal structures.
<figure><img src="image_37.png" /></figure>

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

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

<figure><img src="image_40.png" /></figure>

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

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

<figure><img src="image_43.png" /><figcaption>**Figure S6.7.** Unit-cell parameters changes along with temperature in the **Ni-1a** crystal structures.</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**Figure S6.8.** Unit-cell parameters changes along with the *endo*-nitrito form's population built up in the **Pd-1a** crystal structures.</figcaption></figure>
<figure><img src="image_45.png" /><figcaption>**Figure S6.9.** Unit-cell parameters changes along with temperature in the **Pd-1a** crystal structures.</figcaption></figure>
<figure><img src="image_46.png" /><figcaption>**Figure S6.10.** Unit-cell parameters changes along with the *endo*-nitrito form's population built up in the **Pd-1a-DCM** crystal structures.</figcaption></figure>
<figure><img src="image_47.png" /><figcaption>**Figure S6.11.** Unit-cell parameters changes along with temperature in the Pd-1a-DCM crystal structures.</figcaption></figure>
# 7. Solid-state IR spectroscopy

## 7.1. Tables with numerical data.

**Table S7.1.** Key vibrational modes characteristic for the examined linkage isomers of the nitrite group with their theoretical counterparts for **Ni-1a**. 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 rowspan="3">nitro-(η¹-NO₂)</td><td>ν<sub>as</sub>(NO₂)</td><td>1479</td><td>1340</td></tr><tr><td>ν(N=O)</td><td>1489</td><td>1350</td></tr><tr><td>ν(N=O)</td><td>1497</td><td>1369</td></tr><tr><td rowspan="4">endo-nitrito-(η¹-ONO)</td><td>ν(N=O)</td><td>1091</td><td>1110</td></tr><tr><td>ν(N=O)</td><td>1491</td><td>1404</td></tr><tr><td>ν(N=O)</td><td>1510</td><td>1410</td></tr><tr><td>ν(N=O)</td><td>1517</td><td>1417</td></tr><tr><td>exo-nitrito-(η¹-ONO)</td><td>ν(N=O)</td><td>996</td><td>1062</td></tr></tbody></table>

**Table S7.2.** Key vibrational modes characteristic for the examined linkage isomers of the nitrite group with their theoretical counterparts for **Pd-1a**. 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 rowspan="3">nitro-(η¹-NO₂)</td><td>ν(N=O)</td><td>1475</td><td>1344</td></tr><tr><td>ν<sub>as</sub>(NO₂)</td><td>1490</td><td>1375</td></tr><tr><td>ν<sub>as</sub>(NO₂)</td><td>1504</td><td>1399</td></tr><tr><td rowspan="2">endo-nitrito-(η¹-ONO)</td><td>ν(N=O)</td><td>1057</td><td>1082</td></tr><tr><td>ν(N=O)</td><td>1526</td><td>1425</td></tr><tr><td rowspan="3">exo-nitrito-(η¹-ONO)</td><td>ν(N=O)</td><td>992</td><td></td></tr><tr><td>ν(N=O)</td><td>994</td><td></td></tr><tr><td>ν(N=O)</td><td>1587</td><td></td></tr></tbody></table>

**Table S7.3.** Approximate fluence (power density, $Q = P_{avg}/A$) values of LEDs used in the IR experiments.

<table><thead><tr><th>Wavelength,<br>λ / nm</th><th>Fluence,<br>Q / mW·cm<sup>-2</sup></th></tr></thead><tbody><tr><td>660</td><td>125</td></tr><tr><td>590</td><td>34</td></tr><tr><td>530</td><td>71</td></tr><tr><td>470</td><td>217</td></tr><tr><td>405</td><td>188</td></tr></tbody></table>
## 7.2. Additional IR spectra.

<figure><img src="image_48.png" /><figcaption>**Figure S7.1.** IR spectra of Ni-1a collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>

<figure><img src="image_49.png" /><figcaption>**Figure S7.2.** IR spectra of Ni-1a collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>
<figure><img src="image_50.png" /><figcaption>**Figure S7.3.** IR spectra of **Ni-1a** collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>

<figure><img src="image_51.png" /><figcaption>**Figure S7.4.** IR spectra of **Pd-1a** collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>
<figure><img src="image_52.png" /><figcaption>**Figure S7.5.** IR spectra of Pd-1a collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>

<figure><img src="image_53.png" /><figcaption>**Figure S7.6.** IR spectra of Pd-1a collected at: 280 K (black line), 260 K (red line), 240 K (green line), 220 K (blue line), 200 K (cyan line), 180 K (magenta line), 160 K (yellow line), 140 K (dark yellow line), 120 K (navy line) and 100 K (violet line).</figcaption></figure>
<figure><img src="image_54.png" /><figcaption>**Figure S7.7.** IR spectra of Ni-1a 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_55.png" /><figcaption>**Figure S7.8.** IR spectra of Ni-1a 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).</figcaption></figure>
irradiation with 470 nm LED for 15 minutes (blue line), after irradiation with 405 nm LED for 15 minutes (violet line).

<figure><img src="image_56.png" /></figure>

**Figure S7.9.** IR spectra of **Ni-1a** 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).

<figure><img src="image_57.png" /></figure>

**Figure S7.10.** IR spectra of **Pd-1a** 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).
irradiation with 470 nm LED for 15 minutes (blue line), after irradiation with 405 nm LED for 15 minutes (violet line).

<figure><img src="image_58.png" /></figure>

**Figure S7.11.** IR spectra of Pd-1a 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).

<figure><img src="image_59.png" /></figure>

**Figure S7.12.** IR spectra of Pd-1a 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).
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).

<figure><img src="image_60.png" /><figcaption>**Figure S7.13.** IR spectra of Pd-1a-prim collected during cooling cycle. For colors see legend.</figcaption></figure>

<figure><img src="image_61.png" /><figcaption>**Figure S7.14.** IR spectra of Pd-1a-prim collected during the cooling cycle. For colors see legend.</figcaption></figure>
<figure><img src="image_62.png" /><figcaption>**Figure S7.15.** IR spectra of Pd-1a-prim collected during the cooling cycle. For colors see legend.</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>**Figure S7.16.** IR spectra of Pd-1a-prim 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 (magenta line).</figcaption></figure>
nm LED for 15 minutes (dark blue line), after irradiation with 365 nm LED for 15 minutes (violet line).

<figure><img src="image_64.png" /></figure>

**Figure S7.17.** IR spectra of Pd-1a-prim 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 (dark blue line), after irradiation with 365 nm LED for 15 minutes (violet line).

<figure><img src="image_65.png" /></figure>

**Figure S7.18.** IR spectra of Pd-1a-prim 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).
nm LED for 15 minutes (dark blue line), after irradiation with 365 nm LED for 15 minutes
(violet line).
## 8. Computational part

**Table S8.1.** Energy differences between QM/MM-optimization results obtained for the examined nitrite isomers with respect to the most energetically favored isomer, i.e. the nitro form. Note this results differ from the ones shown in the main text (Table 4) since here the UFF force-field energy is taken into account.

<table><thead><tr><th rowspan="2">Linkage isomer</th><th colspan="3">E<sub>diff</sub><sup>QM/MM+UFF</sup> / kJ·mol<sup>-1</sup></th></tr><tr><th>Ni-1a</th><th>Pd-1a (mol. A)</th><th>Pd-1a (mol. B)</th></tr></thead><tbody><tr><td>nitro</td><td>0.0</td><td>0.0</td><td>0.0</td></tr><tr><td>endo-nitrito</td><td>14.3</td><td>47.6</td><td>39.3</td></tr><tr><td>exo-nitrito</td><td>65.4</td><td>50.7<sup>a</sup></td><td>38.4<sup>b</sup></td></tr></tbody><tfoot><tr><td colspan="4"><sup>a</sup> Converged to a distorted exo-nitrito moiety resembling the transition state species between nitro and exo-nitrito binding modes. <sup>b</sup> Despite starting from an arbitrary exo-nitrito isomer optimization converged to an endo-nitrito binding mode of the opposite conformation to the other one.</td></tr></tfoot></table>

**Table S8.2.** Comparison of interaction energies (E<sub>int</sub>) for the main molecular dimers in the Ni-1a crystal structure depending on chosen basis set. In all cases the same density functional was used: B3LYP. All energies are given in kcal·mol<sup>-1</sup>. Basis set shown in **bold** has been used for further computations, as it gave the comparable accuracy to the other ones with reasonable time.

<table><thead><tr><th rowspan="2">Basis set</th><th colspan="5">Structural motif</th></tr><tr><th>A1</th><th>A2</th><th>A3</th><th>A4</th><th>A5</th></tr></thead><tbody><tr><td>6-311++G**</td><td>-3.80</td><td>-8.75</td><td>-7.68</td><td>-34.52</td><td>-28.13</td></tr><tr><td>LANL2DZ</td><td>-</td><td>-9.70</td><td>-7.79</td><td>-37.02</td><td>-</td></tr><tr><td>pob-TZVP</td><td>-3.86</td><td>-8.83</td><td>-7.22</td><td>-35.43</td><td>-28.03</td></tr><tr><td>def2-TZVPPD</td><td>-3.85</td><td>-8.50</td><td>-7.55</td><td>-34.61</td><td>-28.08</td></tr><tr><td>Average</td><td>-3.84(3)</td><td>-8.9(5)</td><td>-7.6(2)</td><td>-35(1)</td><td>-28.08(4)</td></tr></tbody></table>
<table><caption>**Table S8.3.** Comparison of interaction energies for the main molecular dimers in the **Pd-1a** crystal structure depending on chosen basis set. In all cases the same density functional was used: B3LYP. All energies are given in kcal·mol⁻¹. Basis set shown in **bold** has been used for further computations, as it gave the comparable accuracy to the other ones with reasonable time. *Note the combined set is 6-311++G**\** for C, H, N and O with aug-cc-pVTZ-PP for Pd.*</caption><thead><tr><th rowspan="2">Basis set</th><th colspan="10">Structural motif</th></tr><tr><th>BB1</th><th>BB2</th><th>AB3</th><th>AB4</th><th>AB5</th><th>AB6</th><th>AB7</th><th>BB8</th><th>AA9</th><th>AA10</th></tr></thead><tbody><tr><th>Combined set</th><td>-8.84</td><td>-3.72</td><td>-6.22</td><td>-8.51</td><td>-9.91</td><td>-16.61</td><td>-4.57</td><td>-35.35</td><td>-22.65</td><td>-34.27</td></tr><tr><th>LANL2DZ</th><td>-9.55</td><td>-4.11</td><td>-6.66</td><td>-8.92</td><td>-10.41</td><td>-17.89</td><td>-4.35</td><td>-37.66</td><td>-</td><td>-36.20</td></tr><tr><th>pob-TZVP</th><td>-8.55</td><td>-3.41</td><td>-6.30</td><td>-8.45</td><td>-10.07</td><td>-16.92</td><td>-3.95</td><td>-36.03</td><td>-</td><td>-36.13</td></tr><tr><th>def2-TZVPPD</th><td>-8.54</td><td>-3.63</td><td>-6.20</td><td>-8.41</td><td>-9.86</td><td>-16.26</td><td>-4.58</td><td>-35.58</td><td>-</td><td>-34.58</td></tr><tr><th>Average</th><td>-8.9(4)</td><td>-3.7(3)</td><td>-6.3(2)</td><td>-8.6(2)</td><td>-10.1(2)</td><td>-16.9(6)</td><td>-4.4(3)</td><td>-36.2(9)</td><td>-</td><td>-35.3(9)</td></tr></tbody></table>

<figure><img src="image_66.png" /><figcaption>**Figure S8.1.** Theoretical UV-Vis spectra of **Ni-1a**: nitro isomer (green), *endo*-nitrito isomer (blue), *exo*-nitrito isomer (red).</figcaption></figure>

<figure><img src="image_67.png" /><figcaption>**Figure S8.2.** Theoretical UV-Vis spectra of **Pd-1a**: nitro isomer (green), *endo*-nitrito isomer (blue), *exo*-nitrito isomer (red).</figcaption></figure>
9. Arrhenius plots

<figure><img src="image_68.png" /><figcaption>**Figure S9.1.** Arrhenius plots showing the *endo*-nitrito isomer relaxation for (a) Ni-1a and (b) Pd-1a.</figcaption></figure>