The charge density and force-field view of bonding interactions in crystals with photomechanical properties: [Co(NH ₃ ) ₅ NO ₂ ]ClNO ₃ versus [Co(NH ₃ ) ₅ ONO]ClNO ₃

Charge density and local electrostatic and quantum kinetic electron forces acting on the bound atoms in [Co(NH₃)₅NO₂]ClNO₃ (nitro isomer) and [Co(NH₃)₅ONO]ClNO₃ (nitrito isomer) were studied to elucidate to the role of the crystalline environment in the intramolecular nitro-nitrito linkage photoisomerization. Quantum crystallography and orbital-free DFT approaches were applied. In the more stable nitro isomer, the -NO₂^- group forms two hydrogen bonds with the NH₃ ligands, while in the nitrito isomer the -ONO^- group is not involved in any hydrogen bonds, however, it forms van der Waals contacts with surrounding atoms. No bond paths in the electron density were found between the anions NO₃^- and Cl^-, which were previously supposed to play an important role in photoisomerization. Still, these anions do interact with each other in both isomers via an indirect mechanism involving H atoms of -NH₃ ligands. Whereas there is no direct electrostatic interaction between the anions in the nitro isomer, the part of electrons belonging to the Cl^- anion in the nitrito isomer is pulled towards the nucleus of an oxygen atom of the NO₃^- anion, creating a favorable condition for direct interaction between anions through corresponding local electronic forces directed to the nuclei. The link of cation/anion atomic basin volumes relationship and nature of the intermolecular interactions is established.