A theoretical insight into the role of counter anions and their interactions in nitropentaamminecobalt(III) toward linkage isomerism-induced photochemical motion

Density functional theory (B3LYP, B3LYP-D3, wB97XD, M062X, and M06L) and ab initio methods (MP2 and CCSD(T)) in conjunction with 6-31+G(d,p) and LanL2DZ were employed to investigate the interaction energies between [Co(NH3)(5)NO2](2+) linkage isomers and chloride and nitrate in both gas phase and solid state. The nature of the chemical bonding has been analyzed by means of the atoms in molecules, electron density shift, natural bond orbitals, symmetry adapted perturbation theory, and energy decomposition analysis. The electronic structures of the two lowest laying singlet states (S-o and S-1) of [Co(NH3)(5)NO2](NO3)Cl isomers were also investigated using CASSCF(6,6) with LanL2DZ and 6-31G(d) basis sets. Our results show that [Co(NH3)(5)NO2](2+) linkage isomers interact more strongly with chloride than nitrate. The structures of [Co(NH3)(5)NO2](NO3)Cl linkage isomers and their relative stabilities were examined in gas phase and in solid state and confirmed the nitro-complex as the most stable following by a viable intermediate endo-complex. Study of the nitro-nitrito linkage isomerization in [Co(NH3)(5)NO2](NO3)Cl revealed that anions form strong electrostatic bonds with [Co(NH3)(5)NO2](2+) leading to decrease in an activation energy compared to the [Co(NH3)(5)ONO](2+) isomers. A concerted action of ionic interactions and hydrogen bonds are suspected of regulating the isomerization in solid state. Assessment of various DFT methods with respect to CCSD(T) suggests M062X suitable method for [Co(NH3)(5)NO2](NO3)Cl linkage-isomerization study. Potential energy surface calculations at the CASSCF/6-31G(d) level of theory shows that the conical intersection (S-1/S-o) might play an important role in the photoisomerization of [Co(NH3)(5)NO2](NO3)Cl.