Theoretical investigation on the ground state properties of the hexaamminecobalt(iii) and nitro–nitrito linkage isomerism in pentaamminecobalt(iii) in vacuo

Abstract

Nitro–nitrito isomerization in Co(NH₃)₅NO₂²⁺ linkage isomers was investigated with a focus on the geometries, relative stabilities and chemical bonding using ωB97XD/6-31+G(d,p) to elucidate the photo-salient effect in [Co(NH₃)₅NO₂]NO₃Cl. Different techniques like atoms in molecules (AIM), electron localization function (ELF) and natural bonding orbital (NBO) were used to gain insight into the chemical bonds of the isomers and to identify the key factors influencing their relative stabilities. The study of the ground-state potential energy surface of [Co(NH₃)₅NO₂]²⁺ reveals that the nitro/exo-nitrito isomerization reaction can proceed via the following two paths: (1) nitro → TS1 (38.16 kcal mol⁻¹) → endo-nitrito → TS2 (9.68 kcal mol⁻¹) → exo-nitrito and (2) nitro → TS3 (41.76 kcal mol⁻¹) → exo-nitrito. Pathway (1) through endo-nitrito is the most likely isomerization mechanism because of a lower energy barrier than pathway (2). The intramolecular-resonance-assisted hydrogen bonds (N–H⋯O and N–H⋯N), the orientation of NO₂, and the difference between Co–N and Co–O bond energies are identified as the key factors determining the relative stabilities of the linkage isomers. Co(NH₃)₆³⁺ is less stable compared to Co(NH₃)₅NO₂²⁺ and undergoes a slight geometrical distortion from D_3d to either D₃ or S₆ characterized by a stabilization energy of ∼750 cm⁻¹ at CCSD(T)/6-31+G(d,p).