Infrared photodissociation spectroscopy of Co⁺(NH₃)_n and Ni⁺(NH₃)_n: Preference for tetrahedral or square-planar coordination

Coordination structures of the Co⁺(NH₃)_n and Ni⁺(NH₃)_n ions are probed by infrared (IR) photodissociation spectroscopy with the aid of density functional theory (DFT) calculations. The IR spectra of N₂-tagged Co⁺(NH₃)_n (n = 1-4) exhibit two distinct bands assignable to the symmetric and antisymmetric NH stretches of the NH₃ molecules binding directly to Co⁺. Size-dependent changes in the spectra of Co⁺(NH₃)_n (n = 4-8) indicate that the first shell of Co⁺ is filled with four NH₃ molecules and the resulting 4-coordinated structure forms the central core of further solvation. The spectra of Ni⁺(NH₃)_n (n = 3-8) suggest that the coordination number of Ni⁺ is also four, although a minor 3-coordinated isomer is identified for Ni⁺(NH₃)₄. Despite the same coordination number, the DFT calculations predict a distorted square-planar coordination for Ni⁺(NH₃)₄ and a distorted tetrahedral coordination for Co⁺(NH₃)₄. The coordination of Ni⁺(NH₃)₄ is explainable by using a simple model based on the geometry of a half-filled 3d orbital in Ni⁺. This suggests that the Ni⁺ ion gives priority to the minimization of the metal-ligand repulsion in accommodating four ligands in the first shell. On the other hand, the same model fails to explain the coordination of Co⁺(NH₃)₄. An interpretation for this is that the Co⁺ ion gives priority to the minimization of the ligand-ligand repulsion.