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

Abstract

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.