Coordinatively unsaturated cobalt ion in Co⁺(H ₂O)_n (n = 4-6) probed with infrared photodissociation spectroscopy

The hydrated cobalt ions, Co⁺(H₂O)_n (n = 4-6), are studied with the infrared (IR) photodissociation spectroscopy in the OH-stretch region and density functional theory calculations. The calculations predict a T-shaped coordination structure for Co⁺(H ₂O)₃, which exposes empty coordination sites for additional H₂O ligands. Nevertheless, the IR spectrum of Co ⁺(H₂O)₄ indicates that the fourth H ₂O prefers to occupy the second shell through H-bonding rather than coordinate directly to Co⁺. A comparison between the experimental and theoretical IR spectra suggests that the T-shaped coordination remains intact in the n = 4-6 ions, leaving the direct coordination sites unoccupied.