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+(H2O)(3), which exposes empty coordination sites for additional H2O ligands. Nevertheless, the IR spectrum of Co+(H2O)(4) indicates that the fourth H2O 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. (C) 2011 Elsevier B. V. All rights reserved.