Coordinatively unsaturated cobalt ion in Co+(H2O)n (n = 4-6) probed with infrared photodissociation spectroscopy

► Theoretical calculations predict a T-shaped coordination for Co+(H2O)3. ► Such Co+(H2O)3 ion exposes empty binding sites for additional ligands. ► IR spectroscopy suggests a coordination number of 3 for Co+(H2O)4-6. ► The coordination of Co+ in Co+(H2O)4-6 seems to be unsaturated.