Ni+(CO2)(n) and NiO2+(CO2)(m) ion-molecule complexes are mass-selected and studied with infrared photodissociation spectroscopy. Dissociation is inefficient until clusters exceed a critical size of n = 4 for Ni+(CO2)(n) and m = 2 for NiO2(CO2)(m)(+). Dissociation of larger clusters terminates at these same critical sizes. Just beyond the critical size, the infrared spectra exhibit new vibrational bands associated with ligands not coordinated to the metal. These observations indicate that the coordination of Ni+ ions with respect to CO2 binding is exactly four, and that the coordination for NiO2+ is two CO2 ligands, making the overall coordination around the metal the same value of four. (C) 2003 Published by Elsevier B.V.