Gas-phase carbon dioxide activation by Nb(+) and NbO(+) was studied at the density functional level of theory using the hybrid exchange correlation functional B3LYP. Three reaction profiles corresponding to the quintet, triplet, and singlet multiplicities were investigated in order to ascertain the presence of some spin inversion during the CO2 reduction. Carbon dioxide activation mediated by metal cations was found to be an exothermic spin-forbidden process resulting from a crossing between quintet and triplet energetic profiles. The endothermic reaction of NbO(+) with carbon dioxide was a barrierless process involving spin inversion. Geometries of minima along potential energy surfaces and reaction heats were in agreement with those from experimental studies carried out by using a guided ion beam tandem mass spectrometer.