The reactions of Y^＋, Zr^＋, Nb^＋with CO₂, producing the metal oxide ion and CO, are taken as a representative examples to elucidate the overall mechanisms of reactions of second-row early transition metal ions with CO₂. The reactions in both ground and excited states are studied by using the UB3LYP density functional method with the Stuttgart pseudopotentials and corresponding basis sets for the metals and the standard 6-311＋G(2d) basis sets for C and O. The geometries for reactants, the transition states and the products are completely optimized. The result shows that the reaction mechanism between second-row early transition metal ions and CO₂ is an insertion-elimination mechanism. The potential energy curve crossings, which dramatically affect reaction mechanisms, are discussed in detail. The reactions are all exothermic due to the participation of the metal ions, to be compared with the strong endothermic process of the unimolecular CO₂ decomposition.

Key words: early transition-metal ion, two-state reactivity, potential energy surfaces crossing point (CP), exothermic process