Abstract: The gas-phase reaction of CrO₂⁺(²A₁/⁴A″) with H₂ to yield CrO+(²Σ_g/⁵Σ_g) and H₂O is selected as a representative system of activation of H—H σ bond by MO₂⁺. The reaction mechanism has been investigated with density functional theory (DFT) at the UB3LYP/6-311++G(3df, 3pd)//6-311G(2d, p) level. The geometries for reactants, the transition states, and the products were completely optimized. All the transition states were verified by the vibrational analysis and the intrinsic reaction coordinate calculations. The H—H bond has been activated by CrO₂⁺(²A₁) on the doublet PES(potential energy surface) while dihydrogen transfer from Cr to O (in longer Cr—O) on the quartet PES. The involving potential energy curve-crossing, with dramatically affecs reaction mechanism and reaction rate, has been discussed in detail. The crossing points (CPs) are localized by means of the Hammond postulate and the intrinsic reaction coordinate (IRC) approach. The formation of the transition metal dihydrogen complex (H₂)CrO₂⁺(²IM1 and ⁴IM1) involves donation of the H₂ σ bonding orbital to the metal and the back-donation by the metal electron to the H₂ σ* antibonding orbital, as illustrated by the fragment molecular orbital(FMO). In addition, the orbital analysis on the activation of the H—H bond and dihydrogen transfer has been carried out by FMO.

Key words: Activation H—H bond, Potential energy surfaces crossing, Fragment molecular orbital