Early- versus late-transition-metal-oxo bonds: The electronic structure of VO⁺ and RuO⁺

From all-electron ab initio generalized valence bond calculations (GVBCI-SCF) on VO⁺ and RuO⁺, we find that an accurate description of the bonding is obtained only when important resonance configurations are included self-consistently in the wave function. The ground state of VO⁺(³Σ^-) has a triple bond similar to that of CO, with D_e^calcd(V-O) = 128.3 kcal/mol [D_e^exptl(V-O) = 1.31 ± 5 kcal/mol], while the ground state of RuO⁺(⁴Δ) has a double bond similar to that of O₂, with D_e^calcd(Ru-O) = 67.1 kcal/mol. Vertical excitation energies for a number of low-lying electronic states of VO⁺ and RuO⁺ are also reported. These results indicate fundamental differences in the nature of the metal-oxo bond in early and late metal oxo complexes that explain the observed trends in reactivity (e.g., early metal oxides are thermodynamically stable whereas late metal oxo complexes are highly reactive oxidants). Finally, we have used these results to predict the ground states of MO⁺ for other first-row transition-metal oxides.