Electronic Structures and Gas-Phase Reactivities of Cationic Late-Transition-Metal Oxides

The structures, relative stabilities, and multiplicities of the cationic, late-transition-metal oxides FeO⁺, CoO⁺, NiO⁺, and CuO⁺ are rationalized on the basis of ab initio computations. The bonding situation in these cations is analogous to that in the dioxygen molecule with a biradicaloid π-bonding, and hence the electronic ground states of these metal oxide cations correspond to their high-spin variants, FeO⁺ (⁶Σ⁺), CoO⁺ (⁵Δ), NiO⁺ (⁴Σ^-), CuO⁺ (³Σ^-). Density functional theory augmented with CASPT2D computations is used to explore the reaction surface of FeO⁺ + H₂ and to unravel the roots of the extremely low reactivity observed for this system. According to these calculations, the reaction violates spin-conservation rules and involves a curve crossing from the sextet ground state to the excited quartet surface, giving rise to a multicentered, energetically low-lying transition structure, from which the hydrido iron hydroxide cation H—Fe—OH⁺ is formed as the initial oxidation product. The implications of these results with respect to other ion/molecule processes of metal oxide cations with oxidizable organic substrates are discussed.