Bonding in Transition-Metal-Methylene Complexes. 2. (Ruch₂)⁺, a Complex Exhibiting Low-Lying Methylidene-like and Carbene-like States

The electronic structure for a representative late-transition-metal-methylene complex, Ru=CH₂⁺, has been studied by ab initio methods (generalized Valence bond/configuration interaction). The electronic-state spectrum reveals five states close in energy (spread of 12.9 kcal/mol) that partition into two groups in terms of energy separation and mode of metal-carbon bonding. The ground state has ²A₂symmetry and contains covalent M-C σ and π bonds (“metal-methylidene“); a ² A1 state of the same bond character is only 1.2 kcal/mol higher. A cluster of three degenerate excited states (⁴A₂,⁴B_1,and ⁴B₂) 12.9 kcal/mol above the ground state exhibits completely different bonding character, namely, σ-donor/π-acceptor M-C bonds are formed (“metal-carbene”). We conclude that for highly unsaturated, late-transition-metal systems, metal-carbene bonding may be competitive with metal-alkylidene bonding, leading to donor/acceptor bonds comparable in strength to that of covalent double bonds!.