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=€H2+, 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 *A2 symmetry and contains covalent M-C u and a bonds ('metal-methylidene"); a 2A, state of the same bond character is only 1.2 kcal/mol higher. A cluster of three degenerate excited states (4A2, 4B1, and 4B2) 12.9 kcal/mol above the ground state exhibits completely different bonding character, namely, a-donor/a-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!