The electronic structure of transition metal carbonyl complexes

Discrete variational X α calculations are reported on the neutral carbonyl complexes Ni(CO)4, Fe(CO)5 and Cr(CO)6, as well as on the ions Co(CO)4 -, Fe(CO)4 -, Mn(CO)5 -, Mn(CO)6 + and V(CO)6 -. Valence electron ionization potentials are predicted very well by transition-state calculations. The description of complex binding in Ni(CO)4 according to these calculations differs markedly from the one obtained in MSXα and Hartree-Fock calculations, which contradicted each other. The relative magnitude of σ-donation and π-back-donation is found to be roughly equal in the three neutral complexes (0·2–0·3 and 0·5 e respectively). Large changes occur in the ionic complexes, notably in the π-back-donation. These changes are correlated with the observed shifts in C-O stretching frequencies and force constants.