Understanding the agostic bonding for group 4 metal methylidene complexes: A DFT approach

The effect of mono- and di-halogen substituted group 4 transition metal methylidene on formation of agostic bond was investigated through DFT calculation and NBO analysis. For CH2MHX (X = F, H, Cl, Br and I; M = Ti, Zr and Hf) the increasing agostic interaction was mainly caused by p electron delocalization of halide atoms. For CHXMH2 the p electrons of halogen compete for the vacant orbital of metal atom and the competition becomes stronger with the increasing halogen size. For CH2MX2 metal center is saturated with p orbital electrons of two halogens that prevent agostic bonding. For CHXMHX the strong agostic interaction is originated from the π electron delocalization. Substituted halogen species, halogen number and substituted sites could affect agostic interaction, which clearly evidences delocalization of p lone pair electrons of halogen influences the intensity of agostic interaction and an empty d orbital for the transition metal center without occupation is critical for forming an agostic bond.