METHANE ADDUCTS OF D(0) TRANSITION-METAL IMIDO COMPLEXES

Ab-initio results are presented for a series of methane adducts of d0, three-coordinate imido complexes. The presence of weakly bound adducts on the reaction coordinate for CH activation has been inferred from kinetic and spectroscopic data, although their role, if any, in CH activation is not clear. Methane adducts of pyramidal M(NH2) (=NH)2 (M = Nb, Ta) and M(=NH)3 (M = Mo, W) have appreciable calculated binding enthalpies (almost-equal-to 11-16 kcal mol-1), comparable to values measured for Kubas' H-2 complex. However planarimidos ((NH2)2M=NH, M = Ti, Zr, (OH)2Ti=NH) have much smaller binding enthalpies. The calculations suggest a significant covalent contribution to the bonding between substrate and the formally d0 complex. Upon coordination there is weakening of methane CH bonds, charge transfer from methane to metal, and increased C(delta)-H(delta+) polarization, all indicative of a role for the adduct in the all-important CH scission step to follow. The calculations suggest that a methane adduct of Ta(=NR)2(NHR) or W(=NR)3 affords the best opportunity to have an appreciable lifetime and form in concentrations large enough to facilitate characterization, perhaps by methods similar to those used in agostic complexes, e.g. IR and NMR analyses of shifts in (nu)CH and 1J(CH), respectively.