Normal and Inverse Primary Kinetic Deuterium Isotope Effects for C−H Bond Reductive Elimination and Oxidative Addition Reactions of Molybdenocene and Tungstenocene Complexes:  Evidence for Benzene σ-Complex Intermediates

The overall reductive elimination of RH from the ansa-molybdenocene and -tungstenocene complexes Me(2)Si(C(5)Me(4))(2)Mo(Ph)H and Me(2)Si(C(5)Me(4))(2)W(R)H (R = Me, Ph) is characterized by an inverse primary kinetic isotope effect (KIE) for the tungsten system but a normal KIE for the molybdenum system. Oxidative addition of PhH to [Me(2)Si(C(5)Me(4))(2)M] also differs for the two systems, with the molybdenum system exhibiting a substantial intermolecular KIE, while no effect is observed for the tungsten system. These differences in KIEs indicate a significant difference in the reactivity of the hydrocarbon adducts Me(2)Si(C(5)Me(4))(2)M(RH) for the molybdenum and tungsten systems. Specifically, oxidative cleavage of Me(2)Si(C(5)Me(4))(2)M(RH) is favored over RH dissociation for the tungsten system, whereas RH dissociation is favored for the molybdenum system. A kinetics analysis of the interconversion of Me(2)Si(C(5)Me(4))(2)W(CH(3))D and Me(2)Si(C(5)Me(4))(2)W(CH(2)D)H, accompanied by elimination of methane, provides evidence that the reductive coupling step in this system is characterized by a normal KIE. This observation demonstrates that the inverse KIE for overall reductive elimination is a result of an inverse equilibrium isotope effect (EIE) and is not a result of an inverse KIE for a single step. A previous report of an inverse kinetic isotope effect of 0.76 for C-H reductive coupling in the TpPt(CH(3))H(2) system is shown to be erroneous. Finally, a computational study provides evidence that the reductive coupling of Me(2)Si(C(5)Me(4))(2)W(Ph)H proceeds via the initial formation of a benzene sigma-complex, rather than an eta(2)-pi-benzene complex.