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 [Me₂Si(C₅Me₄)₂]Mo(Ph)H and [Me₂Si(C₅Me₄)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 {[Me₂Si(C₅Me₄)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 [Me₂Si(C₅Me₄)₂]M(RH) for the molybdenum and tungsten systems. Specifically, oxidative cleavage of [Me₂Si(C₅Me₄)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 [Me₂Si(C₅Me₄)₂]W(CH₃)D and [Me₂Si(C₅Me₄)₂]W(CH₂D)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 [Tp]Pt(CH₃)H₂ system is shown to be erroneous. Finally, a computational study provides evidence that the reductive coupling of [Me₂Si(C₅Me₄)₂]W(Ph)H proceeds via the initial formation of a benzene σ-complex, rather than an η²-π-benzene complex.