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)H-2 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 a-complex, rather than an eta(2)-pi-benzene complex.