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.