Chapter 5 Kinetic and Equilibrium Deuterium Isotope Effects for C-H Bond Reductive Elimination and Oxidative Addition Reactions Involving the Ansa-Tungstenocene Methyl-Hydride Complex (Me 2 Si(C 5 Me 4 ) 2 )W(Me)H

The reductive elimination of methane from [Me 2 Si(C 5 Me 4 ) 2 ]W(CH 3 )H and [Me 2 Si(C 5 Me 4 ) 2 ]W(CD 3 )D is characterized by an inverse kinetic isotope effect (KIE). 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 and that the inverse KIE for overall reductive elimination is a result of an inverse equilibrium isotope effect (EIE), rather than being a result of an inverse KIE for a single step. Calculations on [H 2 Si(C 5 H 4 ) 2 ]W(Me)H support these results and further demonstrate that the interconversion between [H 2 Si(C 5 H 4 ) 2 ]W(Me)H and the σ-complex [H 2 Si(C 5 H 4 ) 2 ]-W(σ-HMe) is characterized by normal kinetic isotope effects for both reductive coupling and oxidative cleavage. Interestingly, the temperature dependencies of EIEs for coordination and oxidative addition of methane to the tungstenocene fragment {[H 2 Si(C 5 H 4 ) 2 ]W} are calculated to be very different, with the EIE for coordination approaching zero at OK, while the EIE for oxidative addition approaches infinity.