Density functional study of hydrogen exchange and methane elimination from bis(cyclopentadienyl)tungsten methyl hydrides and their ansa-bridged analogues

Theoretical calculations have been carried out using density functional theory on the hydrogen exchange and methane elimination reactions of [W(eta-C5H5)(2)(Me)H] and [W((eta-C5H4)(2)CH2) (Me)H]. In both cases the hydrogen exchange proceeds along a similar reaction path on which lies a eta(1)-H sigma complex. The midpoint of the exchange reaction is a eta(2)-H,H sigma complex. For [W(eta-C5H5)(2)(Me)H] a transition state is proposed which is calculated to lie 81 kJ mol(-1) above the ground state. This compares with an experimental free energy of activation of 106 kJ mol(?)(-1) for [W(eta-C5H5)(2)(Me)D]. Elimination of methane is calculated to have an overall reaction energy of 19.6 kJ mol(-1) for [W(eta-C5H5)(2)(Me)H] and 78.3 kJ mol(-1) for [W((eta-C5H4)(2)CH2)(Me)H]. The activation energy for elimination from the two compounds is estimated as around 80 and 85 kJ mol(-1) respectively. The principal reason for the relative stability of the [W(eta-C5H5)(2)] product is its relaxation to a parallel ring triplet state which is calculated to lie 67.9 kJ mol(-1) below the lowest excited singlet state. For [W((eta-C5H4)(2)CH2)] the singlet and triplet states are separated by 13.3 kJ mol(-1) with the rings inclined at an angle constrained by the ansa bridge.