Theoretical investigation of the spin crossover transition states of the addition of methane to a series of Group 6 metallocenes using minimum energy crossing points

Density functional calculations are reported on the addition of methane to Group 6 metallocenes, M(eta-C5H5)(2) (M), M(CH2 (eta-C5H4)(2)) (a-M) and M(eta-C5Me5)(2) (M*) where M = Mo and W. Full geometry optimisations were carried out on the singlet and triplet 16 electron complexes, (1)[M] and (3)[M], the eta(2) methane complexes, (1)[M(eta(2)-CH4)], and the hydridomethyl adducts, (1)[M(CH3)(H)]. The triplet state for [M] was found to be more stable for all six metallocenes, the difference being least in the case of the ansa-bridged system. Formation of the hydridomethyl complexes was exoenergetic for all tungsten systems and for a-Mo, the other two Mo systems being endoenergetic. Minumum energy crossing points (MECPs) between the triplet and singlet surfaces were calculated for Mo, W, a-W and W*. These MECPs formed the barrier to formation of the methane complex. Transition states for insertion of M into the C-H bond and exchange between the coordinated H of the methane complex were also calculated for Mo, W, a-W and W*. For W and W* these were of similar height to the MECP. For a-W the insertion barrier was lower than the MECP while for Mo it was higher. Activation of methane was established as being most favourable for a-W. The calculated results are fully in accord with published experimental data on hydrogen exchange in and thermal stability of (1)[M(CH3)( H)] where M = W, a-W and W*.