Cis-trans isomerization in triply-bonded ditungsten complexes: a multitude of possible pathways

We have investigated different possible mechanisms for the cis-trans isomerization in triply bonded ditungsten complexes with stoichiometry W(2)Cl(4)(NHEt)(2)(PMe(3))(2) using static density functional calculations as well as Car-Parrinello simulations. Our studies reveal an unexpected richness of possible reaction pathways that include both unimolecular and bimolecular mechanisms. Among the possible routes that have been identified are processes involving successive dissociation/reassociation of phosphine ligands, intramolecular chloride hopping, intertungsten phosphine exchange as well as numerous combinations of these basic reaction types. All pathways involve maximal activation barriers of less than 35 kcal/mol and include phosphine concentration dependent and independent routes. The energetically most favorable phosphine-dependent pathway is based on the dissociation/reassociation of phosphine ligands. This path is characterized by a maximal dissociation barrier of 18 kcal/mol. The fastest alternative unimolecular route (with a maximal activation barrier of 24 kcal/mol) is based on a direct exchange of phosphine between the two metallic coordination centers. All the identified pathways, with the exception of a previously proposed internal flip mechanism that can be ruled out on energetic grounds, are competitive and may contribute in various combinations to the overall reaction rate. The identified isomerization mechanisms are fully consistent with the experimentally observed 3-state-kinetics and the dependence of the overall reaction rate on the excess concentration of phosphine which is demonstrated with a simplified kinetic model of the process.