Cis-trans isomerization in triply-bonded ditungsten complexes: A multitudeof possible pathways

We have investigated different possible mechanisms for the cis-trans isomer ization in triply bonded ditungsten complexes with stoichiometry W2Cl4(NHEt )(2)(PMe3)(2) using static density functional calculations as well as Car-P arrinello simulations. Our studies reveal an unexpected richness of possibl e reaction pathways that include both unimolecular and bimolecular mechanis ms. Among the possible routes that have been identified are processes invol ving successive dissociation/reassociation of phosphine ligands, intramolec ular chloride hopping, intertungsten phosphine exchange as well as numerous combinations of these basic reaction types. All pathways involve maximal a ctivation barriers of less than 35 kcal/mol and include phosphine concentra tion dependent and independent routes. The energetically most favorable pho sphine-dependent pathway is based on the dissociation/reassociation of phos phine ligands. This path is characterized by a maximal dissociation barrier of IS kcal/mol. The fastest alternative unimolecular route (with a maximal activation barrier of 24 kcal/mol) is based on a direct exchange of phosph ine between the two metallic coordination centers. All the identified pathw ays, with the exception of a previously proposed internal flip mechanism th at can be ruled out on energetic grounds, are competitive and may contribut e in various combinations to the overall reaction rate. The identified isom erization mechanisms are fully consistent with the experimentally observed 3-state-kinetics and the dependence of the overall reaction rate on the exc ess concentration of phosphine which is demonstrated with a simplified kine tic model of the process.