We have investigated different possible mechanisms for the cis-trans isomerization in triply bonded ditungsten complexes with stoichiometry W2Cl4(NHEt)2(PMe3)2 using static d. functional calcns. as well as Car-Parrinello simulations. Our studies reveal an unexpected richness of possible reaction pathways that include both unimol. and bimol. mechanisms. Among the possible routes that have been identified are processes involving successive dissocn./reassocn. of phosphine ligands, intramol. 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 concn. dependent and independent routes. The energetically most favorable phosphine-dependent pathway is based on the dissocn./reassocn. of phosphine ligands. This path is characterized by a maximal dissocn. barrier of 18 kcal/mol. The fastest alternative unimol. 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 exptl. obsd. 3-state-kinetics and the dependence of the overall reaction rate on the excess concn. of phosphine which is demonstrated with a simplified kinetic model of the process. [on SciFinder (R)]