Three- and four-center trans effects in triply bonded ditungsten complexes: An ab initio molecular dynamics study of compounds with stoichiometry W2Cl4(NHEt)(2)(PMe3)(2)

We have performed ab initio molecular dynamics simulations based on density functional theory to characterize the structural, electronic, and dynamic properties of the three major isomeric forms of the title compound. In agre ement with experimental results, calculations with two different parametriz ations of the exchange-correlation functional (BLYP and BP) both indicate t he cis-C-2 form as the most stable isomer. The relative energies of the dif ferent forms are, however, small (less than or similar to1-2 kcal/mol), and the three compounds show overall very similar ground-state properties. Lar ger differences exist in their finite temperature behavior, which is domina ted by the facile dissociation of one or both phosphine ligands. The calcul ated activation energies for phosphine dissociation differ clearly for the trans and the cis isomers and vary in the order trans much less than cis-C- 2 less than or similar to cis-C-i. Analysis of the electronic structure of the transition states shows that the difference in activation energy betwee n cis and trans isomers can be rationalized in terms of a classic trans eff ect caused by a molecular orbital spanning the three atomic centers N-W-P. The subtle difference between the two cis isomers, on the other hand, is li kely due to an analogous four-center trans effect N-W-W-P which is mediated via metal-metal orbitals and involves ligands on both tungsten atoms.