We have performed ab initio mol. dynamics simulations based on d. functional theory to characterize the structural, electronic, and dynamic properties of the three major isomeric forms of the title compd. In agreement with exptl. results, calcns. with two different parametrizations of the exchange-correlation functional (BLYP and BP) both indicate the cis-C2 form as the most stable isomer. The relative energies of the different forms are, however, small (.ltorsim.1-2 kcal/mol), and the three compds. show overall very similar ground-state properties. Larger differences exist in their finite temp. behavior, which is dominated by the facile dissocn. of one or both phosphine ligands. The calcd. activation energies for phosphine dissocn. differ clearly for the trans and the cis isomers and vary in the order trans .mchlt. cis-C2 .ltorsim. cis-Ci. Anal. of the electronic structure of the transition states shows that the difference in activation energy between cis and trans isomers can be rationalized in terms of a classic trans effect caused by a MO spanning the three at. centers N-W-P. The subtle difference between the two cis isomers, on the other hand, is likely 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. [on SciFinder (R)]