Theoretical analysis of bis(ethylene) complexes of molybdenum and tungsten

Density functional calculations on the zero-valent bis(ethylene) trans-[M(C2H4)2(PH3)4] model complexes and the actual trans-[M(C2H4)2(PMe3)4] (M=Mo, W) compounds were carried out at the BPW91 level of theory. The experimentally characterised molecular structures of both molybdenum and tungsten complexes were properly reproduced by the energy optimised geometries only when the trimethylphosphine ligands were used. The experimentally observed ruffling of the phosphine ligands was not detected when the simplified model was considered. This discrepancy was interpreted as a consequence of different steric interactions between the PH3 and PMe3 ligands. The theoretical analysis gives a good account of some chemical properties observed in these compounds. In particular, the low PMe3 dissociation energies calculated for these compounds are in agreement with the experimental observation of free phosphine dissociation in the solutions of trans-[M(C2H4)2(PMe3)4] compounds. The comparison of energy data for PH3 and PMe3 complexes corroborates the experimental hypothesis that phosphine dissociation has a steric origin.