Nature and Strength of Metal-Chalcogen Multiple Bonds in High Oxidation State Complexes

Density functional theory calculations have been carried out on the trigonal complexes OsO3E and MCl3E (M = V. Ta) and the square pyramidal systems MCl4E (M = Cr, Mo, W, Re) for E = O, S, Se, and Te as well as (C5H5)ReO3. All complexes were fully optimized, and the calculated geometrical parameters are in reasonable agreement with gas-phase electron diffraction data where available. The calculated M-E bond energies decrease from oxygen to tellurium, from bottom to top in a metal triad, and from left to right in a transition series. The trend setting factor is the donation from the dσ metal orbital to the pσ acceptor orbital on the chalcogen atom, The contribution from the chalcogen to metal π back-donation has a maximum for sulfur and selenium. However in relative terms, the contribution from the π back-donation to the total M-E bond energy increases from oxygen to tellurium. Comparisons are made to previous calculations and experimental data on M-E bond strengths.