TRANSITION-METAL POLYHYDRIDE COMPLEXES .8. PENTAHYDRIDO(CYCLOPENTADIENYL)OSMIUM(VI)

The recent synthesis of (pentamethylcyclopentadienyl)osmium(VI) pentah ydride (CpOsH5) completes the series of high-oxidation-state cyclopen tadienyl metal polyhydrides. A single hydride resonance in the proton NMR spectrum indicated that the complex contained either five equivale nt hydride ligands or inequivalent hydrides in rapid exchange, Vibrati onal studies were inconclusive as to whether the molecule adopted a C- 5v (pseudo-pentagonal-pyramid with the Cp ring occupying the axial pos ition) or a pseudo-C-4v (pseudo-octahedral with the Cp ring considered as occupying one site) orientation in the ground state. In this study , ab initio calculations with effective core potentials were performed on pentahydrido(cyelopentadienyI)osmium(VI), CpOsH(5), to determine i ts structure, vibrational spectrum, and rearrangement mechanism. Geome try optimizations at the Hartree-Fock (HF) and second-order Moller-Ple sset (MP2) levels of theory and energy calculations at the MP3, config uration interaction singles and doubles (CISD), coupled cluster single s and doubles (CCSD), and coupled cluster singles and doubles with per turbative estimate of triple excitations (CCSD(T)) levels place the ps eudo-C-4v, structure lower in energy than the C-5v structure. When com pared to the experimental results, the calculated frequencies confirm that the pseudo-C-4v geometry is the experimentally observed conformat ion. The topography of the MP2 potential energy surface supports the n otion of a highly fluxional pseudo-C-4v geometry which exchanges hydri de ligands through a Bailor or trigonal-twist mechanism without passin g through the C-5v geometry.