THEORETICAL-STUDIES OF ORGANOMETALLIC COMPOUNDS .5. ALKYNE AND VINYLIDENE COMPLEXES OF MOLYBDENUM AND TUNGSTEN IN HIGH-OXIDATION STATES

The geometries of the acetylene complexes MX4C2H2 and the vinylidene i somers MX4CCH2 (M = W, MO; X = F, Cl) are theoretically predicted usin g quantum mechanical ab initio methods at the Hartree-Fock level of th eory and relativistic effective core potentials for the transition met als. The optimized geometries and energies of the anionic complexes MX 5C2H2- are also reported. The optimization of WX5C2H2- gives geometrie s for the chloro and fluoro complexes, which are in good agreement wit h experiment. The corresponding MoX5C2H2- structures are not minima on the potential energy surface. The geometries and energies of the alky ne complexes are compared with the optimized structures of the vinylid ene complexes. The vinylidene complexes are calculated to be slightly higher in energy than the alkyne complexes, but they become clearly mo re stable than the alkyne complexes when hydrogen is substituted by fl uorine. The electronic structure of the complexes is investigated usin g the natural bond orbital population analysis and the topological ana lysis of the wave function. Detailed information is given about the na ture of the metal-carbon bonds and the hybridization and atomic popula tion of the transition metals in the alkene and vinylidene complexes.