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 isomers MX₄CCH₂ (M = W, Mo; X = F, Cl) are theoretically predicted using quantum mechanical ab initio methods at the Hartree-Fock level of theory and relativistic effective core potentials for the transition metals. The optimized geometries and energies of the anionic complexes MX₅C₂H₂⁻ are also reported. The optimization of WX₅C₂H₂⁻ gives geometries for the chloro and fluoro complexes, which are in good agreement with experiment. The corresponding M0X₅C₂H₂⁻ structures are not minima on the potential energy surface. The geometries and energies of the alkyne complexes are compared with the optimized structures of the vinylidene complexes. The vinylidene complexes are calculated to be slightly higher in energy than the alkyne complexes, but they become clearly more stable than the alkyne complexes when hydrogen is substituted by fluorine. The electronic structure of the complexes is investigated using the natural bond orbital population analysis and the topological analysis of the wave function. Detailed information is given about the nature of the metal-carbon bonds and the hybridization and atomic population of the transition metals in the alkene and vinylidene complexes.