An investigation into the relative influence of alkoxide and thiolate ligands on the metal-carbon triple bond in X3MCH compounds, where M = Cr, Mo and W and X = OH, SH, OCH3, SCH3, OCF3 and SCF3 from electronic structure calculations

Electronic structure calculations using density functional theory with the aid of the GAUSSIAN-98 package have been employed on the model Group 6 metal alkylidyne complexes X(3)Mequivalent toCH, where M = Cr, Mo, and W and X = OH, SH, OCH3, SCH3, OCF3 and SCF3. Collectively, these calculations reveal that the relative energies of the metal-carbon pi-bonding molecular orbitals are greatly influenced by the metal and the ancillary ligand X. In comparing the bonding in related 0 and S containing molecules, the effect of oxygen is to increase the relative energy of the M-C pi bond by approximately 1 eV as a result of M-O pi antibonding interactions. In contrast, the ligand-based lone-pair orbitals lie higher in energy for sulfur and for the series where X = SH, SCH3 and SCF3, the HOMO is a sulfur lone-pair combination of a(2) Symmetry in the C-3v point group. The relative influence of H, CH3 and CF3 within each series is as expected based on their electron releasing properties. A comparison is made with related compounds having metal to metal or metal to nitrogen triple bonds. (C) 2002 Elsevier Science Ltd. All rights reserved.