THE NON-PERPENDICULAR AND NONPARALLEL ALKYNE BRIDGE IN W-2(MU-C2H2)(MU-OCH2,BU-T)(2)(OCH2BU-T)(6)

The bonding in the ethyne adduct W-2(mu-C2H2)(mu-ONp)(2)(ONp)(6) (Np = (CH2Bu)-Bu-t) has been examined by various computational methods [Ext ended Huckel (EHMO), Fenske-Hall, and Gaussian 92 RHF (Restricted Hart ree-Fock) and density functional (Becke-3LYP) calculations] employing the model compound W-2(mu-C2H2)(mu-OH)(2)(OH)(6). EHMO and Fenske-Hall calculations suggest, based on total orbital energy, that a mu-parall el ethyne geometry should have the lowest energy, although traditional frontier orbital arguments agree with the observance of a skewed acet ylene bridge. Gaussian 92 computations reproduce the non-perpendicular /non-parallel mu-C2H2 geometry in close agreement to that observed in the solid-state (X-ray) structure, which leads us to suggest that the distortion is not sterically imposed by the attendant alkoxide ligands . The observed geometry can be rationalized in terms of Jahn-Teller di stortional stabilization from either the mu-parallel or mu-perpendicul ar mode, i.e., the geometry is favored on electronic grounds, though t he potential energy surface is rather shallow. These results are discu ssed in terms of previous studies of the addition of alkynes to d(3)-d (3) dinuclear complexes of tungsten and in terms of relationships betw een d(2)-W(OR)(4) and d(8)-Os(CO)(4) fragments. (C) 1998 Elsevier Scie nce S.A.