Theoretical aspects of the bonding in acetylide-bridged organometallic dinuclear complexes

The electronic and geometrical structures of a variety of acetylide-bridged organometallic dinuclear complexes (ML n ) 2 (μ-C 2 ) (M = Sc, Ti, W, Ru, Pt) are analysed and compared by use of extended Hückel and density functional-theory calculations. It is established that all the studied complexes feature the ethyne valence structure, except the titanium species for which resonance forms of ethyne-type and cumulenic-type must be invoked to rationalise its structure. Results indicate that the σ-bonded framework is similar in all complexes and mainly governs the metal–carbon bonding in these species. In all cases but the titanium compound, the σ-type M–C is hardly complemented by π-back-donation from the metals into acceptor C 2 orbitals. The peculiar energy and composition of the HOMOs, which are in general metal–carbon antibonding and carbon–carbon bonding for the metals at the right of the periodic table, can allow the observation of cumulenic structures upon oxidation. .