Possibility of bond stretch isomerism in [Cp(CO)₂M]₂(μ-E) complexes (M = Mn, Re, Cr and W; E = S, Se and Te); a molecular orbital study

Abstract

Extended Huckel calculations have been carried out on [Cp(CO)₂M]₂(μ-E) complexes (M = Mn, Re, Cr and W; E = S, Se and Te) in linear, bent and triangular geometries. The possibility of double well potential and consequent "bond stretch isomerism" in these complexes is analyzed. All the complexes with a 38 valence electron count (VEC) are calculated to have a minimum corresponding to the triangular geometry. The Mn and Cr complexes with a tellurido bridge have an additional minimum for the bent geometry; only the bent structure is known experimentally. The triangular isomers of these two complexes are marginally higher in energy, but separated by substantial barriers. The other M₂E combinations studied adopt a triangular geometry. The difference in the behaviour between these complexes is explained by the fragment molecular orbital approach and by considering the atomic radii of the M and E species. Single point ab initio calculations using the LANL1MB basis set on [Cp(CO)₂M]₂(μ-E) (M = Mn and Re; E = S, Se and Te) confirm the relative energy orderings produced by the extended Huckel calculations. The relation between the M---E bond orders and bond lengths and the extent of Main Group d-orbital participation is also analyzed. It is predicted that complexes with a VEC of 36 and 40 should adopt linear and bent geometries, respectively.