Investigation of the electronic structures of η-cyclopentadienyl transition-metal dihalide dimers

Abstract

Extended-Hückel molecular-orbital calculations for the model systems [W₂(cp)₂Cl₄], [Mo₂(cp)₂(µ-Cl)₂, and [Re₂(cp)₂Cl₂(µ-Cl)₂](cp =η-C₅H₅) were used to analyse the bonding in the three observed, distinct geometries of metal–metal bonded (η-cyclopentadienyl)transition metal dihalide dimers of the general formula [{M(η-C₅R₅)X₂}₂], and are supported by He I and He II photoelectron spectra for [W₂(η-C₅H₄Prⁱ)₂Cl₄] and by a He I photoelectron spectrum for [Mo₂(η-C₅H₄Me)₂(µ-Br)₄]. The He I and He II photoelectron spectra are also reported for the related complex [Mo₂(η-C₅H₄Prⁱ)₂(µ-SMe)₄]. The calculations show that the metal d-electron count is the most important factor selecting the observed geometry of [{M(η-C₅R₅)X₂}₂](X = Cl or Br) species, but that the extent of metal–metal overlap and non-bonding interactions within the (µ-X)₄ linkages of the quadruply bridged isomers must also be considered.