Methyl group geometries, C–H bond properties and internal rotation in dicarbonyl(η5-cyclopentadienyl)methyl-iron(II) and -ruthenium(II), [MMe(η5-C5H5)(CO)2](M = Fe or Ru)

Abstract

Vibrational spectra have been recorded for the CH₃, CD₃ and CHD₂ isotopomers of [MMe(cp)(CO)₂](M = Fe or Ru; cp = cyclopentadienyl). The effects of Fermi resonance were considered in detail and the resonance-corrected CH and CD stretching frequencies used to predict methyl group geometries. Only one ‘isolated’ CH stretching absorption, ν^isCH, is observed in each CHD₂ isotopomer. The results are not consistent with a model in which the methyl groups are subject to an effective barrier to internal rotation, with significant variation of CH bond length with orientation. The spectra are best interpreted in terms of an essentially freely rotating methyl group in which the individual CH bonds undergo a small variation in bond length with rotational angle. The larger barriers which occur in the tricarbonyl compounds [MMe(cp)(CO)₃](M = Cr, Mo or W) are likely to be primarily steric in origin, whereas that in [TiMe(cp)Cl₂] may arise from an interaction between a CH bonding pair and an unoccupied titanium orbital. The Ru–CH₃ bond appears to be slightly weaker than the Fe–CH₃ bond, in contrast with results for earlier groups in the d block, in which M–CH₃ bond strengths increase down each group.