METHYL-GROUP GEOMETRIES, C-H BOND PROPERTIES AND INTERNAL-ROTATION INDICARBONYL(ETA(5)-CYCLOPENTADIENYL)METHYL-IRON(II) AND ONYL(ETA(5)-CYCLOPENTADIENYL)METHYL-RUTHENIUM(II), [MME(ETA(5)-C5H5)(CO)(2)] (M=FE OR RU)

Vibrational spectra have been recorded for the CH3, CD3 and CHD2 isoto pomers 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 gro up geometries. Only one 'isolated' CH stretching absorption, nu(is)CH, is observed in each CHD, isotopomer. The results are not consistent w ith a model in which the methyl groups are subject to an effective bar rier to internal rotation, with significant variation of CH bond lengt h with orientation. The spectra are best interpreted in terms of an es sentially freely rotating methyl group in which the individual CW bond s undergo a smell 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, whe reas that in [TiMe(cp)Cl-2] may arise from an interaction between a CH bonding pair and an unoccupied titanium orbital. The RU-CH3 bond appe ars to be slightly weaker than the Fe-CH3 bond, in contrast with resul ts for earlier groups in the d block, in which M-CH3 bond strengths in crease down each group.