PARTIAL DEUTERIATION STUDIES, ETHYL-GROUP GEOMETRIES AND C-H BOND PROPERTIES IN [MET(CP)(CO)(2)] (M=FE OR RU) AND [MET(CP)(CO)(3)] (M=MO ORW) (CP=ETA(5)-C5H5)

Vibrational spectra have been obtained for the CH3CH2, CH3CD2, CD3CH2, CHD2CD2 and (for Fe and Mo) CHD2CHD isotopomers of [MEt(cp)(CO)(2)] ( M = Fe or Ru) and [MEt(cp)(CO)(3)] (M = Mo or W) (cp = eta(5)-C5H5). T he isolated CH stretching frequencies v(is)(CH), observed in the spect ra of the CHD(2)CD(2)M isotopomers, show that the terminal methyl grou ps in all four compounds are significantly asymmetric. with one weak a nd two equivalent stronger C-H bonds [v(is)(CH) Mo, 2885. 2941;W, 2888 , 2936; Fe, Ru. 2877, 2927 cm(-1)]. These differences in v(is)(CH) ind icate differences of about 0.005 Angstrom in bond length, or 20 kJ mol (-1) in bond-dissociation energy, between the weaker and stronger C-H bonds in each methyl group. The effect of the metal atom substituents is to weaken all three beta-C-H bonds (relative to ethane), the gauche bonds by a relatively small amount but the trans bonds by as much as 25 kJ mol(-1). These bonds are amongst the weakest yet identified in n on-agostic alkyltransition-metal compounds. The methylene (alpha-C-H) bonds are: also weakened relative to ethane, although to a lesser exte nt than the trans-beta-C-H bonds, and are weaker than the methyl C-H b onds in the corresponding methylmetal compounds. There is no evidence to suggest that the methylene C-H bonds are inequivalent, as appears t o be the case in [TiEt(cp)Cl-2]. The ethyl groups occur in two conform ations, C-s and C-1, but the methyl C-H bond properties are unaffected by the differences in orientation.