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 or W)(cp =η5-C5H5)

Abstract

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 =η⁵-C₅H₅). The isolated CH stretching frequencies ν^is(CH), observed in the spectra of the CHD₂CD₂M isotopomers, show that the terminal methyl groups in all four compounds are significantly asymmetric, with one weak and two equivalent stronger C–H bonds [ν^is(CH) Mo, 2885, 2941; W, 2888, 2936; Fe, Ru, 2877, 2927 cm^–1]. These differences in ν^is(CH) indicate differences of about 0.005 Å 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 β-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 non-agostic alkyltransition-metal compounds. The methylene (α-C–H) bonds are also weakened relative to ethane, although to a lesser extent than the trans-β-C–H bonds, and are weaker than the methyl C–H bonds in the corresponding methylmetal compounds. There is no evidence to suggest that the methylene C–H bonds are inequivalent, as appears to be the case in [TiEt(cp)Cl₂]. The ethyl groups occur in two conformations, C_s and C₁, but the methyl C–H bond properties are unaffected by the differences in orientation.