Partial deuteriation studies, ethyl-group geometries and C–H bond properties in [MEt(cp)(CO)<sub>2</sub>](M = Fe or Ru) and [MEt(cp)(CO)<sub>3</sub>](M = Mo or W)(cp =η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)
作者:A. H. Jean Robertson、Geoffrey P. McQuillan、Donald C. McKean
DOI:10.1039/dt9950003963
日期:——
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). The isolated CH stretching frequencies v(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 [v(is)(CH) Mo, 2885. 2941;W, 2888, 2936; Fe, Ru. 2877, 2927 cm(-1)]. These differences in v(is)(CH) indicate 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 non-agostic alkyltransition-metal compounds. The methylene (alpha-C-H) bonds are: also weakened relative to ethane, although to a lesser extent than the trans-beta-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)Cl-2]. The ethyl groups occur in two conformations, C-s and C-1, but the methyl C-H bond properties are unaffected by the differences in orientation.