Carbon–hydrogen bond properties and alkyl group geometries in dichloro(η5-cyclopentadienyl)-methyltitanium(IV) and dichloro(η5-cyclopentadienyl)-ethyltitanium(IV)[TiR(η5-C5H5)Cl2](R = Me or Et)

Abstract

Infrared spectra have been recorded for various isotopomers of [TiMe(cp)Cl2](cp = cyclopentadienyl, η5-C5H5; Me = CH3, or CHD2) and [TiEt(cp)Cl2](Et = CH3CH2, CD3CH2, CH3CD2 or CHD2CD2) and assignments proposed for the alkyl group vibrations. A method was elaborated for the calculation of Fermi-resonance shifts on CH and CD stretching modes in methyl groups with C3v and Cs symmetry. Estimates of CH bond lengths, bond strengths and HCH angles were derived from the resonance-corrected frequency data. The results show the methyl group in [TiMe(cp)Cl2] to be markedly asymmetric, with the CH bond trans to the cyclopentadienyl ligand being ca. 0.005 Å longer and 15 kJ mol–1 weaker than those trans to chlorine. In the ethyl compound, the terminal methyl group is similarly asymmetric, again with one weak bond and two stronger bonds. The νCH2 and νCD2 frequencies for the methylene group are anomalous and can only be satisfactorily interpreted in terms of a model in which the two methylene CH bonds are inequivalent. The estimated bond lengths are 1.1005 and 1.1059Å, and the corresponding bond dissociation energies are 403 and 383 kJ mol–1, respectively. These results appear to point to a direct α-interaction between at least one of the methylene CH bonds and the titanium atom. A similar effect may also occur in [TiMe(cp)Cl2].