Infrared spectra and methyl group properties in dicyclopentadienyldimethyl-titanium(IV), -zirconium(IV) and -hafnium(IV)

Infrared spectra are reported for CH₃-, CD₃- and CHD₂-substituted Cp₂MMe₂ (Cp = η⁵-C₅H₅, M = Ti, Zr, Hf) in CCl₄ solution. The isolated CH stretching frequencies, ν(^isCH), measured in the CHD₂ species are lower than any previously observed in methylmetal compounds and the methyl CH bonds in Cp₂HfMe₂ are predicted to be the longest and weakest such bonds yet to have been characterised by this method. The methyl groups in Cp₂ZrMe₂ and Cp₂HfMe₂ have all three CH bonds equal, but in Cp₂TiMe₂ each methyl group contains two strong CH bonds and one weak one. This may be the result of steric overcrowding effects around the relatively small titanium atom. The symmetric deformation δ_s(CH₃) rises with increasing atomic number of the metal atom, the reverse of the trend observed for methyl derivatives of Main Group elements.