Properties of CH bonds in alkyl transition metal compounds
This Thesis describes the results of a detailed vibrational study of a representative series of methyl and ethyl derivatives of transition metals. The primary objective of the work was to show that vibrational frequency data can be used to provide information on CH bond lengths, HCH angles and alkyl group geometries with an accuracy and reliability which is not readily attainable by other methods, and that the vibrational technique is generally applicable to a wide variety of organometallic compounds. The work involved the preparation of protonated, partially deuterated and fully deuterated examples of the molecules studied and made particular use of correlations between 'isolated' CH stretching frequencies, v15CH, measured in partially deuterated systems, and CH bond length and HCH angles. In the case of MeM(CO)2Cp (M = Fe,Ru), it is shown that, unlike related molecules of this type, the results show no evidence for a high barrier to internal rotation of the methyl group. The titanium compound MeTiCl2Cp, is shown to be non-agnostic, with the only unusual feature being an unusually low δ5CH3 frequency. In the ethyl series preliminary studies necessitated a careful re-examination of the accepted assignments for ethyl halides, leading to changes in some cases. Following this, detailed assignments are made, wherever possible, for the ethyl metal compounds, with particular emphasis on the CH stretching region. In all of the ethyl metal compounds, the α and β-CH bonds are considerably longer, and weaker, than those in the ethyl halides, and the terminal methyl group invariably contains two dissimilar types of CH bond, differing in the CH stretching frequency by as much as 58cm-1. The longer CH bonds in these ethyl compounds are trans to the metal atoms and are amongst the weakest to have been identified in alkyl metal systems.