The methyl complexes [TpMe2Nb(CH3)2(CH3CCCH3)] (2) and [TpMe2NbCl(CH3)(CH3CCCH3)] (3) have been prepared from the corresponding dichloride, and their structures investigated by X-ray diffraction. In each case, the Nb–CH3 moiety displays a normal geometry in comparison with other methylniobium derivatives, with no evidence for the structural distortions which attend an α-agostic interaction. The NMR spectra of 2 and 3, including those of the CH2D isotopomers, also point to a conventional methyl–metal complex, with no isotopic perturbation of resonance which can signal agostic behaviour. Likewise, the IR spectra of the CH3, CH2D and CD3 isotopomers of 2 and 3 indicate a conventional bonding situation, although the asymmetric Nb–CH3 potential leads to two distinct νisCD modes in the CH2D spectrum. Taken together, these results lead us to conclude that there is no intrinsic α-agostic interaction in the methyl complexes 2 and 3, and that the α-agostic structures deduced for their longer chain alkyl congeners are predominantly the result of steric interactions between the alkyl moiety and the bulky TpMe2 ligand.
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