C-H and C-C agostic interactions in cycloalkyl tris(pyrazolyl)boratoniobium complexes

Cycloalkyl niobium complexes, TpMe2NbX(R)(MeC(identical)CMe)(TpMe2= hydrotris(3,5-dimethylpyrazolyl)borate; X = Cl, R =c-C3H5 (2a); X = Br, R =c-C3H5(2b), X = Cl, R =c-C5H9(3), X = Cl, R =c-C6H11(4),) have been prepared from TpMe2NbCl2(MeC(identical)CMe). The cyclopropyl complex 2a shows no sign of C-H agostic interactions either in the solid state (X-ray) or in solution. In contrast, the NMR spectra of 3 and 4 are temperature dependent as a consequence of an equilibrium between a major (alpha)-agostic species and a minor non-agostic one. Hybrid QM/MM calculations are used to rationalise the behaviour of these cycloalkyl species, and illustrate the subtle interplay of steric and electronic effects in these systems.