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

Cycloalkyl niobium complexes, Tp(Me2)NbX(R)(MeC=CMe) (Tp(Me2) = 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 Tp(Me2)NbCl(2)(MeC=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.