Agostic interactions in alkyl derivatives of sterically hindered tris(pyrazolyl)borate complexes of niobium

This review describes how the joint experimental and computational study of a series of niobium(tris(pyrazolyl))(chloride)(alkyne)(alkyl) complexes has unearthed a very rich chemistry involving a variety of agostic interactions between the alkyl chain and the niobium centre. alpha and beta C-H agostic, as well as alpha C-C agostic coordination, have been observed, in some cases in dynamic equilibrium within the same complex. The application of experimental and computational (DFT and DFT/MM) techniques reveals the subtle interplay of steric and electronic interactions that control the relative stability of these species. This analysis also sheds light or) the electronic origin of agostic distortions in early transition metal centres. (C) 2008 Elsevier B.V. All rights reserved.