Structure and bonding in a cyclobutyl tris(pyrazolyl)boratoniobium complex and the variation in agostic behaviour with ring size in the series Tp^Me2NbCl(c-C<inf>n</inf>H<inf>2n − 1</inf>)(MeC≡.tbd;CMe), n = 3–6

The synthesis and characterisation of the cyclobutyl complex TpMe2NbCl(c-C4H7)(MeC≡.tbd;CMe) completes the family of cycloalkyl complexes TpMe2NbCl(c-CnH2n − 1), n = 3–6. The properties of the cyclobutyl complex are qualitatively similar to those of its cyclopentyl and cyclohexyl analogues, and dramatically different from those of the cyclopropyl derivative. Most conspicuously, the cyclobutyl system has an α-C–H agostic interaction in the dominant isomer, with no evidence for the α-C–C agostic character found for the smaller ring. C–C agostic character therefore seems to be unique to the cyclopropyl complex, where the acute C–C–C angles destabilise the C–C bonding orbitals. © 2006 The Royal Society of Chemistry.