Syntheses and structures of asymmetric bis(silyl) niobocene hydrides

This paper deals with the preparation and structural investigation of asymmetric bis(silyl) niobocene hydrides, Cp2Nb(SiHMe2)(H)(SiXMe2) (2; X = F (a), Cl (b), Br (c), I (d)) and Cp2Nb(SiXMe2)(H)(SiYMe2) (X,Y= F-I; X not equal Y). Complexes 2a-d were prepared by selective electrophilic activation of the Si-H bond in Cp2Nb(SiHMe2)2(H). The Cp2Nb(SiXMe2)(H)(SiYMe2) complexes were prepared by electrophilic activation of the Si-H bond in 2a-d and, in some cases, by electrophilic exchange of the X halides in Cp2Nb(SiXMe2)2(H) (1) for other halides, Y. The structures of complexes 2b and 2c have been studied by X-ray and neutron diffraction (ND). The ND results unequivocally established that the hydride ligand in 2c is shifted toward the SiBrMe2 ligand and that in 2b is positioned symmetrically between two nonequivalent silyl groups, with the H...SiClMe2 distance being shorter because of the shorter Nb-SiClMe2 bond length. Analysis of the X-ray structures of complexes 2a-d and complexes Cp2Nb(SiXMe2)(H)(SiYMe2) shows that the largest structural distortions are observed for the silyl groups substituted by heavy halogen atoms. These trends are rationalized in terms of stronger interligand hypervalent interactions (IHI) Nb-H...Si-X for heavy atoms X from Group 7.