Combined single crystal neutron diffraction and solution NMR relaxation studies of mono- and bis(silyl) substituted niobocene hydrides with nonclassical interligand interactions

A neutron diffraction study of the bis(silyl) complex Cp2NbH(SiMe2Cl)(2) (3) provides unambiguous localization of the hydride ligand in a central position in the bisecting plane of the niobocene moiety, which is thus in accordance with the molecular symmetry group and the results of recent density functional theory (DFT) calculations. This result is compared with the quantitative localization of the hydride ligands in solutions of the isomeric mono(silyl) complexes Cp2NbH2(SiMe2Cl) (1 and 2) and the bis(silyl) complex 3 by means of T-1min, T-1, T-1sel and T-1bis NMR measurements. A good agreement between the solution and solid state structural data is observed. It is found that the presence of a neighbouring SiMe2Cl ligand increases the Nb-hydride bond length remarkably, probably through the mechanism of Si-H interligand hypervalent interaction (IHI). This effect is especially pronounced in the mono(hydride) complex 3 containing two Si groups, where the Nb-H distance is determined as 1.781(1) Angstrom (NMR relaxation, solution) and 1.816(8) Angstrom (neutron diffraction, solid state).