Metal-hydride and hydride-hydride distances from measurements of selective and nonselective 1H T1 relaxation times of hydride ligands:: Relaxation in NbCp2H3

The detailed H-1 NMR relaxation study, performed for (Cp2NbH2HA)-H-X (1) and its partially-deuterated derivatives in toluene-d(8), has demonstrated how T-I and T-1sel measurements can be used for accurate determination of the hydride-hydride and metal-hydride distances in Nb complexes. The T-1sel method was independently supported by the relaxation studies of the partially deuterated derivatives of 1. The application of the T-1sel approach is limited by the presence in the H-X/H-A pair of the slow hydride/hydride site exchange characterized by saturation transfer experiments: k(296) = 1.34 s(-1), Delta G(double dagger)(298 K) = 17.3 kcal/mol, Delta H-double dagger = 13.1 kcal/mol, and Delta S-double dagger = -14.2 eu. The exchange does not show a pronounced isotopic effect, and k(H)/k(D) = 1.1 at 295 K. It has been suggested that the T-1sel approach can be used for indication of slow hydride/hydride exchanges. The experimental estimations have shown Nb-H dipole-dipole interactions to be very important for the hydride relaxation in 1. The Nb-H contribution shortens significantly the hydride T-1 times, providing, for example, up to 64% of the H-X relaxation rate. The relaxation H-X-H-A and Nb-H distances have been discussed.