Metal−Hydride and Hydride−Hydride Distances from Measurements of Selective and Nonselective 1H T1 Relaxation Times of Hydride Ligands:  Relaxation in NbCp2H3

The detailed 1 H NMR relaxation study, performed for Cp 2 NbH X 2 H A ( 1 ) and its partially-deuterated derivatives in toluene- d 8, has demonstrated how T 1 and T 1 sel measurements can be used for accurate determination of the hydride−hydride and metal−hydride distances in Nb complexes. The T 1 sel method was independently supported by the relaxation studies of the partially deuterated derivatives of 1 . The application of the T 1 sel 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, Δ G ⧧ (298 K) = 17.3 kcal/mol, Δ H ⧧ = 13.1 kcal/mol, and Δ S ⧧ = −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 1 sel 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.