EFFECT OF A LIBRATION OR HOPPING MOTION OF THE ETA(2)-DIHYDROGEN LIGAND ON LONGITUDINAL NUCLEAR-MAGNETIC-RESONANCE RELAXATION

In the past, H-H distances of the eta(2)-dihydrogen ligand in transiti on metal complexes have been determined in the solid state by crystall ographic or NMR studies (d(H2)(cryst)) and have been estimated in solu tion from (1)J(HD) coupling constants (d(H2)(HD)) or minimum T-1 value s (d(H2)(slow)) for slow internal motion of the H-2 or d(H2)(fast) for fast spinning of a free rotor H-2 where d(H2)(fast) = 0.793d(H2)(slow ) as determined by H-1 NMR. The best estimate of the H-H distance in s olution was found to be d(H2)(HD). This work shows that d(H2)(HD) is f ound to lie between d(H2)(slow) and d(H2)(fast) for many dihydrogen co mplexes reported in the literature, In certain cases this will be true if the correlation time of the H-2 is similar to that of the molecula r complex. Two other cases are considered here for the first time: (1) torsional oscillation of the H-2 in a twofold potential energy surfac e and (2) hydrogens undergoing rapid 90 degrees hops between sites of unequal population in a potential surface with a fourfold component, T he (1)J(HD) and T-1(min) data from the literature for 73 dihydrogen co mplexes are examined in light of these two other possible cases. Dihyd rogen in fast rotation is proposed for 32 complexes, Six complexes app ear to have an H-2 ligand with slow internal motion, Either torsional libration or fast hopping might have a significant influence on the T- 1 relaxation of the remaining 35 complexes.