In the past, HÈH distances of the g2-dihydrogen ligand in transition metal complexes have been determined in the solid state by crystallographic or NMR studies and have been estimated in solution from coupling (d H2 cryst) 1J HD constants or minimum values for slow internal motion of the or for fast spinning of a free (d H2 HD) T 1 (d H2 slow H 2 d H2 fast rotor where as determined by 1H NMR. The best estimate of the HÈH distance in solution H 2 d H2 fast = 0.793d H2 slow) was found to be This work shows that is found to lie between and for many dihydrogen d H2 HD . d H2 HD d H2 slow d H2 fast complexes reported in the literature. In certain cases this will be true if the correlation time of the is similar to H 2 that of the molecular complex. Two other cases are considered here for the Ðrst time : (1) torsional oscillation of the in a twofold potential energy surface and (2) hydrogens undergoing rapid 90Ä hops between sites of unequal H 2 population in a potential surface with a fourfold component. The and data from the literature for 73 1J HD T 1 min dihydrogen complexes are examined in light of these two other possible cases. Dihydrogen in fast rotation is proposed for 32 complexes. Six complexes appear to have an ligand with slow internal motion. Either torsional H 2 libration or fast hopping might have a signiÐcant inÑuence on the relaxation of the remaining 35 complexes. T 1