The ruthenium(II) hexaaqua complex [Ru(H2O)6]2+ reacts with dihydrogen under pressure to give the h2-dihydrogen ruthenium(II) pentaaqua complex [Ru(H2)(H2O)5]2+.The complex was characterized by 1H, 2H and 17O NMR: d H = -7.65 ppm, J HD = 31.2 Hz, d O = -80.4 ppm (trans to H2) and d O = -177.4 ppm (cis to H2).The H-H distance in coordinated dihydrogen was estd. to 0.889 .ANG. from J HD, which is close to the value obtained from DFT calcns. (0.940 .ANG.). Kinetic studies were performed by 1H and 2H NMR as well as by UV-Vis spectroscopy, yielding the complex formation rate and equil. consts.: k f = (1.7 +- 0.2) * 10-3 kg mol-1 s-1 and K eq = 4.0 +- 0.5 mol kg-1.The complex formation rate with dihydrogen is close to values reported for other ligands and thus it is assumed that the reaction with dihydrogen follows the same mechanisn (I d).In deuterated water, one can observe that [Ru(H2)(H2O)5]2+ catalyzes the hydrogen exchange between the solvent and the dissolved dihydrogen.A hydride is proposed as the intermediate for this exchange.Using isotope labeling, the rate const. for the hydrogen exchange on the h2-dihydrogen ligand was detd. as k 1 = (0.24 +- 0.04) * 10-3 s-1.The upper and lower limits of the pK a of the coordinated dihydrogen ligand have been estd.:3 < pK a < 14.