The reactivity of a series of trans-Ru(P-2)(2)Cl-2 complexes with H-2 was explored. The complexes reacted with H-2 via a stepwise H-2 addition/heterolysis pathway to form the trans-[Ru(P-2)(2)(H-2)H](+) dihydrogen complexes. Some of the resulting eta(2)-H-2 complexes were surprisingly inert to substitution by water, even at concentrations as high as 55 M; however, the identity of the bidentate phosphine ligand greatly influenced the lability of the coordinated eta(2)-H-2 ligand. With less electron-donating phosphine ligands, the H-2 ligand was susceptible to substitution by H2O, whereas with more electron-rich phosphine ligands, the H-2 ligand was inert to substitution by water. Density functional theory (DFT) calculations of the ligand substitution reactions showed that the Ru-H-2 and Ru-H2O complexes are very close in energy, and therefore slight changes in the donor properties of the bidentate phosphine ligand can inhibit or promote the substitution of H2O for H-2.