Aqueous coordination chemistry of H2: why is coordinated H2 inert to substitution by water in trans-Ru(P2)2(H2)H+-type complexes (P2 = a chelating phosphine)?

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 H(2)O, 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-H(2)O 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 H(2)O for H(2).