The acidity of transition-metal dihydrogen complexes of the general type trans-[LM(H2PCH2CH2PH2)(2)(eta(2)-H-2)](n+) (M = Ru, OS; L = H-, CH3-, F-, CF3-, CN-, Cl-, Br-, CO, NCH, NH3, PH3; n = 1, 2) has been modeled by calculating deprotonation energies using the B3LYP density functional method and effective core potentials. Calculated DP energies correlate well with experimental pK(a) values. For anionic L with strong sigma-donor properties the DP energies are higher and acidities lower, due to significant destabilization of the conjugate hydride. When L is neutral, the complexes are dicationic and have much lower DP energies. The acidities of these complexes are governed by the pi-accepting ability of L, as the less positively charged conjugate hydride can be stabilized through metal(d) --> pi* back-donation. The effect of the metal center on acidity is also dependent upon the electronic properties of L, Dihydrogen complexes of Ru with L that is either a sigma-donor or pi-acceptor are less acidic when compared with those of Os. The effect of a strong sigma-donor leads to greater destabilization of the Ru conjugate hydrides, while the pi-accepting property of L preferentially stabilizes the Os conjugate hydrides.
