DFT calculations were performed on model complexes of MH(NHCMe2 CMe2NH2)(PPh3)2, M = Ru (1), Os (2) to explore the mechanism of catalytic homogeneous hydrogenation of polar bonds using ruthenium based diamine/diphosphine or bisphosphine systems. The ligands PPh3 and 2,3,-diamino-2,3-dimethylbutane were simplified to PH3 and ethylenediamine. Dihydrogen activation by 1 and 2 was found to have five essential steps: free hydrido-amido species and H2(g), approach of H2 to the metal, formation of an eta2-H2 complex, elongation and breaking of the H2 bond, and formation of a trans-dihydride. Two stable conformers exist for the eta2-H 2 complex with osmium, but only one was located with ruthenium. In the presence of alcohol, the activation barrier for the H2-splitting was calculated to be 6 kcal/mol lower.A possible correlation was investigated between the pKalpha THF of 13 acids and the gas-phase proton affinity of their conjugate bases as calculated by DFT methods. A poor correlation was found.