We present an account of our research concerning the chemistry and catalytic properties of ruthenium and osmium complexes in homogeneous hydrogenation reactions. In particular we describe the use of metal complexes of the type MHX(CO)(ER₃)_2,3, MX(CO)(OCOR‘)(EPh₃)₂ MH(CO)(OCOR‘)(PPh₃)₂ (M = Ru, Os; X = Cl, Br; E = P, As) and other related cationic and dimeric species as catalyst precursors for homogeneous hydrogenation reactions.