Complexes of the general formula [MXH(2)(dppp)(2)](n+) (M=Ru, Os; X=H, Cl, CO; dppp=1,3-bis (diphenylphosphino)propane) have been prepared and characterized, and the effect of the donor/acceptor properties of X on their structure and acidity has been studied. The five-coordinate complexes [MCl(dppp)(2)](+) (M=Ru (1a), Os (1b)) react with H-2 gas in CH2Cl2 to give the complexes [MCl(eta(2)-H-2)(dppp)(2)](+) (M=Ru (2a), Os (2b)) containing elongated dihydrogen ligands. The molecular structure of 2b has been determined by X-ray crystallography (monoclinic, space group P2(1)/n with a=13.314(7) Angstrom, b=18.63(2) Angstrom, c=23.20(2) Angstrom, beta=94.58(6)degrees, and Z=4). Chlorohydride [OsH(Cl)(dppp)(2)] (3b) reacts with H-2 gas in the presence of Na[BPh(4)] forming [OsH3(dppp)(2)](+) (4b). Protonation of [OsH2(dppp)(2)] (5b) with HBF4 . Et(2)O also gives 4b. A combination of X-ray crystallography (monoclinic, space group P2(1)/n with a=13.392(3) Angstrom, b=25.306(7) Angstrom, c=21.247(7) Angstrom, beta=91.15(2)degrees, and Z=4) and H-1 and P-31 NMR studies indicate that 4b is a classical trihydride. Hydridocarbonyls [MH(CO)(dppp)(2)](+) (M=Ru (6a), Os (6b)) are protonated by F3CSO3H in CD2Cl2 to yield [M(CO)(eta(2)-H-2)(dppp)(2)](2+) (M=Ru (7a), Os (7b)), which were characterized in solution. 7a is stable only at low temperature. Compound 7b is a highly acidic dihydrogen complex with an estimated pK(a) of -6.