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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 CH 2 Cl 2 to give the complexes [MCl(η 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) Å, b ) 18.63(2) Å, c ) 23.20(2) Å, ) 94.58(6)°, and Z ) 4). Chlorohydride [OsH(Cl)(dppp) 2 ] (3b) reacts with H 2 gas in the presence of Na[BPh 4 ] forming [OsH 3 (dppp) 2 ] + (4b). Protonation of [OsH 2 (dppp) 2 ] (5b) with HBF 4 ‚Et 2 O also gives 4b. A combination of X-ray crystallography (monoclinic, space group P2 1 /n with a ) 13.392(3) Å, b ) 25.306(7) Å, c ) 21.247(7) Å, ) 91.15(2)°, and Z ) 4) and 1 H and 31 P NMR studies indicate that 4b is a classical trihydride. Hydridocarbonyls [MH(CO)(dppp) 2 ] + (M ) Ru (6a), Os (6b)) are protonated by F 3 CSO 3 H in CD 2 Cl 2 to yield [M(CO)(η 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.

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