Heterolytic H₂ Activation by Dihydrogen Complexes. Effects of the Ligand X in [M(X)H₂{Ph₂P(CH₂)₃PPh₂}₂]n⁺ (M = Ru, Os; X = CO, Cl, H)

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 P 2 1 / n with a = 13.314(7) Å, b = 18.63(2) Å, c = 23.20(2) Å, β = 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 P 2 1 / n with a = 13.392(3) Å, b = 25.306(7) Å, c = 21.247(7) Å, β = 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 p K a of −6.