Preparation and reactivity of dihydrogen complexes [MX(η2-H2)P4]BF4 (M = Ru or Os;: X = halogenide or SEt-;: P = phosphite)

Monohydride complexes MHXP4 [M = Ru or Os; X = Cl-, Br-, I-, SEt- or N-3(-); P = P(OEt)(3), PPh(OEt)(2) or PPh2OEt] were prepared by treating dihydride species MH2P4 first with CF3SO3Me and then with an excess of the anionic ligand X. In an argon atmosphere, protonation of MHXP4 with HBF4. Et2O gives dihydrogen cations [MX(eta(2)-H-2)P-4](+), with X = Cl, Br, I or SEt; the classical dihydride [MH2(N-3)P-4](+) was obtained with the azide ligand. Instead, in a hydrogen atmosphere, protonation of MHXP4 with HBF4. Et2O gives hydride-dihydrogen [MH(eta(2)-H-2)P-4](+) species, according to a proposed mechanism involving interaction of Bronsted acid with ligand X. Some [MX(eta(2)-H-2)P-4](+) cations were thermally unstable and fully characterised in solution (H-1 and P-31 NMR, variable temperature T-1 measurements), whereas the [OsX(eta(2)-H-2){PPh(OEt)(2)}(4)]BF4 complexes were stable and isolated as solids. Treatment of [MX(eta(2)-H-2)P-4](+) cations with alkyne PhC=CH gave evolution of H-2 and formation of the vinylidene intermediate [MX{=C=C(H)Ph}P-4](+) which, by reaction with base, afforded the final acetylide M(C=CPh)XP4 derivatives. Treatment with propargyl alcohols HC=CC(OH)RR' of the [MX(eta(2)-H-2)P-4](+) cations, instead, gave propadienylidene derivatives [MX(=C=C=CRR')P-4]BPh4 (M = Ru or Os; R = R' = Ph or R = Ph, R' = Me). Hydrazine complexes [MX(NH2NH2)P-4]BPh4 were also prepared by substitution of the dihydrogen ligand in the new eta(2)-H-2 derivatives.