Complex OsH 4 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (1) activates the Si−H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)trihydride derivatives OsH 3 (SiR 3 ){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} [SiR 3 = SiEt 3 (2), SiPh 3 (3), SiMe(OSiMe 3 ) 2 (4)] and H 2 . The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant-(P i Pr 2 ) 2 ). This intermediate, which has been trapped to form OsH 4 {κ 2 -P,P-[xant(P i Pr 2 ) 2 ]}(P i Pr 3 ) (5), coordinates the Si−H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si−H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C�CC(OH)Ph 2 } 2 {�C�CHC(OH)Ph 2 }{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C�CC(OH)Ph 2 } 2 {�C�C�CPh 2 }{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ 1 -C,η 2 -[C 6 H 4 CH 2 CH�CH 2 ]}{κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (8) and PhCH 2 CH�CH(SiEt 3 ).

An unusual 1,3-hydrogen shift from the metal center to the C β atom of the C 3 -chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}) 2 (μ-Cl) 2 ][BF 4 ] 2 ( 1 ). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh 2 ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}]BF 4 ( 2 ), which yields OsHCl(=C=C=CPh 2 ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 3 ) by selective abstraction of the C β –H hydrogen atom of the alkenylcarbyne ligand with K t BuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the C β atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=C IndPh ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 4 ) and the osmanaphthalene OsCl(C 9 H 6 Ph){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 5 ). Protonation of 4 with HBF 4 leads to the elongated dihydrogen complex [OsCl(η 2 -H 2 )(=C IndPh ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ...

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