The dihydrogen complex [OsTp(η 2 -H 2 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 (1; Tp ) hydridotris(pyrazolyl)borate) reacts with tert-butylacetylene to give the hydride-carbyne derivative [OsHTp(tCCH 2 t Bu)(P i Pr 3 )]BF 4 (2). Similarly, the treatment of 1 with (trimethylsilyl)acetylene in the presence of methanol leads to [OsHTp(tCCH 3 )(P i Pr 3 )]BF 4 (3). In chloroform at 60 °C, complexes 2 and 3 evolve into the corresponding chloro derivatives [OsClTp(tCR)(P i Pr 3 )]BF 4 (R ) CH 2 t Bu (4), CH 3 ( 5)), whereas in acetonitrile the carbenes [OsTp(dCHR)(NCCH 3 )(P i Pr 3 )]BF 4 (R ) CH 2 t Bu (6), CH 3 ( 7)) are formed. Complex 1 also reacts with phenylacetylene. The reaction initially gives [OsHTp(tCCH 2 Ph)(P i Pr 3 )]BF 4 (8). Subsequently, the insertion of a second molecule of alkyne into the Os-H bond of 8 and the migration of the resulting alkenyl group from the metal center to the carbyne carbon atom take place, to afford the carbene [OsTp{dC(CH 2 Ph)C(Ph)dCH 2 }(P i Pr 3 )]BF 4 (9). The metal center of 9 is saturated by means of a strong agostic interaction (r bp ) 0.4(1)) between the osmium atom and one of the ortho-CH bonds of the phenyl substituent of the alkenyl unit of the alkenylcarbene ligand. Treatment of 1 with methylpropiolate leads to the carbene [OsTp{dCHCH 2 C(O)OMe}(P i Pr 3 )]BF 4 (10) in a one-pot synthesis. Complexes 2, 4, 9, and 10 have been characterized by X-ray diffraction analysis. † Dedicated to Professor Jose ´Gimeno on the occasion of his 60th birthday.

Complex OsH 3 Cl(P i Pr 3 ) 2 (1) reacts with KTp (Tp ) hydridotris(pyrazolyl)borate) in tetrahydrofuran at room temperature to give OsH 3 (κ 2 -Tp)(P i Pr 3 ) 2 (2). In toluene at 80 °C, the κ 2 -Tp complex 2 is transformed to the κ 3 -Tp derivative OsH 3 Tp(P i Pr 3 ) (3) in quantitative yield after 7 h. Protonation of 3 with HBF 4 affords the bis(dihydrogen) compound [OsTp(η 2 -H 2 ) 2 (P i Pr 3 )]BF 4 (4). In acetone complex 4 releases the coordinated hydrogen molecules in a sequential manner. At room temperature, the dihydrogensolvate complex [OsTp(η 2 -H 2 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 (5) is formed, while at 56 °C the loss of both hydrogen molecules gives rise to the bis-solvento derivative [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 (6). Complexes 2-6 have been characterized by X-ray diffraction analysis, and DFT calculations support their formulation. The structures of 3-5 have been compared with those of their Cp counterparts OsH 3 Cp(P i Pr 3 ), [OsH 2 -Cp(η 2 -H 2 )(P i Pr 3 )]BF 4 , and [OsH 2 Cp(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 . The results show that the Tp ligand avoids piano stool geometries, while it enforces dispositions allowing N-Os-N angles close to 90°such as octahedron and pentagonal bipyramid.

The complex [OsTp(kappa(1)-OCMe)(2)(P(i)Pr(3))]BF(4) [Tp = hydridotris(pyrazolyl)borate] promotes the cleavage of both C(sp(3))-C(sp(2)) bonds of benzylidenecyclopropane and 3-phenylpropylidenecyclopropane to yield the complexes [OsTp(=C=CHR)(eta(2)-CH(2)=CH(2))(P(i)Pr(3))]BF(4) (R = Ph, CH(2)CH(2)Ph). The process is proposed to take place via metallacyclopropene intermediates stabilized by an ethylene chelation assistant. The driving force for the fragmentation is the high stability of the resulting ethylene-Os-vinylidene species.