Treatment of OsH 6 (P i Pr 3 ) 2 (1) with benzophenone and acetophenone in toluene under reflux affords OsH 3 {C 6 H 4 C(O)R}(P i Pr 3 ) 2 (R ) Ph (2), CH 3 (3)), as a result of the ortho-CH activation of the aromatic group of the ketones. Complex 1 is also capable of activating ortho-CF bonds of fluorinated aromatic ketones. Thus, the reactions of this complex with pentafluoroacetophenone, decafluorobenzophenone, and 2,6-difluoroacetophenone give OsH 3 {C 6 F 4 C(O)R}-(PiPr 3 ) 2 (R ) CH 3 (4), C 6 F 5 (5)) and OsH 3 {C 6 H 3 FC(O)CH 3 }(P i Pr 3 ) 2 ( 6). The structure of 4 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. Complexes 4 and 6 can be also obtained by reaction of 1 with 2,3,4,5tetrafluoroacetophenone and 2-fluoroacetophenone, respectively. This selective C-H activation of the ortho-CH bond of the above-mentioned ketones is in contrast with the selective C-F activation observed for the reaction of 1 with 2,3,4,5,6-pentafluorobenzophenone, which affords OsH 3 {C 6 F 4 C(O)C 6 H 5 }(P i Pr 3 ) 2 ( 7). The structure of 7 has also been determined by X-ray diffraction. The geometry around the osmium is the same as that of 4. DFT calculations suggest that in fluorinated aromatic ketones the ortho-CF activation is thermodynamically favored over the ortho-CH activation and that the kinetically preferred ortho-CH activation of 2,3,4,5-tetrafluoroacetophenone and 2-fluoroacetophenone is in part due to the preferred anti arrangement of the F-C-C-CdO unit of the starting ketones. In solution, the hydride ligands of the OsH 3 unit of 2-7 undergo two different thermally activated exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The exchange involving the hydride ligand disposed cis to the carbonyl group is faster than the other one in all the cases. For 2, 3, and 6, quantum exchange coupling is also observed between the hydride ligands involved in the faster thermally activated exchange process.

The elongated dihydrogen complex [Os{C6H4C(O) CH3}(eta2-H2)(H2O)(PiPr3)2]BF4 reacts with phenylacetylene and HBF4.OEt2 to give the unsaturated compound [Os{(E)-CH=CHPh}(CCPh)(CCH2Ph)(PiPr3)2]BF4 containing alkenyl, alkynyl, and carbyne ligands. The addition of sodium chloride to this compound leads to the cyclic allene Os{=C=C(Ph)CH(Ph)CH=C(CH2Ph)}Cl(PiPr3)2, which is the first isometallabenzene with the structure of a 1,2,4-cyclohexatriene.

Treatment of the complex (1) with HBF4·OEt2 in diethyl ether−acetone (2:1) affords the elongated dihydrogen derivative (2), which reacts with NaCl and CsF to give (X = Cl (3), F (4)). The X-ray diffraction studies on 2 and 4 and DFT calculations on the model complexes (1t), (2t), and (X = Cl (3t), F (4t)) suggest that one of the hydrogen atoms bonded to the osmium atom undergoes a cis electrostatic attraction with the L ligand (L = H (1, 1t), (CH3)2CO or H2O (2, 2t), Cl (3, 3t), F (4, 4t)), which increases in the sequence (CH3)2CO or H2O < Cl ≤ F < H. This interaction provokes a lengthening of the hydrogen−hydrogen bond and an increase in the rotation barrier of the elongated dihydrogen ligand. Thus, it is observed that the separation between the hydrogen atoms of the dihydrogen decreases in the sequence 1t (1.695 Å) > 4t (1.544 Å) > 3t (1.489 Å) > 2t (1.455 Å), whereas the rotation barrier of the dihydrogen increases in the sequence 2 (<9 kcal·mol-1) < 3 (≅9 kcal·mol-1) < 4 (10.1 ± 0.8 kcal·mol-1) < 1 (13.9 ± 0.3 kcal·mol-1).