Influence of the Cis Ligand on the H-H Separation and the Rotation Barrier of the Dihydrogen in Osmium-Elongated Dihydrogen Complexes Containing an Ortho-Metalated Ketone † Pilar Barrio, ‡ Miguel A. Esteruelas,* ,‡ Agustı ´ Lledo ´s,* ,§ Enrique On ˜ ate, ‡ and Jaume Toma `s § Departamento de Quı ´mica Inorga ´ nica, Instituto de Ciencia de Materiales de Arago ´ n, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, and Unitat de Quı ´mica Fı ´sica, Departament de Quı ´mica, Universitat Auto ´ noma de Barcelona, 08193 Bellaterra, Barcelona, Spain Received October 22, 2003 Treatment of the complex OsH 3 {C 6 H 4 C(O)CH 3 }(P i Pr 3 ) 2 (1) with HBF 4 ‚OEt 2 in diethyl ether-acetone (2:1) affords the elongated dihydrogen derivative [Os{C 6 H 4 C(O)CH 3 }(η 2 -H 2 ) - {η 1 -(CH 3 ) 2 CO}(P i Pr 3 ) 2 ]BF 4 (2), which reacts with NaCl and CsF to give Os{C 6 H 4 C(O)CH 3 } - X(η 2 -H 2 )(P i Pr 3 ) 2 (X ) Cl (3), F (4)). The X-ray diffraction studies on 2 and 4 and DFT calculations on the model complexes OsH 3 {C 6 H 4 C(O)CH 3 }(PH 3 ) 2 (1t), [Os{C 6 H 4 C(O)CH 3 } - (η 2 -H 2 )(H 2 O)(PH 3 ) 2 ] + (2t), and Os{C 6 H 4 C(O)CH 3 }X(η 2 -H 2 )(PH 3 ) 2 (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), (CH 3 ) 2 CO or H 2 O(2, 2t), Cl (3, 3t), F (4, 4t)), which increases in the sequence (CH 3 ) 2 CO or H 2 O < Cl e 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 ). Introduction The coordination of molecular hydrogen to an asym- metrical metallic fragment promotes the polarization of the hydrogen-hydrogen bond. Thus, one of the hydro- gen centers of the coordinated H 2 is positively charged. 1 In cis-hydride-dihydrogen complexes, it has been rec- ognized that the coordinated H 2 ligand has the capabil- ity of interacting with the hydride. Theoretical studies have not characterized any form of covalent bonding between the hydride and the dihydrogen. The bonding is described as an electrostatic attraction or a dipole/ induced dipole interaction between the negatively charged hydride and the positively charged extremity of the coordinated H 2 . 1d,f,g From an electronic point of view the situation seems to be similar to that in the hydrogen bond. 2 This interaction provokes structural distortions in the coordination of the dihydrogen and perturbations in its rotation barrier. 3 Elongated dihydrogen complexes are a group of di- hydrogen compounds with the hydrogen-hydrogen separation ranging from 1.0 to 1.6 Å. They present as a distinctive characteristic a very flat energy potential curve for the H-H stretch. 4 In such a situation small perturbations, like those caused by weak interactions, could induce severe changes in the H‚‚‚H distance. Thus, this kind of dihydrogen complex is very sensitive to detection and comparison of the strength of weak interactions involving the H 2 unit. The osmium complexes of this type have been shown to be useful templates for carbon-carbon and carbon- heteroatom coupling reactions. 5 However, their reactiv- ity toward unsaturated organic molecules is difficult to * To whom correspondence should be addressed. E-mail: maester@ posta.unizar.es (M.A.E.). † Dedicated to Prof. Helmut Werner on the occasion of his 70th birthday. ‡ Universidad de Zaragoza-CSIC. § Universitat Auto `noma de Barcelona. (1) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155. (b) Kristja ´ nsdo ´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New York, 1992; pp 309-359. (c) Jia, G.; Lau, C.-P. Coord. Chem. Rev. 1999, 190-192, 83. (d) Maseras, F.; Lledo ´s, A.; Clot, E.; Eisenstein, O. Chem. Rev. 2000, 100, 601. (e) Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000, 122, 9155. (f) Kubas, G. J. J. Organomet. Chem. 2001, 635, 37. (g) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer Academic/Plenum: New York, 2001. (2) (a) Richmond, T. G. Coord. Chem. Rev. 1990, 105, 221. (b) Peris, E.; Lee, J. C., Jr.; Rambo, J. R.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 1995, 117, 3485. (c) Crabtree, R. H. J. Organomet. Chem. 1998, 557, 111. (d) Crabtree, R. H.; Eisenstein, O.; Sini, G.; Peris, E. J. Organomet. Chem. 1998, 567, 7. (e) Custelcean, R.; Jackson, J. E. Chem. Rev. 2001, 101, 1963. 3008 Organometallics 2004, 23, 3008-3015 10.1021/om0342513 CCC: $27.50 © 2004 American Chemical Society Publication on Web 05/13/2004