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14-September-2008 18:38:49 - complex Formation and equilibrium structures of metal dihydrogen and dihydride complexes L = ligand. Formation and equilibrium structures of metal dihydrogen and dihydride complexes L = ligand. Dihydrogen complexes are coordination complexes containing intact H2 as a ligand.1 The prototypical complex is WCO3PCy32H2. This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metals complexes with octahedral geometry. Upon complexation, the H-H bond is extended to 0.81-0.82 Å as indicated by neutron diffraction, about a 10% extension relative to the H-H bond in free H2. Some complexes containing multiple hydrogen ligands, i.e. polyhydrides, also exhibit short H---H contacts. It has been suggested that distances 1.00 Å indicates signficant dihydrogen character, where separations 1 Å are better described as dihydrido complexes see figure. Contents 1 Characterization 2 Synthesis 3 History 4 References 5 Further reading Characterization The preferred method of characterization of dihydrogen complexes is neutron diffraction. Neutrons interact strongly with hydrogen atoms, which allows one to infer their location in a crystal. In some cases, hydrogen ligands are usefully characterized by X-ray crystallography, but often the presence of metals, which strongly scatter X-rays, complicates the analysis. NMR techniques are also widely used. The magnitude of spin-spin coupling is a useful indicator of the strength of the bond between the hydrogen and deuterium in HD complexes. Dihydrogen-complexes typically have longer 1H-spin-lattice relaxation times than the corresponding dihydrides. Synthesis Two preparation methods involve the direct reactions with H2 gas. The first entails the addition of H2 to an unsaturated metal center, as originally reported for WCO3P-i-Pr32H2. In some cases, H2 will displace weakly bound ligands, sometimes even halides in favorable cases: LnMX + H2 → LnMH2+ + X- Many metal hydrides can be protonated to give dihydrogen complexes: LnM-H + H+ → LnMH2+ In such cases the acid usually is derived from a weakly coordinating anion. History In 1984, Kubas et al. discovered that the addition of H2 to purple-colored species MCO3PR32 gave a yellow precipitate of mer-trans- MCO3PR32H2 M = Mo or W; R = cyclohexyl, iso-propyl.2 This result rapidly led to the discovery of a variety of related complexes such as CrH2CO53 and FeH2Hdppe2+.4 Kubas et al's findings also led to a reevaluation of previously described compounds. For example, the complex RuH4PPh33 described in 1968 was reformulated as a dihydrogen complex. References ^ Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenum Publishers: New York, 2001. ISBN 0-306-46465-9 ^ Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. Characterization of the First Examples of Isolable Molecular Hydrogen Complexes, M CO3PR32H2 M = Mo, W; R = Cy, i-Pr. Evidence for a Side-on Bonded H2 Ligand Journal of the American Chemical Society 1984, volume 100, 451-2. ^ Sweany, R. L. Photolysis of Hexacarbonylchromium in Hydrogen-Containing matrixes: evidence of simple adducts of molecular hydrogen Journal of the American Chemical Society 1985, volume 107, 2374-9. DOI: 10.1021/ja00294a030 ^ Morris, R. H.; Sawyer, J. F.; Shiralian, M. and Zubkowski, J., Two Molecular Hydrogen Complexes: trans-Mη2-H2HPPh2CH2CH2PPh22BF4 M = Fe, Ru. The Crystal Structure Determination of the Iron Complex, Journal of the American Chemical Society, 1985, volume 107, 5581-2.DOI|10.1021/ja00305a071 1 # Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenum Publishers: New York, 2001. ISBN 0-306-46465-9 2 ^ Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. Characterization of the First Examples of Isolable Molecular Hydrogen Complexes, M CO3PR32H2 M = Mo, W; R = Cy, i-Pr. Evidence for a Side-on Bonded H2 Ligand Journal of the American Chemical Society 1984, volume 106, 451-2 3 ^ Sweany, R. L. Photolysis of Hexacarbonylchromium in Hydrogen-Containing matrixes: evidence of simple adducts of molecular hydrogen Journal of the American Chemical Society 1985, volume 107, 2374-9. DOI: 10.1021/ja00294a030 4 ^ Morris, R. H.; Sawyer, J. F.; Shiralian, M. and Zubkowski, J., Two Molecular Hydrogen Complexes: trans-Mη2-H2HPPh2CH2CH2PPh22BF4 M = Fe, Ru. The Crystal Structure Determination of the Iron Complex, Journal of the American Chemical Society, 1985, volume 107, 5581-2.DOI|10.1021/ja00305a071 Further reading J. K. Burdett, O. Eisenstein, and S. A. Jackson, Transition Metal Didydrogen Complexes: Theoretical Studies, in A. Dedieu, ed., Transition Metal Hydrides, VCH, New York, 1992, pp. 149-184 Burdett, J. K.; Phillips, J. R.; Pourian, M. R.; Poliakoff, M.; Turner, J. J.; Upmacis, R. K. Inorg. Chem. 1987, 26, 3054 Lyons, D.; Wilkinson, G..; Thornton-Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1986, 695 Kubas, G. J.; Ryan, R. R. Polyhedron 1986, 5, 473 Crabtree, R. H.; Lavin, M. J. Chem. Soc.; Chem. Commun. 1985, 1661. v d e Chemical bonds Strong Covalent bonds Antibonding Sigma bonds: 3c-2e bent bond · 3c-4e Hydrogen bond, Dihydrogen bond, Agostic interaction · 4c-2e Pi bonds: π backbonding · Conjugation · Hyperconjugation · Aromaticity · Metal aromaticity Delta bond: Quadruple bond · Quintuple bond · Sextuple bond Coordinate covalent bond · Hapticity Ionic bonds Cation-pi interaction · Salt bridge Metallic bonds Metal aromaticity Weak Hydrogen bond Dihydrogen bond · Dihydrogen complex · Low-barrier hydrogen bond · Symmetric hydrogen bond · Hydrophile Other noncovalent van der Waals force · Mechanical bond · Halogen bond · Aurophilicity · Intercalation · Stacking · Entropic force · Chemical polarity other Disulfide bond · Peptide bond · Phosphodiester bond Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds Retrieved from http://en..org/wiki/Dihydrogen_complex Categories: Chemical bonding | Inorganic chemistry Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page This page was last modified on 17 June 2008, at 22:13
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