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BoggleDihydrogen complex
From Wikipedia, the free encyclopedia
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 W(CO)3(PCy3)2(H2). 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 significant dihydrogen character, where separations > 1 Å are better described as dihydrido complexes (see figure).
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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, JHD, is a useful indicator of the strength of the bond between the hydrogen and deuterium in HD complexes. For example, JHD is 43.2 Hz in HD but 33.5 Hz in W(HD)(CO)3(PiPr3)2. Dihydrogen-complexes typically have longer 1H-spin-lattice relaxation times than the corresponding dihydrides.
The triangular MH2 subunit has six normal modes of vibration, one of which is mainly of νH-H character. In free H2, this very strong bond absorbs at 4300 cm-1, whereas in dihydrogen complexes the frequency drops to around 2800 cm-1.
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 W(CO)3(P-i-Pr3)2(H2). In some cases, H2 will displace weakly bound ligands, sometimes even halides in favorable cases:
- LnMX + H2 → [LnM(H2)]+ + X-
Many metal hydrides can be protonated to give dihydrogen complexes:
- LnM-H + H+ → [LnM(H2)]+
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 M(CO)3(PR3)2 gave a yellow precipitate of mer-trans- M(CO)3(PR3)2(H2) (M = Mo or W; R = cyclohexyl, iso-propyl).[2] This result rapidly led to the discovery of a variety of related complexes such as Cr(H2)(CO)5[3] and [Fe(H2)(H)(dppe)2]+.[4] Kubas et al.'s findings also led to a reevaluation of previously described compounds. For example, the complex "RuH4(PPh3)3" described in 1968 was reformulated as a dihydrogen complex.
References
- ^ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes - Structure, Theory, and Reactivity (1 ed.). Springer. ISBN 0306464659.
- ^ Kubas, G. J.; R. R. Ryan, B. I. Swanson, P. J. Vergamini, H. J. Wasserman (1984-01-01). "Characterization of the first examples of isolable molecular hydrogen complexes, M( CO)3(PR3)2(H2) (M = molybdenum or tungsten; R = Cy or isopropyl). Evidence for a side-on bonded dihydrogen ligand". Journal of the American Chemical Society 106 (2): 451–452. doi:. http://dx.doi.org/10.1021/ja00314a049. Retrieved 2009-04-17.
- ^ Sweany, Ray L. (1985-04-01). "Photolysis of hexacarbonylchromium in hydrogen-containing matrixes: evidence of simple adducts of molecular hydrogen". Journal of the American Chemical Society 107 (8): 2374–2379. doi:. http://dx.doi.org/10.1021/ja00294a030. Retrieved 2009-05-02.
- ^ Morris, Robert H.; Jeffery F. Sawyer, Mahmoud Shiralian, Jeffrey Zubkowski (1985). "Two molecular hydrogen complexes: trans-[M(η2-H2)(H)(PPh2CH2CH2PPh2)2BF4 (M = Fe, Ru). The crystal structure determination of the iron complex"]. Journal of the American Chemical Society 107 (19): 5581–5582. doi:. http://dx.doi.org/10.1021/ja00305a071. Retrieved 2009-05-02.
Further reading
- Burdett, J. K.; O. Eisenstein, S. A. Jackson (1991-12-06). "Transition Metal Didydrogen Complexes: Theoretical Studies". in A. Dedieu (ed.). Transition Metal Hydrides. Wiley-VCH. pp. 149–184. ISBN 0471187682.
- Burdett, Jeremy K.; John R. Phillips, Mohammad R. Pourian, Martyn Poliakoff, James J. Turner, Rita Upmacis (1987). "Electronic stability of metal-dihydrogen and polyhydrogen complexes". Inorganic Chemistry 26 (18): 3054–3063. doi:. http://dx.doi.org/10.1021/ic00265a026. Retrieved 2009-05-02.
- Lyons, David; Geoffrey Wilkinson, Mark Thornton-Pett, Michael B. Hursthouse (1984). "Synthesis, X-ray crystal structure, and reactions of dihydridopentakis(trimethylphosphine)molybdenum(II): crystal structure of the carbon dioxide insertion product, (formato-O,O')hydridotetrakis(trimethylphosphine)molybdenum(II)". Journal of the Chemical Society, Dalton Transactions (4): 695–700. http://dx.doi.org/10.1039/DT9840000695. Retrieved 2009-05-02.
- Kubas, G. J.; R. R. Ryan (1986). "Activation of H2 and SO2 by Mo and W complexes: first examples of molecular-H2 complexes, SO2 insertion into metal-hydride bonds, and homogeneous hydrogenation of SO2". Polyhedron 5 (1-2): 473–485. doi:. http://www.sciencedirect.com/science?_ob=GatewayURL&_method=citationSearch&_uoikey=B6TH8-430XDT8-2N&_origin=SDEMFRASCII&_version=1&md5=d553eaf170ce2410f4339e0d461c8a21. Retrieved 2009-05-02.
- Crabtree, Robert H.; Maryellen Lavin (1985). "[IrH2(H2)2L2]+{L = P(C6H11)3}: A non-classical polyhydride complex". Journal of the Chemical Society, Chemical Communications (23): 1661–1662. http://dx.doi.org/10.1039/C39850001661. Retrieved 2009-05-02.
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