Dihydrogen complex

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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 W(CO)3(P(Cy)3)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 complexaton, 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

[edit] 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.

[edit] 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.

[edit] 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.

[edit] References

  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( CO)3(PR3)2(H2) (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.
  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-[M(η2-H2)(H)(PPh2CH2CH2PPh2)2]BF4 (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( CO)3(PR3)2(H2) (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-[M(η2-H2)(H)(PPh2CH2CH2PPh2)2]BF4 (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}

[edit] Further reading

  1. 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

  1. Burdett, J. K.; Phillips, J. R.; Pourian, M. R.; Poliakoff, M.; Turner, J. J.; Upmacis, R. K. Inorg. Chem. 1987, 26, 3054
  2. Lyons, D.; Wilkinson, G..; Thornton-Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1986, 695
  3. Kubas, G. J.; Ryan, R. R. Polyhedron 1986, 5, 473
  4. Crabtree, R. H.; Lavin, M. J. Chem. Soc.; Chem. Commun. 1985, 1661.
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Chemical bonds
"Strong"
-
"Weak"
other
Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds