Dihydrogen complex

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).

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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 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 W(CO)₃(P-i-Pr₃)₂(H₂). In some cases, H₂ will displace weakly bound ligands, sometimes even halides in favorable cases:

L_nMX + H₂ → [L_nM(H₂)]⁺ + X^-

Many metal hydrides can be protonated to give dihydrogen complexes:

L_nM-H + H⁺ → [L_nM(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.

See also Dihydrogen bond See also Agostic interaction

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}

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.