Metal carbonyl hydride

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Cobalt tetracarbonyl hydride

Metal carbonyl hydrides are complexes of transition metals with carbon monoxide and hydride as ligands. These complexes are useful in organic synthesis as catalysts in homogeneous catalysis, such as hydroformylation.

Preparation

Walter Hieber prepared the first metal carbonyl hydride in 1931 by the so called Hieber base reaction of metal carbonyls. In this reaction a hydroxide ion reacts with the carbon monoxide ligand of a metal carbonyl such as iron pentacarbonyl in a nucleophilic attack to form a metallacarboxylic acid. This intermedia releases of carbon dioxide in a second step, giving the iron tetracarbonyl hydride anion. The synthesis of Cobalt tetracarbonyl hydride (HCo(CO)4) proceeds in the same way.[1]

Fe(CO)5 + NaOH → Na[Fe(CO)4CO2H]
Na[Fe(CO)4CO2H] → Na[HFe(CO)4] + CO2

A further synthetic route is the reaction of the metal carbonyl with hydrogen.[2] The protonation of metal carbonylates leads also to the formation of metal carbonyl hydrides.

Properties

Metal Carbonyl hydride pKa
HCo(CO)4 "strong"
HCo(CO)3(P(OPh)3) 5.0
HCo(CO)3(PPh3) 7.0
HMn(CO)5 7.1
H2Fe(CO)4 4.4, 14
[HCo(dmgH)2PBu3] 10.5

The neutral metal carbonyl hydrides are often volatile and can be quite acidic.[3] The hydrogen atom is directly bounded to the metal. The metal-hydrogen bond length is for Cobalt 114 pm, the metal-carbon bond length is for axial ligands 176  and 182  for the equatorial ligands.[4]

Applications

Metal carbonyl hydride are used as catalysts in the hydroformylation of olefins. Under industrial conditions the catalyst is usually formed in situ in a reaction of a metal salt precursor with the syngas. The hydroformylation starts with the generation of a 16-electronn, coodinative unsaturated metal carbonyl hydride complex like HCo(CO)3 or HRh(CO)(PPh3)2 by dissociation of a carbon monoxide ligand. Such complexes bind olefins in a first step in via π-bonding. In a second step an alkyl complex is formed by insertion of the olefin in the metal-hydrogen bond, leading once again to a 16-electron species. This complex can bind another carbon monoxide, which can insert into the metal-carbon bond of the alkyl ligand to form a acyl complex. By oxidative addition of hydrogen and elimination of the aldehyde the initial metal carbonyl hydride complex is formed again.

Analytical characterization

It has been uncertain for a long time whether the Metal carbonyl hydride contained a direct metal-hydrogen bond, although this has been suspected by Hieber. The precise structure cannot be identified by X-ray diffraction, particularly the length of a possible metal-hydrogen bond remained uncertain.[5] The exact structure of the metal carbonyl hydrides has been determined by using neutron diffraction and nuclear magnetic resonance spectroscopy. [6]

Literature

  • Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5 

References

  1. ↑ W. Hieber, F. Leutert: Zur Kenntnis des koordinativ gebundenen Kohlenoxyds: Bildung von Eisencarbonylwasserstoff. In: Die Naturwissenschaften. 19, 1931, pp. 360–361, doi:10.1007/BF01522286.
  2. ↑ H. D. Kaesz, S. A. R. Knox, J. W. Koepke, R. B. Saillant: Synthesis of metal carbonyl hydrides from metal carbonyls and hydrogen at atmospheric pressure. In: Journal of the Chemical Society D: Chemical Communications. 1971, S. 477, doi:10.1039/C29710000477.
  3. ↑ Ralph G. Pearson: The transition-metal-hydrogen bond. In: Chemical Reviews. 85, 1985, S. 41–49, doi:10.1021/cr00065a002.
  4. ↑ E. A. McNeill, F. R. Scholer: Molecular structure of the gaseous metal carbonyl hydrides of manganese, iron, and cobalt. In: Journal of the American Chemical Society. 99, 1977, S. 6243–6249, doi:10.1021/ja00461a011.
  5. ↑ F. A. Cotton: Structure and Bonding in Metal Carbonyls and Related Compounds. In: Helvetica Chimica Acta. 50, 1967, S. 117–130, doi:10.1002/hlca.19670500910.
  6. ↑ E. A. McNeill, F. R. Scholer: Molecular structure of the gaseous metal carbonyl hydrides of manganese, iron, and cobalt. In: Journal of the American Chemical Society. 99, 1977, S. 6243–6249, doi:10.1021/ja00461a011.

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