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.[1]

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.[2]

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.[3] The protonation of metal carbonyl anions, e.g. [Co(CO)4]−, leads also to the formation of metal carbonyl hydrides.

Properties

Some Metal Carbonyl Hydrides
Metal Carbonyl hydride pKa
HCo(CO)4 1[4]
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
HRh(CO)(PPh3)3 unknown

The neutral metal carbonyl hydrides are often volatile and can be quite acidic.[5] 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.[6]

Applications and occurrence

Metal carbonyl hydrides are used as catalysts in the hydroformylation of olefins. 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 coordinatively unsaturated 16-electron metal carbonyl hydride complex like HCo(CO)3 or HRh(CO)(PPh3)2 by dissociation of a ligand. Such complexes bind olefins in a first step via π-complexation, thus beginning the transformation of the alkene to the aldehyde.

Iron carbonyl hydrides occur in nature at the active sites of hydrogenase enzymes.

Analytical characterization

It has been uncertain for a long time whether metal carbonyl hydrides contain a direct metal-hydrogen bond, although this has been suspected by Hieber for H2Fe(CO)4. The precise structure cannot be identified by X-ray diffraction, particularly the length of a possible metal-hydrogen bond remained uncertain.[7] The exact structure of the metal carbonyl hydrides has been determined by using neutron diffraction and nuclear magnetic resonance spectroscopy.[6][8]

Further reading

  • Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN 0-12-352651-5

References

  1. ↑ J. F. Hartwig; Organotransition metal chemistry - from bonding to catalysis. University Science Books. 2009. 753, 757-578. ISBN 978-1-891-38953-5.
  2. ↑ Hieber, W.; Leutert, F. (1931). "Zur Kenntnis des koordinativ gebundenen Kohlenoxyds: Bildung von Eisencarbonylwasserstoff". Die Naturwissenschaften. 19 (17): 360–361. Bibcode:1931NW.....19..360H. doi:10.1007/BF01522286. S2CID 791569.
  3. ↑ Kaesz, H. D.; Saillant, R. B. (1972). "Hydride complexes of the transition metals". Chemical Reviews. 72 (3): 231–281. doi:10.1021/cr60277a003.
  4. ↑ Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). Lehrbuch der anorganischen Chemie (in German). Berlin. ISBN 978-3-11-012641-9. OCLC 237142268.{{cite book}}: CS1 maint: location missing publisher (link)
  5. ↑ Ralph G. Pearson The Transition-Metal-Hydrogen Bond. Chemical Reviews. volume 85, 1985, S. 41–49, doi:10.1021/cr00065a002.
  6. 1 2 McNeill, E. A.; Scholer, F. R. (1977). "Molecular Structure of the Gaseous Metal Carbonyl Hydrides of Manganese, Iron, and Cobalt". Journal of the American Chemical Society. 99 (19): 6243–6249. doi:10.1021/ja00461a011.
  7. ↑ Cotton, F. A. (1967). "Structure and Bonding in Metal Carbonyls and Related Compounds". Helvetica Chimica Acta. 50: 117–130. doi:10.1002/hlca.19670500910.}
  8. ↑ Bau, Robert; Drabnis, Mary H. (1997). "Structures of transition metal hydrides determined by neutron diffraction". Inorganica Chimica Acta. 259 (1–2): 27–50. doi:10.1016/S0020-1693(97)89125-6.
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