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]
See main article: article and metal hydride complex. thumb|left|Decacarbonyldihydridotriosmium is one of many polymetallic carbonyl hydrides.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)<sub>4</sub>CO<sub>2</sub>H]
Na[Fe(CO)<sub>4</sub>CO<sub>2</sub>H] → Na[HFe(CO)<sub>4</sub>] + 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)<sub>4</sub>]−, leads also to the formation of 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 |
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.
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]