Cobalt tetracarbonyl hydride


Cobalt tetracarbonyl hydride is an organometallic compound with the formula HCo4. It is a volatile, yellow liquid that forms a colorless vapor and has an intolerable odor. The compound readily decomposes upon melt and in absentia of high CO partial pressures forms Co28. Despite operational challenges associated with its handling, the compound has received considerable attention for its ability to function as a catalyst in hydroformylation. In this respect, HCo4 and related derivatives have received significant academic interest for their ability to mediate a variety of carbonylation reactions.

Structure and properties

HCo4 adopts trigonal bipyrimidal structure with the equatorial CO ligands slightly bent out of the equatorial plane. The hydride ligand occupies one of the axial positions, thus the symmetry of the molecule is C3v. The Co–CO and Co–H bond distances were determined by gas-phase electron diffraction to be 1.764 and 1.556 Å, respectively. Assuming the presence of a formal hydride ion, the oxidation state of cobalt in this compound is +1.
Unlike some other transition-metal hydrides complexes, HCo4 is highly acidic, with a pKa of 8.5. It readily undergoes substitution by tertiary phosphines and other Lewis-bases. For example, triphenylphosphine gives HCo3PPh3 and HCo22. These derivatives are more stable than HCo4 and are used industrially to improve catalyst selectivity in hydroformylation. These derivatives are generally less acidic than HCo4.

Preparation

Tetracarbonylhydrocobalt was first described by Hieber in the early 1930s. It was the second transition metal hydride to be discovered, after H2Fe4. It is prepared by reducing Co28 with sodium amalgam or a similar reducing agent followed by acidification.
Since HCo4 decomposes so readily, it is usually generated in situ by hydrogenation of Co28.
The thermodynamic parameters for the equilibrium reaction were determined by infrared spectroscopy to be ΔH = 4.054 kcal mol−1, ΔS = −3.067 cal mol−1 K−1.

Applications

Tetracarbonylhydridocobalt was the first transition metal hydride to be used in industry. In 1953 evidence was disclosed that it is the active catalyst for the conversion of alkenes, CO, and H2 to aldehydes, a process known as hydroformylation. Although the use of cobalt-based hydroformylation has since been largely superseded by rhodium-based catalysts, the world output of C3–C18 aldehydes produced by tetracarbonylhydrocobalt catalysis is about 100,000 tons/year, roughly 2% of the total.