Cobalt tetracarbonyl hydride

Cobalt tetracarbonyl hydride is an organometallic compound with the formula HCo(CO)₄. It is a volatile, yellow liquid that forms a colorless vapor and has an intolerable odor.^[4] The compound readily decomposes upon melt and in absentia of high CO partial pressures forms Co₂(CO)₈. 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, HCo(CO)₄ and related derivatives have received significant academic interest for their ability to mediate a variety of carbonylation (introduction of CO into inorganic compounds) reactions.

Cobalt tetracarbonyl hydride

Names
Other names cobalt hydrocarbonyl tetracarbonylhydridocobalt Tetracarbonylhydrocobalt Hydrocobalt tetracarbonyl
Identifiers
CAS Number	16842-03-8 Y
3D model (JSmol)	Interactive image
ECHA InfoCard	100.290.757
PubChem CID	61848
UNII	33B757V1YU Y
CompTox Dashboard (EPA)	DTXSID70937490
SMILES O=C=[CoH](=C=O)(=C=O)=C=O
Properties
Chemical formula	C4HCoO4
Molar mass	171.98 g/mol
Appearance	Light yellow liquid
Odor	offensive[1]
Melting point	−33 °C (−27 °F; 240 K)
Boiling point	47 °C (117 °F; 320 K)
Solubility in water	0.05% (20°C)[1]
Solubility	soluble in hexane, toluene, ethanol
Vapor pressure	>1 atm (20°C)[1]
Acidity (pKa)	1 (in water)[2] 8.3 (in acetonitrile)[3]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	flammable, decomposes in air[1]
NIOSH (US health exposure limits):
PEL (Permissible)	none[1]
REL (Recommended)	TWA 0.1 mg/m3[1]
IDLH (Immediate danger)	N.D.[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Structure and properties

HCo(CO)₄ adopts trigonal bipyramidal structure, with the hydride ligand occupying one of the axial positions, giving an overall symmetry of C_3v. The three equatorial CO ligands are slightly bent out of the equatorial plane.^[5] The Co–CO and Co–H bond distances were determined by gas-phase electron diffraction to be 1.764 and 1.556 Å, respectively.^[6] Assuming the presence of a formal hydride ion, the oxidation state of cobalt in this compound is +1.

But unlike some other transition-metal hydrides complexes, HCo(CO)₄ is highly acidic, with a pK_a of 8.5.^[7] It readily undergoes substitution by tertiary phosphines and other Lewis-bases. For example, triphenylphosphine gives HCo(CO)₃PPh₃ and HCo(CO)₂(PPh₃)₂. These derivatives are more stable than HCo(CO)₄ and are used industrially to improve catalyst selectivity in hydroformylation.^[8] These derivatives are generally less acidic than HCo(CO)₄.^[7]

Preparation

Tetracarbonylhydrocobalt was first described by Hieber in the early 1930s.^[9] It was the second transition metal hydride to be discovered, after H₂Fe(CO)₄. It is prepared by reducing Co₂(CO)₈ with sodium amalgam or a similar reducing agent followed by acidification.^[5]

Co₂(CO)₈ + 2 Na → 2 NaCo(CO)₄

NaCo(CO)₄ + H⁺ → HCo(CO)₄ + Na⁺

Since HCo(CO)₄ decomposes so readily, it is usually generated in situ by hydrogenation of Co₂(CO)₈.^[8][10]

Co₂(CO)₈ + H₂ ⇌ 2 HCo(CO)₄

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.^[8]

Applications

Tetracarbonylhydridocobalt was the first transition metal hydride to be used in industry.^[11] 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 (oxo reaction).^[12] 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 per year, roughly 2% of the total.^[11]

References

1. NIOSH Pocket Guide to Chemical Hazards. "#0148". National Institute for Occupational Safety and Health (NIOSH).
2. Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). Lehrbuch der anorganischen Chemie (in German). Berlin: de Gruyter. ISBN 978-3-11-012641-9. OCLC 237142268.
3. Moore, Eric J.; Sullivan, Jeffrey M.; Norton, Jack R. (1986). "Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 3. Thermodynamic acidity of common mononuclear carbonyl hydrides". Journal of the American Chemical Society. 108 (9). American Chemical Society (ACS): 2257–2263. doi:10.1021/ja00269a022. ISSN 0002-7863. PMID 22175569.
4. Kerr, W. J. (2001). "Sodium Tetracarbonylcobaltate". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rs105. ISBN 0471936235.
5. Donaldson, J. D.; Beyersmann, D. (2005). "Cobalt and Cobalt Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a07_281.pub2. ISBN 3527306730.
6. 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. doi:10.1021/ja00461a011.
7. Moore, E. J.; Sullivan, J. M.; Norton, J. R. (1986). "Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 3. Thermodynamic acidity of common mononuclear carbonyl hydrides". Journal of the American Chemical Society. 108 (9): 2257–2263. doi:10.1021/ja00269a022. PMID 22175569.
8. Pfeffer, M.; Grellier, M. (2007). "Cobalt Organometallics". Comprehensive Organometallic Chemistry III. Elsevier. pp. 1–119. doi:10.1016/B0-08-045047-4/00096-0. ISBN 9780080450476.
9. Hieber, W.; Mühlbauer, F.; Ehmann, E. A. (1932). "Derivate des Kobalt- und Nickelcarbonyls (XVI. Mitteil. über Metallcarbonyle)". Berichte der Deutschen Chemischen Gesellschaft (A and B Series). 65 (7): 1090. doi:10.1002/cber.19320650709.
10. Eisenberg, David C.; Lawrie, Christophe J. C.; Moody, Anne E.; Norton, Jack R. (1991). "Relative Rates of Hydrogen Atom (H^.) Transfer from Transition-Metal Hydrides to Trityl Radicals". Journal of the American Chemical Society. 113 (13): 4888–4895. doi:10.1021/ja00013a026.
11. Rittmeyer, P.; Wietelmann, U. (2000). "Hydrides". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a13_199. ISBN 3527306730.
12. Wender, I.; Sternberg, H. W.; Orchin, M. (1953). "Evidence for Cobalt Hydrocarbonyl as the Hydroformylation Catalyst". J. Am. Chem. Soc. 75 (12): 3041–3042. doi:10.1021/ja01108a528.