Cobalt tetracarbonyl hydride

Cobalt tetracarbonyl hydride

Names
Other names cobalt hydrocarbonyl tetracarbonylhydridocobalt Tetracarbonylhydrocobalt Hydrocobalt tetracarbonyl
Identifiers
CAS Number	16842-03-8=
PubChem	61848
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)	8.5
Hazards
Main hazards	flammable, decomposes in air[1]
US health exposure limits (NIOSH):
PEL (Permissible)	none[1]
REL (Recommended)	TWA 0.1 mg/m^3[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).
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Infobox references

Cobalt tetracarbonyl hydride is the organometallic compound with the formula HCo(CO)₄. It is a yellow liquid that forms a colorless vapor and has an intolerable odor.^[2] Its main use is as a catalyst in hydroformylation.

Structure and properties

HCo(CO)₄ is a trigonal bipyrimidal molecule. The hydride ligand occupies one of the axial positions, thus the symmetry of the molecule is C_3v.^[3] The Co–CO and Co–H bond distances were determined by gas-phase electron diffraction to be 1.764 and 1.556 Å, respectively.^[4] Assuming the presence of a formal hydride ion, the oxidation state of cobalt in this compound is +1.

Like some other metal carbonyl hydrides, HCo(CO)₄ is acidic, with a pK_a of 8.5.^[5] HCo(CO)₄ melts at −33 °C and above that temperature decomposes to Co₂(CO)₈ and H₂.^[2] It undergoes substitution by tertiary phosphines. 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.^[6] These derivatives are generally less acidic than HCo(CO)₄.^[5]

Preparation

Tetracarbonylhydrocobalt was first described by Hieber in the early 1930s.^[7] 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.^[3]

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.^[6]

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.^[6]

Applications

Tetracarbonylhydridocobalt was the first transition metal hydride to be used in industry.^[8] 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).^[9] 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.^[8]

References

1. "NIOSH Pocket Guide to Chemical Hazards #0148". National Institute for Occupational Safety and Health (NIOSH).
2. Kerr, W. J. (2001). "Sodium Tetracarbonylcobaltate". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rs105.
3. 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.
4. 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.
5. 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.
6. Pfeffer, M.; Grellier, M. (2007). "Cobalt Organometallics". Comprehensive Organometallic Chemistry III. Elsevier. doi:10.1016/B0-08-045047-4/00096-0.
7. 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.
8. Rittmeyer, P.; Wietelmann, U. (2000). "Hydrides". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a13_199.
9. Wender, I.; Sternberg, H. W.; Orchin, M. (1953). "Evidence for Cobalt Hydrocarbonyl as the Hydroformylation Catalyst". J. Am. Chem. Soc. 75: 3041–3042. doi:10.1021/ja01108a528.