cobalt tetracarbonyl hydride

File:HCo(CO)4-3D-balls.png

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. The Co–CO and Co–H bond distances were determined by gas-phase electron diffraction to be 1.764 and 1.556 Å, respectively.{{cite journal |title=Molecular structure of the gaseous metal carbonyl hydrides of manganese, iron, and cobalt |year=1977 |last1=McNeill |first1=E. A. |last2=Scholer |first2=F. R. |journal=Journal of the American Chemical Society |volume=99 |issue=19 |pages=6243 |doi=10.1021/ja00461a011}} 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.{{cite journal |pmid=22175569 |title=Kinetic and thermodynamic acidity of hydrido transition-metal complexes. 3. Thermodynamic acidity of common mononuclear carbonyl hydrides |year=1986 |last1=Moore |first1=E. J. |last2=Sullivan |first2=J. M. |last3=Norton |first3=J. R. |journal=Journal of the American Chemical Society |volume=108 |issue=9 |pages=2257–2263 |doi=10.1021/ja00269a022}} 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. These derivatives are generally less acidic than HCo(CO)₄.

The compound decomposes easily. It is colorless when pure, but, soon after melting, develops a yellow tinge due to decomposition to cobalt tetracarbonyl dimer.

Tetracarbonylhydrocobalt was first described by Hieber in the early 1930s.{{cite journal |title=Derivate des Kobalt- und Nickelcarbonyls (XVI. Mitteil. über Metallcarbonyle) |year=1932 |last1=Hieber |first1=W. |last2=Mühlbauer |first2=F. |last3=Ehmann |first3=E. A. |journal=Berichte der Deutschen Chemischen Gesellschaft (A and B Series) |volume=65 |issue=7 |pages=1090 |doi=10.1002/cber.19320650709}} It was the second transition metal hydride to be discovered, after H₂Fe(CO)₄. Laboratory samples are prepared typically from the Brønsted conjugate base, Co(CO){{su|b=4|p=−}}. The latter can be produced from direct carbonylation of cobaltous salts in base,{{cite encyclopedia|article=Cobalt tetracarbonyl hydride|pp=1753-1754|first=F.|last=Seel|encyclopedia=Handbook of Preparative Inorganic Chemistry|volume=2 (3 in original German)|edition=2|editor-first=Georg|editor-last=Bauer|translator-first=Paul G.|translator-last=Stecher|year=1965|publisher=Academic Press / Scripta Technica|location=New York, NY|lccn=63-14307}} possibly with a cysteine catalyst...{{cite journal|title=Metal carbonyl anions|last=Ellis|first=J. E.|journal=Organometallics|year=2003|volume=22|issue=17|pages=3322–3338|doi=10.1021/om030105l}}

:2 Co(NO₃)₂ + 11 CO + 12 KOH → 2 KCo(CO)₄ + 4 KNO₃ + 3 K₂CO₃ + 6 H₂O

...or applying a reducing agent like sodium amalgam to Co₂(CO)₈, which gives sodium tetracarbonylcobaltate:{{cite book |last1=Donaldson |first1=J. D. |last2=Beyersmann |first2=D. |year=2005 |chapter=Cobalt and Cobalt Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |isbn=3527306730 |doi=10.1002/14356007.a07_281.pub2}}

: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.{{cite book |last1=Pfeffer |first1=M. |last2=Grellier |first2=M. |chapter=Cobalt Organometallics |title=Comprehensive Organometallic Chemistry III |pages=1–119 |date=2007 |publisher=Elsevier |isbn=9780080450476 |doi=10.1016/B0-08-045047-4/00096-0}}{{cite journal |title=Relative Rates of Hydrogen Atom (H^.) Transfer from Transition-Metal Hydrides to Trityl Radicals |year=1991 |last1=Eisenberg |first1=David C. |last2=Lawrie |first2=Christophe J. C. |last3=Moody |first3=Anne E. |last4=Norton |first4=Jack R. |journal=Journal of the American Chemical Society |volume=113 |issue=13 |pages=4888–4895 |doi=10.1021/ja00013a026}}

:Co₂(CO)₈ + H₂ {{eqm}} 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.

Tetracarbonylhydridocobalt was the first transition metal hydride to be used in industry.{{cite book |last1=Rittmeyer |first1=P. |last2=Wietelmann |first2=U. |year=2000 |chapter=Hydrides |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |isbn=3527306730 |doi=10.1002/14356007.a13_199}} 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).{{cite journal |last1=Wender |first1=I. |last2=Sternberg |first2=H. W. |last3=Orchin |first3=M. |title=Evidence for Cobalt Hydrocarbonyl as the Hydroformylation Catalyst |journal=J. Am. Chem. Soc. |date=1953 |volume=75 |issue=12 |pages=3041–3042 |doi=10.1021/ja01108a528}} 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.

{{Reflist}}

{{Hydrogen compounds}}

{{Cobalt compounds}}

Category:Cobalt carbonyl complexes

Category:Hydrido complexes

Category:Substances discovered in the 1930s