Cobalt tetracarbonyl hydride
|Cobalt tetracarbonyl hydride|
|Molar mass||171.98 g/mol|
|Appearance||Light yellow liquid|
-33 °C, 240 K, -27 °F
47 °C, 320 K, 117 °F
|Solubility in water||Partially|
|Solubility||soluble in hexane, toluene, ethanol|
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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. 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 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. The oxidation state of cobalt in this compound is +1.
Like some other metal carbonyl hydrides, HCo(CO)4 is acidic, with a pKa of 8.5. HCo(CO)4 melts at -33 °C and above that temperature decomposes to Co2(CO)8 and H2. 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. These derivatives are generally less acidic than HCo(CO)4.
Tetracarbonylhydrocobalt was first described by Hieber in the early 1930s. 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.
- Co2(CO)8 + 2 Na → 2 NaCo(CO)4
- NaCo(CO)4 + H+ → HCo(CO)4 + Na+
- 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.
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 (Oxo Reaction). Although the use of cobalt-based hydroformylation has since been largely superseded by rhodium-based catalysts, the world output of C3-C18 aldehydes produced by tetracarbonylhydrocobalt catalysis is about 100,000 tons/year, roughly 2% of the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.
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