Dicobalt octacarbonyl
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| Names | |
|---|---|
| IUPAC name
Octacarbonyldicobalt(Co—Co)
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| Other names
Cobalt carbonyl
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| Identifiers | |
| ECHA InfoCard | 100.030.454 |
| RTECS number |
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| UN number | 3281 |
CompTox Dashboard (EPA)
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| Properties | |
| Co2(CO)8 | |
| Molar mass | 341.95 g/mol |
| Appearance | red-orange crystals when pure |
| Density | 1.87 g/cm3 |
| Melting point | 51–52 °C |
| Boiling point | decomp. ca. 52 ºC |
| insoluble | |
| Structure | |
| 1.33 D (C2v isomer) 0 D (D3d isomer) | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards
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Toxic |
| Related compounds | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Dicobalt octacarbonyl is the inorganic compound Co2(CO)8. This metal carbonyl is a reagent and catalyst in organometallic chemistry and organic synthesis.[1] It is used as a catalyst for hydroformylation, the conversion of alkenes to aldehydes.[2]
Synthesis, structure, properties
The high pressure carbonylation of cobalt(II) salts, often in the presence of cyanide, affords this compound. It is an orange-colored, pyrophoric solid that is thermally unstable. It exists as two isomers in solution:[3]
The predominant isomer resembles Fe2(CO)9, less one bridging CO. The Co-Co distance is 2.52 Å, and the Co-COterminal and Co-CObridge distances are 1.80 and 1.90 Å, respectively.[4] These isomers rapidly interconvert. The minor isomer has no bridging CO ligands, it is described (CO)4Co-Co(CO)4 (D3d symmetry group). The major isomer contains two bridging CO ligand and features octahedral cobalt, describable as (CO)3Co(μ-CO)2Co(CO)3 (C2v symmetry group). The minor isomer has been crystallized together with C60.[5]
Reactions
The most characteristic reaction of Co2(CO)8 is its hydrogenation to tetracarbonylhydrocobalt, [HCo(CO)4]:
- Co2(CO)8 + H2 → 2 HCo(CO)4
This hydride is the active agent in hydroformylation. It adds to alkenes to give an alkyl-Co(CO)4 product that then proceeds to insert CO and undergo hydrogenolysis to afford the aldehyde. Reduction of Co2(CO)8 gives the conjugate base of HCo(CO)4:
- Co2(CO)8 + 2 Na → 2 NaCo(CO)4
The CO ligands can be replaced with tertiary phosphine ligands to give Co2(CO)8-x(PR3)x. These bulky derivatives are more selective catalysts for hydroformylation reactions. "Hard" Lewis bases, e.g. pyridine, cause disproportionation:
- 6 C6H5N + 1.5 Co2(CO)8 → [Co(C6H5N)6][Co(CO)4]2 + 4 CO
Co2(CO)8 catalyzes the Pauson–Khand reaction of an alkyne, an alkene, and CO to give cyclopentenones.
Heating causes decarbonylation and formation of the tetrahedral cluster:
- 2 Co2(CO)8 → Co4(CO)12 + 4 CO
Safety
Co2(CO)8 is pyrophoric, a volatile source of cobalt, and releases carbon monoxide upon decomposition.
References
- ^ Pauson, P. L. “Octacarbonyldicobalt” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289[dead link].
- ^ Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
- ^ Ray L. Sweany and Theodore L. Brown "Infrared spectra of matrix-isolated dicobalt octacarbonyl. Evidence for the third isomer" Inorganic Chemistry 1977, 16, pp 415 - 421. doi:10.1021/ic50168a037
- ^ G.G. Sumner, HP Klug, LE Alexander "The crystal structure of dicobalt octacarbonyl" Acta Crystallographica, 1964 Volume 17 Part 6 Pages 732-742. doi:10.1107/S0365110X64001803
- ^ "Splendid symmetry: crystallization of an unbridged isomer of Co2(CO)8 in Co2(CO)8·C60" Thelma Y. Garcia, James C. Fettinger, Marilyn M. Olmstead, Alan L. Balch, Chem. Commun., 2009. doi:10.1039/b915083h.