Dicobalt octacarbonyl
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| Dicobalt octacarbonyl | |
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| IUPAC name | Dicobalt octacarbonyl |
| Other names | Cobalt carbonyl Octacarbonyldicobalt |
| Identifiers | |
| CAS number | [10210-68-1] |
| RTECS number | GG0300000 |
| Properties | |
| Molecular formula | Co2CO8 |
| Molar mass | 341.95 g/mol |
| Appearance | red-orange crystals when pure |
| Density | 1.87 g/cm3 |
| Melting point |
51–52 °C |
| Boiling point |
sublimes > 40 °C (vacuum) |
| Solubility in water | insoluble |
| Structure | |
| Dipole moment | 0 D |
| Hazards | |
| Main hazards | CO source pyrophoric |
| R-phrases | 11-22-40-43-48/ 20-52/53-62 |
| S-phrases | 36/37-61 |
| Related compounds | |
| Related compounds | Ni(CO)4 Fe3(CO)12 Fe2(CO)9, Co4(CO)12 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Dicobalt octacarbonyl is the chemical compound Co2(CO)8. This metal carbonyl is a versatile reagent in organometallic chemistry and organic synthesis.[1] It continues to be used as a catalyst for hydroformylation.[2]
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[edit] Synthesis and structure
The high pressure reaction of cobalt(II) salts with CO, often in the presence of cyanide, affords this compound. It is an orange-colored, pyrophoric material. It is one of the more structurally complex of the binary metal carbonyls because it exists as isomers:[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).
[edit] Reactions
- The characteristic reaction of this species entails its hydrogenation:
- Co2(CO)8 + H2 → 2 HCo(CO)4
This hydride is the active agent in hydroformylation. It adds to alkenes to give an alkylCo(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 tertiaryphosphine ligands to give Co2(CO)8-x(PR3)x. These bulky derivatives are more selective catalysts for hydroformylation reactions.
- Co2(CO)8 catalyzes the Pauson–Khand reaction of an alkyne, an alkene, and CO to give a cyclopentenone.
- Lewis bases cause disproportionation:
- 6 pyridine + 1.5 Co2(CO)8 → [Co(pyridine)6][Co(CO)4]2 + 4 CO
- Heating causes decarbonylation and formation of the tetrahedral cluster Co4(CO)12.
[edit] Safety
Co2(CO)8 is a volatile source of cobalt; it releases carbon monoxide upon decomposition. This is one of the more reactive binary metal carbonyls, and some samples will contain pyrophoric particles of cobalt that can inflame in air. Samples are refrigerated during storage.
[edit] 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.
- ^ 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
