Dicobalt octacarbonyl
| Dicobalt octacarbonyl | |
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Octacarbonyldicobalt(Co—Co) |
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Other names
Cobalt carbonyl |
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| Identifiers | |
| CAS number | 10210-68-1  |
| ChemSpider | 2007057 |
| UN number | 3281 |
| RTECS number | GG0300000 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | 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, 324-325 K, 124-126 °F |
| Boiling point |
52 °C, 325 K, 126 °F (c.a. decomposes) |
| Solubility in water | insoluble |
| Structure | |
| Dipole moment | 1.33 D (C2v isomer) 0 D (D3d isomer) |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | Toxic |
| Related compounds | |
| Related metal carbonyls | Iron pentacarbonyl Diiron nonacarbonyl Nickel tetracarbonyl |
 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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 into aldehydes.[2]
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Synthesis, structure, properties [edit]
It is synthesised by the high pressure carbonylation of cobalt(II) salts, often in the presence of cyanide. It is an orange-colored, pyrophoric solid that is thermally unstable. It exists as two isomers in solution[3] that rapidly interconvert.
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The major isomer contains two bridging CO ligands and three terminal CO ligands, described as (CO)3Co(μ-CO)2Co(CO)3 (C2v symmetry group). It resembles Fe2(CO)9, but with 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] Analysis of the bonding suggests the absence of a direct Co–Co bond.[5]
The minor isomer has no bridging CO ligands; it is described (CO)4Co-Co(CO)4 (D3d symmetry group). It features an unbridged Co-Co bond that is 2.70 Å in length.[6] The minor isomer has been crystallized together with C60.
Reactions [edit]
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 for hydroformylation reactions. It adds to alkenes to give an alkyl-Co(CO)4 product that then proceeds to insert CO and undergo hydrogenolysis to produce 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
The Pauson–Khand reaction, in which an alkyne, an alkene, and carbon monoxide cyclize to give a cyclopentenone, can be catalyzed by Co2(CO)8:
Co2(CO)8 reacts with alkynes to form a stable covalent complex, which is useful as a protective group for the alkyne. This complex itself can also be used in the Pauson–Khand reaction.
Heating causes decarbonylation and formation of the tetrahedral cluster:
- 2 Co2(CO)8 → Co4(CO)12 + 4 CO
Safety [edit]
Co2(CO)8 a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.[7]
See also [edit]
References [edit]
- ^ 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, H. P. Klug, L. E. Alexander "The crystal structure of dicobalt octacarbonyl" Acta Crystallographica, 1964 Volume 17 Part 6 Pages 732-742. doi:10.1107/S0365110X64001803
- ^ Jennifer C. Green, Malcolm L. H. Green, Gerard Parkin "The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds" Chem. Commun. 2012, 11481-11503. doi:10.1039/c2cc35304k
- ^ Thelma Y. Garcia, James C. Fettinger, Marilyn M. Olmstead, Alan L. Balch, "Splendid symmetry: crystallization of an unbridged isomer of Co2(CO)8 in Co2(CO)8·C60" Chem. Commun., 2009, p. 7143-7145. doi:10.1039/b915083h.
- ^ Cole Parmer MSDS
