|Jmol-3D images||Image 1|
|Molar mass||341.95 g/mol|
51-52 °C, 324-325 K, 124-126 °F
52 °C, 325 K, 126 °F (c.a. decomposes)
|Solubility in water||insoluble|
|Dipole moment||1.33 D (C2v isomer)
0 D (D3d isomer)
|Related metal carbonyls||Iron pentacarbonyl
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Dicobalt octacarbonyl is the inorganic compound Co2(CO)8. This metal carbonyl is a reagent and catalyst in organometallic chemistry and organic synthesis. It is used as a catalyst for hydroformylation, the conversion of alkenes into aldehydes.
Synthesis, structure, properties 
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 that rapidly interconvert.
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. Analysis of the bonding suggests the absence of a direct Co–Co bond.
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. The minor isomer has been crystallized together with C60.
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
Heating causes decarbonylation and formation of the tetrahedral cluster:
- 2 Co2(CO)8 → Co4(CO)12 + 4 CO
See also 
- 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