Dicobalt octacarbonyl

Dicobalt octacarbonyl is the chemical compound Co2(CO)8. This metal carbonyl is a versatile reagent in organometallic chemistry and organic synthesis. It continues to be used as a catalyst for hydroformylation catalysis.

Synthesis
The high pressure reaction of cobalt(II) salts with CO, often in the presence of cyanide, affords this compound.

Structure
Co2(CO)8 is the most structurally complex of the binary metal carbonyls because it exists in two isomers. These isomers rapidly interconvert. The minor isomer has no bridging CO ligands, it is described (CO)4Co-Co(CO)4. The major isomer contains two bridging CO ligand and is described as (CO)3Co(μ-CO)2Co(CO)3.

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 affort 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.

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.