Dicobalt octacarbonyl is a metal compound with composition Co2(CO)8. This metal carbonyl is used as a reagent and catalyst in organometallic chemistry and organic synthesis, and is central to much known organocobalt chemistry.It is the precursor to a hydroformylation catalyst, cobalt tetracarbonyl hydride.Each molecule consists of two cobalt atoms bound to eight carbon monoxide ligands, though multiple distinct structural arrangements are known.Some of the carbonyl ligands are highly labile. The compound is highly reactive towards alkynes, and is sometimes used as an alkyne protecting group. As the cobalt-alkyne complex, it plays a role in promoting both the Nicholas reaction and the Pauson–Khand reaction.

Reactions

Reduction

Dicobalt octacarbonyl is reductively cleaved by alkali metals and relatived reagents. The resulting alkali metal salts protonate to give tetracarbonyl cobalt hydride:

Co2(CO)8 + 2 Na → 2 NaCo(CO)4

NaCo(CO)4 + H+ → HCo(CO)4 + Na+

Reactions with electrophiles

Halogens and related reagents cleave the Co-Co bond to give pentacoordinated halotetracarbonyls:

Co2(CO)8 + Br2 → 2 BrCo(CO)4

Cobalt tricarbonyl nitrosyl is produced by treatment of dicobalt octacarbonyl with nitric oxide:

Co2(CO)8 + 2 NO → 2 Co(CO)3NO + 2 CO

Nicholas reaction

The Nicholas reaction is a substitution reaction whereby an alkoxy group located on the α-carbon of an alkyne is replaced by another nucleophile.The alkyne reacts first with dicobalt octacarbonyl, from which is generated a stabilized propargylic cation that reacts with the incoming nucleophile and the product then forms by oxidative demetallation.

Pauson–Khand reaction

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,though newer methods that are more efficient have since been developed: