Dicobalt octacarbonyl an orange-colored, pyrophoric solid.^[6] It is synthesised by the high pressure carbonylation of cobalt(II) salts:^[6]

2 (CH₃COO)₂Co + 8 CO + 2 H₂ → Co₂(CO)₈ + 4 CH₃COOH

The preparation is often carried out in the presence of cyanide, converting the cobalt(II) salt into a pentacyanocobaltate(II) complex that reacts with carbon monoxide to yield K[Co(CO)₄]. Acidification produces cobalt tetracarbonyl hydride, HCo(CO)₄, which degrades near room temperature to dicobalt octacarbonyl and hydrogen.^[3][7] It can also be prepared by heating cobalt metal to above 250 °C in a stream of carbon monoxide gas at about 200 to 300 atm:^[3]

2 Co + 8 CO → Co₂(CO)₈

It exist as a mixture of rapidly interconverting isomers.^[2][3] In solution, there are two isomers known that rapidly interconvert:^[5]

The major isomer (on the left in the above equilibrium process) contains two bridging carbonyl ligands linking the cobalt centres and six terminal carbonyl ligands, three on each metal.^[5] It can be summarised by the formula (CO)₃Co(μ-CO)₂Co(CO)₃ and has C_2v symmetry. This structure resembles diiron nonacarbonyl (Fe₂(CO)₉) but with one fewer bridging carbonyl. The Co–Co distance is 2.52 Å, and the Co–CO_terminal and Co–CO_bridge distances are 1.80 and 1.90 Å, respectively.^[8] Analysis of the bonding suggests the absence of a direct cobalt–cobalt bond.^[9]

The minor isomer has no bridging carbonyl ligands, but instead has a direct bond between the cobalt centres and eight terminal carbonyl ligands, four on each metal atom.^[5] It can be summarised by the formula (CO)₄Co-Co(CO)₄ and has D_4d symmetry. It features an unbridged cobalt–cobalt bond that is 2.70 Å in length in the solid structure when crystallized together with C₆₀.^[10]

Reduction edit

Dicobalt octacarbonyl is reductively cleaved by alkali metals and related reagents, such as sodium amalgam. The resulting salts protonate to give tetracarbonyl cobalt hydride:^[3]

Co₂(CO)₈ + 2 Na → 2 Na[Co(CO)₄]

Na[Co(CO)₄] + H⁺ → H[Co(CO)₄] + Na⁺

Salts of this form are also intermediates in the cyanide synthesis pathway for dicobalt octacarbonyl.^[7]

Reactions with electrophiles edit

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

Co₂(CO)₈ + Br₂ → 2 Br[Co(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

Reactions with alkynes edit

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.^[11][12]

The Pauson–Khand reaction,^[13] in which an alkyne, an alkene, and carbon monoxide cyclize to give a cyclopentenone, can be catalyzed by Co₂(CO)₈,^[3][14] though newer methods that are more efficient have since been developed:^[15][16]

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.^[13]

Intramolecular Pauson–Khand reactions, where the starting material contains both the alkene and alkyne moieties, are possible. In the asymmetric synthesis of the Lycopodium alkaloid huperzine-Q, Takayama and co-workers used an intramolecular Pauson–Khand reaction to cyclise an enyne containing a tert-butyldiphenylsilyl (TBDPS) protected primary alcohol.^[17] The preparation of the cyclic siloxane moiety immediately prior to the introduction of the dicobalt octacarbonyl ensures that the product is formed with the desired conformation.^[18]

Dicobalt octacarbonyl can catalyze alkyne trimerisation of diphenylacetylene and its derivatives to hexaphenylbenzenes.^[19] Symmetrical diphenylacetylenes form 6-substituted hexaphenylbenzenes, while asymmetrical diphenylacetylenes form a mixture of two isomers.^[20]

Hydroformylation edit

Hydrogenation of Co₂(CO)₈ produces cobalt tetracarbonyl hydride H[Co(CO)₄]:^[23]

Co₂(CO)₈ + H₂ → 2 H[Co(CO)₄]

This hydride is a catalyst for hydroformylation – the conversion of alkenes to aldehydes.^[4][23] The catalytic cycle for this hydroformylation is shown in the diagram.^[4][21][22]

Substitution reactions edit

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.^[3] "Hard" Lewis bases, e.g. pyridine, cause disproportionation:

12 C₅H₅N + 3 Co₂(CO)₈ → 2 [Co(C₅H₅N)₆][Co(CO)₄]₂ + 8 CO

Conversion to higher carbonyls edit

Heating causes decarbonylation and formation of tetracobalt dodecacarbonyl:^[3][24]

2 Co₂(CO)₈ → Co₄(CO)₁₂ + 4 CO

Like many metal carbonyls, dicobalt octacarbonyl abstracts halides from alkyl halides. Upon reaction with bromoform, it converts to methylidynetricobaltnonacarbonyl, HCCo₃(CO)₉, by a reaction that can be idealised as:^[25]

9 Co₂(CO)₈ + 4 CHBr₃ → 4 HCCo₃(CO)₉ + 36 CO + 6 CoBr₂

Co₂(CO)₈ a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.^[26] The National Institute for Occupational Safety and Health has recommended that workers should not be exposed to concentrations greater than 0.1 mg/m³ over an eight-hour time-weighted average, without the proper respiratory gear.^[27]