Dicobalt octacarbonyl

Dicobalt octacarbonyl is the chemical compound Co₂(CO)₈. This metal carbonyl is a versatile reagent in organometallic chemistry and organic synthesis.^[1] It continues to be used as a catalyst for hydroformylation catalysis.^[2]

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Synthesis and structure

The high pressure reaction of cobalt(II) salts with CO, often in the presence of cyanide, affords this compound. It is an orange-colored, pyrophoric material. It is one of the more structurally complex of the binary metal carbonyls because it exists as isomers:^[3]

The predominant isomer resembles Fe₂(CO)₉, less one bridging CO. 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.^[4] These isomers rapidly interconvert. The minor isomer has no bridging CO ligands, it is described (CO)₄Co-Co(CO)₄ (D_3d symmetry group). The major isomer contains two bridging CO ligand and features octahedral cobalt, describable as (CO)₃Co(μ-CO)₂Co(CO)₃ (C_2v symmetry group).

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.

References

1. ^ Pauson, P. L. “Octacarbonyldicobalt” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
2. ^ Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
3. ^ 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
4. ^ G.G. Sumner, HP Klug, LE Alexander "The crystal structure of dicobalt octacarbonyl" Acta Crystallographica, 1964 Volume 17 Part 6 Pages 732-742. doi:10.1107/S0365110X64001803

See also

• Nicholas reaction