Dicobalt octacarbonyl

Dicobalt octacarbonyl
IUPAC name Octacarbonyldicobalt(Co—Co)
Other names Cobalt carbonyl
Identifiers
CAS number	10210-68-1 Y
ChemSpider	2007057 Y
UN number	3281
RTECS number	GG0300000
Jmol-3D images	Image 1
SMILES [Co+2].[Co+2].[C-]#[O+].[O+]#[C-].[O+]#[C-].[O+]#[C-].[O+]#[C-].[O+]#[C-].[O+]#[C-].[O+]#[C-]
InChI InChI=1S/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2 Y Key: MQIKJSYMMJWAMP-UHFFFAOYSA-N Y InChI=1/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2 Key: MQIKJSYMMJWAMP-UHFFFAOYAG
Properties
Molecular formula	Co2(CO)8
Molar mass	341.95 g/mol
Appearance	red-orange crystals when pure
Density	1.87 g/cm3
Melting point	51-52 °C, 324-325 K, 124-126 °F
Boiling point	52 °C, 325 K, 126 °F (decomp. ca.)
Solubility in water	insoluble
Structure
Dipole moment	1.33 D (C2v isomer) 0 D (D3d isomer)
Hazards
MSDS	External MSDS
Main hazards	Toxic
Related compounds
Related metal carbonyls	Iron pentacarbonyl Diiron nonacarbonyl Nickel tetracarbonyl
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Dicobalt octacarbonyl is the inorganic compound Co₂(CO)₈. This metal carbonyl is a reagent and catalyst in organometallic chemistry and organic synthesis.^[1] It is used as a catalyst for hydroformylation, the conversion of alkenes to aldehydes.^[2]

Synthesis, structure, properties

The high pressure carbonylation of cobalt(II) salts, often in the presence of cyanide, affords this compound. It is an orange-colored, pyrophoric solid that is thermally unstable. It exists as two isomers in solution:^[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). The minor isomer has been crystallized together with C₆₀.^[5]

Reactions

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 in hydroformylation. It adds to alkenes to give an alkyl-Co(CO)₄ product that then proceeds to insert CO and undergo hydrogenolysis to afford 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

Co₂(CO)₈ catalyzes the Pauson–Khand reaction of an alkyne, an alkene, and CO to give cyclopentenones.

Heating causes decarbonylation and formation of the tetrahedral cluster:

2 Co₂(CO)₈ → Co₄(CO)₁₂ + 4 CO

Safety

Co₂(CO)₈ a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.^[6]

See also

• Nicholas reaction

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^{[dead link]}.
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
5. ^ "Splendid symmetry: crystallization of an unbridged isomer of Co₂(CO)₈ in Co₂(CO)₈^·C₆₀" Thelma Y. Garcia, James C. Fettinger, Marilyn M. Olmstead, Alan L. Balch, Chem. Commun., 2009. doi:10.1039/b915083h.
6. ^ Cole Parmer MSDS