Cobaltocene

Names

IUPAC names

Cobaltocene
Bis(η⁵-cyclopentadienyl)cobalt

Other names

Cp₂Co

Identifiers

CAS Number

1277-43-6^Y

3D model (JSmol)

Interactive image

ChEBI

CHEBI:30678^N

ChemSpider

83848^Y

ECHA InfoCard

100.013.692

EC Number

215-061-0

PubChem CID

22045960

RTECS number

GG0350000

UNII

4W7LGU89CV^Y

CompTox Dashboard (EPA)

DTXSID0024840

InChI

InChI=1S/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2^Y
Key: ILZSSCVGGYJLOG-UHFFFAOYSA-N^Y
InChI=1/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
Key: ILZSSCVGGYJLOG-UHFFFAOYAM

SMILES

[cH-]1cccc1.[cH-]1cccc1.[Co+2]

Properties

Chemical formula

[Co(η⁵-C₅H₅)₂]

Molar mass

189.12 g/mol

Appearance

Dark purple solid

Melting point

171–173 °C (340–343 °F; 444–446 K)

Solubility in water

Insoluble

Structure

Coordination geometry

sandwich

Dipole moment

zero

Thermochemistry

Std molar
entropy (S^⦵₂₉₈)

236 J K⁻¹ mol⁻¹

Std enthalpy of
formation (Δ_fH^⦵₂₉₈)

+237 kJ/mol (uncertain)

Std enthalpy of
combustion (Δ_cH^⦵₂₉₈)

−5839 kJ/mol

Hazards

GHS labelling:

Pictograms

^[1]

Signal word

Danger^[1]

Hazard statements

H228, H317, H351^[1]

Precautionary statements

P210, P261, P280, P363, P405, P501^[1]

NFPA 704 (fire diamond)

Safety data sheet (SDS)

External SDS

Related compounds

Related metallocenes

Ferrocene
Nickelocene
Rhodocene

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

N verify (what is ^YN ?)

Infobox references

Chemical compound

Cobaltocene, known also as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt", is an organocobalt compound with the formula Co(C₅H₅)₂. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovered shortly after ferrocene, the first metallocene. Due to the ease with which it reacts with oxygen, the compound must be handled and stored using air-free techniques.

Synthesis

Cobaltocene is prepared by the reaction of sodium cyclopentadienide (NaC₅H₅) with anhydrous cobalt(II) chloride in THF solution. Sodium chloride is cogenerated, and the organometallic product is usually purified by vacuum sublimation.^[2]

Structure and bonding

In Co(C₅H₅)₂ the Co centre is "sandwiched" between two cyclopentadienyl (Cp) rings. The Co–C bond lengths are about 2.1 Å, slightly longer than the Fe–C bond in ferrocene.^[3]

Co(C₅H₅)₂ belongs to a group of organometallic compounds called metallocenes or sandwich compounds.^[4] Cobaltocene has 19 valence electrons, one more than usually found in organotransition metal complexes such as its very stable relative ferrocene. (See 18-electron rule.) This additional electron occupies an orbital that is antibonding with respect to the Co–C bonds. Consequently, the Co–C distances are slightly longer than the Fe–C bonds in ferrocene. Many chemical reactions of Co(C₅H₅)₂ are characterized by its tendency to lose this "extra" electron, yielding an 18-electron cation known as cobaltocenium:

{\ce {\underbrace {2Co(C5H5)2} _{19e-}+I2->\underbrace {2Co(C5H5)2+} _{18e-}+2I-}}

The otherwise close relative of cobaltocene, rhodocene does not exist as a monomer, but spontaneously dimerizes by formation of a C–C bond between Cp rings.

Reactions

Redox properties

Co(C₅H₅)₂ is a common one-electron reducing agent in the laboratory.^[5] In fact, the reversibility of the Co(C₅H₅)₂ redox couple is so well-behaved that Co(C₅H₅)₂ may be used in cyclic voltammetry as an internal standard. Its permethylated analogue decamethylcobaltocene (Co(C₅Me₅)₂) is an especially powerful reducing agent, due to inductive donation of electron density from the 10 methyl groups, prompting the cobalt to give up its "extra" electron even more so. These two compounds are rare examples of reductants that dissolve in non-polar organic solvents. The reduction potentials of these compounds follow, using the ferrocene-ferrocenium couple as the reference:

Half-reaction	E⁰ (V)
Fe(C ₅H ₅)⁺ ₂ + e⁻ ⇌ Fe(C₅H₅)₂	0.00 (by definition)
Fe(C ₅Me ₅)⁺ ₂ + e⁻ ⇌ Fe(C₅Me₅)₂	−0.59
Co(C ₅H ₅)⁺ ₂ + e⁻ ⇌ Co(C₅H₅)₂	−1.33
Co(C ₅Me ₅)⁺ ₂ + e⁻ ⇌ Co(C₅Me₅)₂	−1.94

The data show that the decamethyl compounds are around 600 mV more reducing than the parent metallocenes. This substituent effect is, however, overshadowed by the influence of the metal: changing from Fe to Co renders the reduction more favorable by over 1.3 volts.

Carbonylation

Treatment of Co(C₅H₅)₂ with carbon monoxide gives the cobalt(I) derivative Co(C₅H₅)(CO)₂, concomitant with loss of one Cp ligand. This conversion is conducted near 130 °C with 500 psi of CO.^[2]^[6]

References

^ ^a ^b ^c ^d "Bis(cyclopentadienyl)cobalt(II)". American Elements. Retrieved 2018-08-24.
^ ^a ^b King, R. B. (1965). Organometallic Syntheses. Vol. 1. New York, NY: Academic Press.
^ Antipin, M. Yu.; Boese, R.; Augart, N.; Schmid, G. (1993). "Redetermination of the cobaltocene crystal structure at 100 K and 297 K: Comparison with ferrocene and nickelocene". Struct. Chem. 4 (2): 91–101. doi:10.1007/BF00677370. S2CID 93871667.
^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-28165-7.
^ Connelly, N. G.; Geiger, W. E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chem. Rev. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774.
^ King, R. B.; Stone, F. G. A. (1967). Cyclopentadienyl Metal Carbonyls and Some Derivatives. Inorganic Syntheses. Vol. 7. pp. 99–115. doi:10.1002/9780470132388.ch31. ISBN 9780470132388.