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 Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...

, the first

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

. Due to the ease with which it reacts with oxygen, the compound must be handled and stored using

air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less com ...

Synthesis

Cobaltocene is prepared by the reaction of

sodium cyclopentadienide Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pin ...

(NaC₅H₅) with anhydrous cobalt(II) chloride in THF solution. Sodium chloride is cogenerated, and the

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

product is usually purified by vacuum sublimation.

Structure and bonding

In Co(C₅H₅)₂ the Co centre is "sandwiched" between two

cyclopentadienyl Cyclopentadienyl can refer to * Cyclopentadienyl anion, or cyclopentadienide, ** Cyclopentadienyl ligand * Cyclopentadienyl radical, • * Cyclopentadienyl cation, See also * Pentadienyl {{Chemistry index ...

(Cp) rings. The Co–C bond lengths are about 2.1 Å, slightly longer than the Fe–C bond in ferrocene. Co(C₅H₅)₂ belongs to a group of organometallic compounds called

s or sandwich compounds. 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: :\underbrace_ + I2 -> \underbrace_ + 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. In fact, the reversibility of the Co(C₅H₅)₂

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

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: 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 Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

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.

References

External links

IARC Monograph "Cobalt and Cobalt Compounds"

NIST Standard Reference Database
{{Cyclopentadienide complexes Organocobalt compounds Metallocenes IARC Group 2B carcinogens Cyclopentadienyl complexes One-electron reducing agents