Cobaltocene


Cobaltocene, known also as biscobalt or even "bis Cp cobalt", is an organocobalt compound with the formula Co2. 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 with anhydrous cobalt chloride in THF solution. Sodium chloride is cogenerated, and the organometallic product is usually purified by vacuum sublimation.

Structure and bonding

In Co2 the Co centre is "sandwiched" between two cyclopentadienyl rings. The Co–C bond lengths are about 2.1 Å, slightly longer than the Fe–C bond in ferrocene.
Co2 belongs to a group of organometallic compounds called metallocenes or sandwich compounds. Cobaltocene has 19 valence electrons, one more than usually found in organotransition metal complexes such as its very stable relative ferrocene. 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 Co2 are characterized by its tendency to lose this "extra" electron, yielding an 18-electron cation known as cobaltocenium:
+ 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

Co2 is a common one-electron reducing agent in the laboratory. In fact, the reversibility of the Co2 redox couple is so well-behaved that Co2 may be used in cyclic voltammetry as an internal standard. Its permethylated analogue decamethylcobaltocene 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-reactionE0
+ e− ⇌ Fe20.00
+ e− ⇌ Fe2−0.59
+ e− ⇌ Co2−1.33
+ e− ⇌ Co2−1.94

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 Co2 with carbon monoxide gives the cobalt derivative Co2, concomitant with loss of one Cp ligand. This conversion is conducted near 130 °C with 500 psi of CO.