Cobaltocene

Overview
Cobaltocene, Co(C5H5)2, is known as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt." This dark purple compound is solid at room temperature that sublimes at 40 °C in a good vacuum, ca. 0.1 mm. Cobaltocene was discovered shortly after ferrocene, the first "metallocene." The compound must be handled and stored in the absence of air due to the ease with which it reacts with O2.

Cobaltocene is prepared by the reaction of sodium cyclopentadienide, NaC5H5, with anhydrous CoCl2 in THF solution. Sodium chloride is generated, and the organometallic product is usually purified by vacuum sublimation.

Structure and bonding
Co(C5H5)2 belongs to a group of organometallic compounds called metallocenes that consist of a metal atom sandwiched between two cyclopentadienyl (Cp) rings. Thus, metallocenes are sometimes referred as 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 Co(C5H5)2 are characterized by its tendency lose this "extra" electron, yielding 18-electron cation known as cobaltocenium:

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 couple is so well behaved that Co(C5H5)2 may be used in cyclic voltammetry as an internal standard. One of its analogues called 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:


 * Fe(C5H5)2+/Fe(C5H5)2: 0 V
 * Fe(C5Me5)2+/Fe(C5Me5)2: -0.59
 * Co(C5H5)2+/Co(C5H5)2: -1.33
 * Co(C5Me5)2+/Co(C5Me5)2: -1.94

We can see from this data that the decamethyl compounds are ca. 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.

Other reactions
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 air-stable, distillable liquid has "two-legged piano-stool" structure.