Cobaltocene
Encyclopedia
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 O2, the compound must be handled and stored using air-free technique
s.
(NaC5H5) with anhydrous cobalt(II) chloride
in THF
solution. Sodium chloride is cogenerated, and the organometallic product is usually purified by vacuum sublimation.
(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 metallocene
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 lose this "extra" electron, yielding 18-electron cation known as cobaltocenium:
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.
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 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.
gives the cobalt(I) derivative Co(C5H5)(CO)2
, concomitant with loss of one Cp ligand.
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
, the first metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
. Due to the ease with which it reacts with O2, the compound must be handled and stored using air-free technique
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 commonly carbon dioxide and nitrogen...
s.
Synthesis
Cobaltocene is prepared by the reaction of sodium cyclopentadienideSodium cyclopentadienide
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp or CpNa, where Cp− is the cyclopentadienide anion. Cp is also used as an abbreviation for the cyclopentadienyl ligand in coordination chemistry.-Preparation:Sodium...
(NaC5H5) with anhydrous cobalt(II) chloride
Cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...
in THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
solution. Sodium chloride is cogenerated, and the organometallic product is usually purified by vacuum sublimation.
Structure and bonding
In Co(C5H5)2 the Co centre is "sandwiched" between two cyclopentadienylCyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...
(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 metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
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
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
.) 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:
The otherwise close relative of cobaltocene, rhodocene
Rhodocene
Rhodocene, formally known as bisrhodium, is a chemical compound with the formula [Rh2]. Each molecule contains an atom of rhodium bound between two planar systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has covalent...
does not exist as a monomer, but spontaneously dimerizes by formation of a C-C bond between Cp rings.
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 voltammetryCyclic voltammetry
Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
as an internal standard
Internal standard
An internal standard in analytical chemistry is a chemical substance that is added in a constant amount to samples, the blank and calibration standards in a chemical analysis. This substance can then be used for calibration by plotting the ratio of the analyte signal to the internal standard signal...
. Its permethylated analogue decamethylcobaltocene
Decamethylcobaltocene
Decamethylcobaltocene is an organocobalt compound with the formula Co2, abbreviated CoCp*2. It is dark brown solid. This compound is used as a strong reducing agent in organometallic chemistry.-Synthesis:...
(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 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.
Carbonylation
Treatment of Co(C5H5)2 with carbon monoxideCarbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
gives the cobalt(I) derivative Co(C5H5)(CO)2
Cyclopentadienylcobalt dicarbonyl
Cyclopentadienylcobalt dicarbonyl is an organocobalt compound with formula Co2, abbreviated CpCo2. It is an example of a half-sandwich complex. It is a dark red air sensitive liquid. This compound features one cyclopentadienyl ring that is bound in an η5-manner and two carbonyl ligands...
, concomitant with loss of one Cp ligand.