View source for Cobaltocene

{{Chembox new
|   Name = Cobaltocene
|   ImageFile = Cobaltocene.png
<!-- | ImageSize = 100px -->
|   ImageName = Skeletal formula of cobaltocene
|   ImageFile1 = Cobaltocene-3D-balls.png
<!-- | ImageSize1 = 150px  -->
|   ImageName1 = Ball-and-stick model of cobaltocene
|   IUPACName = Bis(η<sup>5</sup>-cyclopentadienyl)-<br />cobalt(II)
|   OtherNames = Cp<sub>2</sub>Co
|   Name = Safety data
| Section1 = {{Chembox Identifiers
|   CASNo = 1277-43-6
|   EINECS = ???-???-?
|   RTECS = GG0350000
  }}
| Section2 = {{Chembox Properties
|   Formula = [Co(η<sup>5</sup>C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]
|   MolarMass = 189.12 g/mol
|   Appearance = 
|   Density = ? g/cm<sup>3</sup>, ?
|   Solubility = not soluble
|   MeltingPt = °C (? K)
|   BoilingPt = sublimes
  }}
| Section3 = {{Chembox Structure
|   Coordination = sandwich
|   Dipole = zero
  }}
| Section4 = {{Chembox Thermochemistry
|   DeltaHf = +237 kJ/mol (uncertain)
|   DeltaHc = -5839 kJ/mol
|   Entropy = 236 J.K<sup>&minus;1</sup>.mol<sup>&minus;1</sup>
  }}
| Section7 = {{Chembox Hazards
|   EUClass = not listed
|   FlashPt =
  }}
| Section8 = {{Chembox Related
|   Function = [[metallocene]]s
|   OtherFunctn = [[Ferrocene]]<br />[[Nickelocene]]}}
}}
{{SI}}


==Overview==
'''Cobaltocene''', Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>, 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 O<sub>2</sub>. 

Cobaltocene is prepared by the reaction of sodium cyclopentadienide, NaC<sub>5</sub>H<sub>5</sub>, with anhydrous CoCl<sub>2</sub> in [[THF]] solution.  Sodium chloride is generated, and the [[organometallic]] product is usually purified by vacuum sublimation.<ref>King, R. B. “Organometallic Syntheses” Volume 1: Academic Press: New York, 1965.</ref>

==Structure and bonding==
Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> belongs to a group of organometallic compounds called [[metallocene]]s that consist of a metal atom sandwiched between two [[Cyclopentadiene|cyclopentadienyl]] (Cp) rings.<ref>C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed)  (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7</ref>  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(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> are characterized by its tendency lose this "extra" electron, yielding 18-electron cation known as cobaltocenium:

<div align=center>

{|
|-
| 2Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>
| + I<sub>2</sub>
| → 
| 2[Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]<sup>+</sup>
| + 2I<sup>&minus;</sup>
|-
|<center>19e<sup>&minus;</sup></center>
| &nbsp;
| &nbsp;
|<center>18e<sup>&minus;</sup></center>
| &nbsp;
|-
|}

</div>

==Redox properties==
Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> is a common one-electron reducing agent in the laboratory.<ref>Connelly, N. G. and Geiger, W. E., "Chemical Redox Agents for Organometallic Chemistry", Chemical Reviews, 1996, volume 96, 877-922
</ref>  In fact, the reversibility of the Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> [[oxidation|redox]] couple is so well behaved that Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> may be used in [[cyclic voltammetry]] as an [[internal standard]].
One of its analogues called decamethylcobaltocene Co(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub> 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:

<div align=center>

:Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub><sup>+</sup>/Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>: 0 V
:Fe(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub><sup>+</sup>/Fe(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>: -0.59
:Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub><sup>+</sup>/Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>: -1.33
:Co(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub><sup>+</sup>/Co(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>: -1.94

</div>

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(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> with [[carbon monoxide]] gives the cobalt(I) derivative Co(C<sub>5</sub>H<sub>5</sub>)(CO)<sub>2</sub>, concomitant with loss of one Cp ligand.  This air-stable, distillable liquid has "two-legged piano-stool" structure.

==References==
<references/>

==External links==
*[http://www-cie.iarc.fr/htdocs/monographs/vol52/11-cobaltandcobaltcomp.htm IARC Monograph "Cobalt and Cobalt Compounds"]
* [http://www.npi.gov.au/database/substance-info/profiles/26.html National Pollutant Inventory - Cobalt fact sheet]
*[http://webbook.nist.gov/chemistry/ NIST Standard Reference Database]

[[Category:Cobalt compounds]]
[[Category:Metallocenes]]
[[Category:IARC Group 2B carcinogens]]

[[de:Cobaltocen]]
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