:Cobaltocene
{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 456661727
| ImageFileL1 = Cobaltocene.svg
| ImageSizeL1 = 100px
| ImageNameL1 = Skeletal formula of cobaltocene
| ImageFileR1 = Cobaltocene-3D-balls.png
| ImageSizeR1 = 150px
| ImageNameR1 = Ball-and-stick model of cobaltocene
| IUPACName = Cobaltocene
Bis(η5-cyclopentadienyl)cobalt
| OtherNames = Cp2Co
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 83848
| InChI = 1/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
| SMILES = [cH-]1cccc1.[cH-]1cccc1.[Co+2]
| InChIKey = ILZSSCVGGYJLOG-UHFFFAOYAM
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = ILZSSCVGGYJLOG-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 1277-43-6
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 4W7LGU89CV
| ChEBI_Ref = {{ebicite|changed|EBI}}
| ChEBI = 30678
| EINECS = 215-061-0
| RTECS = GG0350000
| PubChem = 22045960
}}
|Section2={{Chembox Properties
| Formula = [Co(η5-C5H5)2]
| MolarMass = 189.12 g/mol
| Appearance = Dark purple solid
| Density =
| Solubility = Insoluble
| MeltingPtC = 171-173
| BoilingPt =
}}
|Section3={{Chembox Structure
| Coordination = sandwich
| Dipole = zero
}}
|Section4={{Chembox Thermochemistry
| DeltaHf = +237 kJ/mol (uncertain)
| DeltaHc = −5839 kJ/mol
| Entropy = 236 J K−1 mol−1
}}
|Section7={{Chembox Hazards
| GHSPictograms = {{GHS02}}{{GHS08}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|228|317|351}}
| PPhrases = {{P-phrases|210|261|280|363|405|501}}
| NFPA-H = 1
| NFPA-F = 2
| NFPA-R = 0
}}
|Section8={{Chembox Related
| OtherFunction_label = metallocenes
| OtherFunction = Ferrocene
Nickelocene
Rhodocene}}
}}
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 oxygen, the compound must be handled and stored using air-free techniques.
Synthesis
Cobaltocene is prepared by the reaction of sodium cyclopentadienide (NaC5H5) with anhydrous cobalt(II) chloride in THF solution. Sodium chloride is cogenerated, and the organometallic product is usually purified by vacuum sublimation.{{cite book|last=King|first= R. B. |title=Organometallic Syntheses|volume=1|publisher= Academic Press|location=New York, NY|date=1965}}
Structure and bonding
In Co(C5H5)2 the Co centre is "sandwiched" between two cyclopentadienyl (Cp) rings. The Co–C bond lengths are about 2.1 Å, slightly longer than the Fe–C bond in ferrocene.{{cite journal|first1=M. Yu. |last1=Antipin|first2= R. |last2=Boese|first3= N. |last3=Augart|first4= G.|last4= Schmid |title=Redetermination of the cobaltocene crystal structure at 100 K and 297 K: Comparison with ferrocene and nickelocene |journal=Struct. Chem.|date= 1993 |volume=4 |issue=2 |pages=91–101|doi=10.1007/BF00677370|s2cid=93871667 }}
Co(C5H5)2 belongs to a group of organometallic compounds called metallocenes or sandwich compounds.{{cite book|first1=C. |last1=Elschenbroich |first2=A. |last2=Salzer |title=Organometallics: A Concise Introduction|edition=2nd |date=1992 |publisher=Wiley-VCH |location=Weinheim |isbn= 978-3-527-28165-7}} 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:
:
+ I2
->
\underbrace{2Co(C5H5)2+}_{18e-}
+ 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.{{cite journal | first1= N. G.|last1= Connelly|first2= W. E.|last2= Geiger | title = Chemical Redox Agents for Organometallic Chemistry | journal = Chem. Rev. | year = 1996 | volume = 96 | pages = 877–910 | doi = 10.1021/cr940053x | pmid=11848774 | issue = 2}} 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. 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:
| class="wikitable"
! Half-reaction !! E0 (V) | |
| {{chem|Fe(C|5|H|5|)|2|+}} + e− ⇌ Fe(C5H5)2 | 0.00 (by definition) |
| {{chem|Fe(C|5|Me|5|)|2|+}} + e− ⇌ Fe(C5Me5)2 | −0.59 |
| {{chem|Co(C|5|H|5|)|2|+}} + e− ⇌ Co(C5H5)2 | −1.33 |
| {{chem|Co(C|5|Me|5|)|2|+}} + e− ⇌ Co(C5Me5)2 | −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 Co(C5H5)2 with carbon monoxide 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.{{cite book |doi=10.1002/9780470132388.ch31|title=Cyclopentadienyl Metal Carbonyls and Some Derivatives|series=Inorganic Syntheses|year=1967|last1=King|first1=R. B.|last2=Stone|first2=F. G. A.|volume=7|pages=99–115|isbn=9780470132388}}
See also
References
{{reflist}}
External links
- [https://web.archive.org/web/20051012230942/http://www-cie.iarc.fr/htdocs/monographs/vol52/11-cobaltandcobaltcomp.htm IARC Monograph "Cobalt and Cobalt Compounds"]
- [https://web.archive.org/web/20060219230038/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]
{{Cyclopentadienide complexes}}
Category:Organocobalt compounds
Category:IARC Group 2B carcinogens