Cobaltocene
Skeletal formula of cobaltocene
Skeletal formula of cobaltocene
Ball-and-stick model of cobaltocene
Ball-and-stick model of cobaltocene
Names
IUPAC names
Cobaltocene
Bis(η5-cyclopentadienyl)cobalt
Other names
Cp2Co
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.692 Edit this at Wikidata
EC Number
  • 215-061-0
RTECS number
  • GG0350000
UNII
  • InChI=1S/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2 checkY
    Key: ILZSSCVGGYJLOG-UHFFFAOYSA-N checkY
  • InChI=1/2C5H5.Co/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2
    Key: ILZSSCVGGYJLOG-UHFFFAOYAM
  • [cH-]1cccc1.[cH-]1cccc1.[Co+2]
Properties
[Co(η5-C5H5)2]
Molar mass 189.12 g/mol
Appearance Dark purple solid
Melting point 171–173 °C (340–343 °F; 444–446 K)
Insoluble
Structure
sandwich
zero
Thermochemistry
236 J K−1 mol−1
+237 kJ/mol (uncertain)
−5839 kJ/mol
Hazards
GHS labelling:
GHS02: FlammableGHS08: Health hazard[1]
Danger[1]
H228, H317, H351[1]
P210, P261, P280, P363, P405, P501[1]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
2
0
Safety data sheet (SDS) External SDS
Related compounds
Related metallocenes
 Ferrocene
Nickelocene
Rhodocene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

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

Vacuum sublimated cobaltocene under nitrogen atmosphere

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.[2]

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.[3]

Co(C5H5)2 belongs to a group of organometallic compounds called metallocenes or sandwich compounds.[4] 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:

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.[5] 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:

Half-reaction E0 (V)
Fe(C
5H
5)+
2 + e− ⇌ Fe(C5H5)2		 0.00 (by definition)
Fe(C
5Me
5)+
2 + e− ⇌ Fe(C5Me5)2		 −0.59
Co(C
5H
5)+
2 + e− ⇌ Co(C5H5)2		 −1.33
Co(C
5Me
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.[2][6]

See also

References

  1. ^ a b c d "Bis(cyclopentadienyl)cobalt(II)". American Elements. Retrieved 2018-08-24.
  2. ^ a b King, R. B. (1965). Organometallic Syntheses. Vol. 1. New York, NY: Academic Press.
  3. ^ Antipin, M. Yu.; Boese, R.; Augart, N.; Schmid, G. (1993). "Redetermination of the cobaltocene crystal structure at 100 K and 297 K: Comparison with ferrocene and nickelocene". Struct. Chem. 4 (2): 91–101. doi:10.1007/BF00677370. S2CID 93871667.
  4. ^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-28165-7.
  5. ^ Connelly, N. G.; Geiger, W. E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chem. Rev. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774.
  6. ^ King, R. B.; Stone, F. G. A. (1967). Cyclopentadienyl Metal Carbonyls and Some Derivatives. Inorganic Syntheses. Vol. 7. pp. 99–115. doi:10.1002/9780470132388.ch31. ISBN 9780470132388.

External links

Salts and covalent derivatives of the Cyclopentadienide ion
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