Chromocene

Chromocene is the organochromium compound with the formula [Cr(C5H5)2]. Like structurally related metallocenes, chromocene readily sublimes in a vacuum and is soluble in non-polar organic solvents. It is more formally known as bis(η5-cyclopentadienyl)chromium(II).[1]

Chromocene
Chromocene
Chromocene
Names
IUPAC name
Bis(η5-cyclopentadienyl)chromium(II)
Other names
Dicyclopentadienylchromium(II)
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.670
RTECS number
  • GB7600000
UNII
Properties
C10H10Cr
Molar mass 182.186 g·mol−1
Appearance dark red crystals
Density 1.43 g/cm3
Melting point 168 to 170 °C (334 to 338 °F; 441 to 443 K)
Boiling point Sublimes (under vacuum)
Insoluble
Structure
Pseudooctahedral
see Ferrocene
0 D
Hazards
Main hazards Pyrophoric
Xi
R-phrases (outdated) R20/21/22-R36/37/38
S-phrases (outdated) S26-S37/39-S45
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
1
0
Related compounds
Related compounds
 Fe(C5H5)2
Ni(C5H5)2
bis(benzene)chromium
chromium(II) acetate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Synthesis

Ernst Otto Fischer, who shared the 1973 Nobel Prize in Chemistry for work on sandwich compounds,[2] first described the synthesis of chromocene.[3][4] One simple method of preparation involves the reaction of chromium(II) chloride with sodium cyclopentadienide:

CrCl2 + 2 NaC5H5 → Cr(C5H5)2 + 2 NaCl

Such syntheses are typically conducted in tetrahydrofuran. Decamethylchromocene, Cr[C5(CH3)5]2, can be prepared analogously from LiC5(CH3)5. Chromocene can also be prepared from chromium(III) chloride in a redox process:[5]

2 CrCl3 + 6 NaC5H5 → 2 Cr(C5H5)2 + C10H10 + 6 NaCl

Structure and bonding

The structure of chromocene has been verified by X-ray crystallography. The average Cr–C bond length is 215.1(13) pm.[6] Each molecule contains an atom of chromium bound between two planar systems of five carbon atoms known as cyclopentadienyl (Cp) rings in a sandwich arrangement, which is the reason its formula is often abbreviated as Cp2Cr. Chromocene is structurally similar to ferrocene, the prototype for the metallocene class of compounds.

With only 16 valence electrons, it does not follow the 18-electron rule.[7] It is a paramagnetic compound.

Reactions

The main reactivity associated with chromocene follow from it being highly reducing and the lability of the Cp ligands.

The complex exhibits diverse reactions, usually involving displacement of one cyclopentadienyl ring. Carbonylation has been examined in detail, leads ultimately to chromium hexacarbonyl. An intermediate is cyclopentadienylchromium tricarbonyl dimer:[8]

2 Cr(C5H5)2 + 6 CO → [Cr(C5H5)(CO)3]2 + "(C5H5)2"

Chromocene provides a convenient route for preparing the anhydrous form of chromium(II) acetate,[9] a useful precursor to other chromium(II) compounds. The reaction involves the displacement of cyclopentadienyl ligands by the formation of cyclopentadiene:

4 CH3CO2H + 2 Cr(C5H5)2 → Cr2(O2CCH3)4 + 4 C5H6

Chromocene decomposes on contact with silica gel to give the Union Carbide catalyst for ethylene polymerization, although other synthetic routes exist for the formation of this important catalyst.

Safety

Chromocene is highly reactive toward air and could ignite upon exposure to the atmosphere.

References
  1. Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). Hoboken, NJ: John Wiley and Sons. p. 2. ISBN 978-0-470-25762-3.
  2. "The Nobel Prize in Chemistry 1973". Nobel Foundation. Retrieved 3 December 2012.
  3. Fischer, E. O.; Hafner, W. (1953). "Di-cyclopentadienyl-chrom". Z. Naturforsch. B (in German). 8 (8): 444–445.
  4. Fischer, E. O.; Hafner, W. (1955). "Cyclopentadienyl-Chrom-Tricarbonyl-Wasserstoff". Z. Naturforsch. B (in German). 10 (3): 140–143. doi:10.1515/znb-1955-0303.
  5. Long, N. J. (1998). Metallocenes: Introduction to Sandwich Complexes. London: Wiley-Blackwell. ISBN 978-0632041626.
  6. Flower, K. R.; Hitchcock, P. B. (1996). "Crystal and Molecular Structure of Chromocene (η5-C5H5)2Cr". J. Organomet. Chem. 507: 275–277. doi:10.1016/0022-328X(95)05747-D.
  7. Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Wiley-VCH: Weinheim. ISBN 3-527-28165-7.
  8. Kalousová, Jaroslava; Holeček, Jaroslav; Votinský, Jiři; Beneš, Ludvík (2010). "Das Reaktionsverhalten von Chromocen". Zeitschrift für Chemie. 23 (9): 327–331. doi:10.1002/zfch.19830230903.
  9. Beneš, L.; Kalousová, J.; Votinský, J. (1985). "Reaction of chromocene with carboxylic acids and some derivatives of acetic acid". J. Organomet. Chem. 290 (2): 147–151. doi:10.1016/0022-328X(85)87428-3.
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