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Chromocene

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Title: Chromocene  
Author: World Heritage Encyclopedia
Language: English
Subject: Metallocenes, Ferrocene, Bis(benzene)chromium, Potassium tetraperoxochromate(V), Chromium(III) iodide
Collection: Cyclopentadienyl Complexes, Metallocenes, Organochromium Compounds
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Chromocene

Chromocene
Chromocene
Chromocene
Names
IUPAC name
Bis(η5-cyclopentadienyl)chromium(II)
Other names
Dicyclopentadienylchromium(II)
Identifiers
ChEBI
ChemSpider
Jmol-3D images Image
RTECS number GB7600000
Properties
C10H10Cr
Molar mass 182.19 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
Irritant Xi
R-phrases R20/21/22-R36/37/38
S-phrases S26-S37/39-S45
NFPA 704
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).

Chromocene, formally known as bis(η5-cyclopentadienyl)chromium(II), is a haptic) covalent chromium–carbon bonds.[1] Chromocene is structurally similar to ferrocene, the prototype for the metallocene class of compounds; however, as it has only 16 valence electrons, it does not follow the 18-electron rule.[2] It is a paramagnetic compound and also highly reducing, both consequences of its low valence electron count. Like structurally related metallocenes, chromocene readily sublimes in a vacuum and is soluble in non-polar organic solvents.

Ernst Otto Fischer, who shared the 1973 Nobel Prize in Chemistry for his work on sandwich compounds,[3] was the first to report a synthesis for chromocene.[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 THF; 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

The chromium(0) organometallic complex chromium hexacarbonyl can be oxidised by cyclopentadiene in the presence of diethylamine to produce chromocene, the removed protons being reduced to produce hydrogen gas.[6]

Cr(CO)6 + 2 C5H6 → Cr(C5H5)2 + 6 CO + H2

The structure of chromocene has been verified by X-ray crystallography; the average Cr–C bond length is 215.1(13) pm.[7]

Like some other metallocenes, the cyclopentadienyl ligands of chromocene are displaceable. When combined with catalysis.

Chromocene provides a convenient route for preparing the anhydrous form of chromium(II) acetate,[8] a useful precursor to other chromium(II) compounds. The reaction involves the displacement of cyclopentadienyl ligands by the formation of cyclopentadiene:

4 CH3COOH + 2 Cr(C5H5)2 → Cr2(O2CCH3)4 + 4 C5H6

Safety

Chromium compounds are toxic, although Cr(VI) species are usually considered more dangerous than reduced chromium compounds. Chromocene is highly reactive toward air and could ignite upon exposure to the atmosphere.

References

  1. ^  
  2. ^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.).  
  3. ^ "The Nobel Prize in Chemistry 1973".  
  4. ^  
  5. ^ Long, N. J. (1998). Metallocenes: Introduction to Sandwich Complexes. London:  
  6. ^  
  7. ^ Flower, K. R.; Hitchcock, P. B. (1996). "Crystal and molecular structure of chromocene (η5-C5H5)2Cr".  
  8. ^ Beneš, L.; Kalousová, J.; Votinský, J. (1985). "Reaction of chromocene with carboxylic acids and some derivatives of acetic acid".  
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