Chromocene

Chromocene

Chromocene
Names
IUPAC name Bis(η5-cyclopentadienyl)chromium(II)
Other names Dicyclopentadienylchromium(II)
Identifiers
CAS Number	1271-24-5
ChEBI	CHEBI:30677
ChemSpider	71485
Jmol 3D model	Interactive image
RTECS number	GB7600000
InChI InChI=1/2C5H5.Cr/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2 Key: TYYBBNOTQFVVKN-UHFFFAOYAS
SMILES [Cr+2].[c-]1cccc1.c1[c-]ccc1
Properties
Chemical formula	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)
Solubility in water	Insoluble
Structure
Coordination geometry	Pseudooctahedral see Ferrocene
Dipole moment	0 D
Vapor pressure	{{{value}}}
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

Chromocene, formally known as bis(η⁵-cyclopentadienyl)chromium(II), is a chemical compound with the condensed structural formula [Cr(C₅H₅)₂]. 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. It is an organometallic compound as it has (haptic) covalent chromium–carbon bonds. 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. 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, was the first to report a synthesis for chromocene. 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:

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.

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.

Like some other metallocenes, the cyclopentadienyl ligands of chromocene are displaceable. When combined with silica gel, chromocene gives the Union Carbide catalyst for ethylene polymerization, although other synthetic routes exist for the formation of this important catalyst. The chromocene decomposes on the silica surface to generate high reactive organometallic centers that are responsible for the catalysis.

Chromocene provides a convenient route for preparing the anhydrous form of chromium(II) acetate, 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.