Chromocene

Chromocene
IUPAC name chromocene, bis(η5-cyclopentadienyl)chromium
Other names CrCp2
Identifiers
CAS number	1271-24-5
ChemSpider	71485
ChEBI	CHEBI:30677
RTECS number	GB7600000
Jmol-3D images	Image 1
SMILES [Cr+2].[c-]1cccc1.c1[c-]ccc1
InChI InChI=1/2C5H5.Cr/c2*1-2-4-5-3-1;/h2*1-5H;/q2*-1;+2 Key: TYYBBNOTQFVVKN-UHFFFAOYAS
Properties
Molecular formula	C10H10Cr
Molar mass	182.18 g/mol
Appearance	dark red crystals
Density	1.43 g/cm3, solid
Melting point	168-170 °C
Boiling point	sublimes (under vacuum)
Solubility in water	decomposes in water
Structure
Coordination geometry	pseudooctahedral see Ferrocene
Dipole moment	0 D
Hazards
EU classification	Xi
R-phrases	R20/21/22, R36/37/38
S-phrases	S26, S37/39, S45
Main hazards	pyrophoric
NFPA 704	0 1 0
Related compounds
Related compounds	Fe(C5H5)2 Ni(C5H5)2 Chromium(II) acetate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Chromocene is an organochromium compound with the formula Cr(C₅H₅)₂, often abbreviated Cp₂Cr. This sandwich compound is structurally similar to ferrocene but does not follow the 18-electron rule because it only has 16 electrons.^[1] It also paramagnetic and highly reducing. Also like structurally related metallocenes, chromocene sublimes readily in a vacuum and is soluble in non-polar organic solvents.

It is prepared from chromium(II) chloride and cyclopentadienylsodium:

CrCl₂ + 2 NaC₅H₅ → Cr(C₅H₅)₂ + 2 NaCl

Such syntheses are typically conducted in THF solution. The structure of the compound has been verified by X-ray crystallography; the average Cr-C bond length is 215.1(13) pm.^[2]

The strongly reducing analogue decamethylchromocene, Cr[C₅(CH₃)₅]₂, is prepared analogously from LiC₅(CH₃)₅.

Like some other metallocenes, the C₅H₅ ligands are displaceable in chromacene. Thus, chromocene in combination with silica gel gives the Union Carbide catalyst for the polymerization of ethylene, although other preparations exist for this important catalyst. The chromocene decomposes on the silica surface to generate high reactive organometallic centers that are responsible for the catalysis.

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 inflame upon exposure to the atmosphere.

References

1. ^ C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
2. ^ Kevin R. Flower, Peter B. Hitchcock "Crystal and molecular structure of chromocene (η⁵-C₅H₅)₂Cr" Journal of Organometallic Chemistry, 1996, volume 507, p. 275-277. doi:10.1016/0022-328X(95)05747-D.