Chromocene
Encyclopedia
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. It also paramagnetic and highly reducing. Also like structurally related metallocene
s, chromocene sublimes readily in a vacuum and is soluble in non-polar organic solvents.
It is prepared from chromium(II) chloride
and cyclopentadienylsodium:
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.
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.
species are usually considered more dangerous than reduced chromium compounds. Chromocene is highly reactive toward air and could inflame upon exposure to the atmosphere.
Sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives...
is structurally similar to ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
but does not follow the 18-electron rule
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
because it only has 16 electrons. It also paramagnetic and highly reducing. Also like structurally related metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s, chromocene sublimes readily in a vacuum and is soluble in non-polar organic solvents.
It is prepared from chromium(II) chloride
Chromium(II) chloride
Chromium chloride is the chemical compound with the formula CrCl2. This white, crystalline solid is used for the synthesis of other chromium complexes. CrCl2 is hygroscopic. It dissolves in water to give bright blue solutions that are easily oxidized by air to give Cr-containing products...
and cyclopentadienylsodium:
- CrCl2 + 2 NaC5H5 → Cr(C5H5)2 + 2 NaCl
Such syntheses are typically conducted in THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
solution. The structure of the compound has been verified by X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
; the average Cr-C bond length is 215.1(13) pm.
The strongly reducing analogue decamethylchromocene, Cr[C5(CH3)5]2, is prepared analogously from LiC5(CH3)5
Pentamethylcyclopentadiene
1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H . 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp*...
.
Like some other metallocenes, the C5H5 ligands are displaceable in chromacene. Thus, chromocene in combination with silica gel
Silica gel
Silica gel is a granular, vitreous, porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid. It is a naturally occurring mineral that is purified and processed into either granular or beaded form...
gives the Union Carbide catalyst for the polymerization of ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, 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)Chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...
species are usually considered more dangerous than reduced chromium compounds. Chromocene is highly reactive toward air and could inflame upon exposure to the atmosphere.