Chromocene

Chromocene is an organometallic compound with the formula Cr(C5H5)2. It is also often abbreviated Cp2Cr. This sandwich compound is structurally similar to ferrocene but paramagnetic and far more reactive. Also like structurally related metallocenes, chromocene sublimes readily in a vacuum and is soluble in non-polar organic solvents.

It is prepared by the following route:
 * CrCl2 + 2 NaC5H5  →  Cr(C5H5)2  +  2 NaCl

Such syntheses are typically conducted in THF solution. 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. Thus, chromocene in combination with silica gel gives the Phillips 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. A large percentage of the industrial production of polyethylene is attributable to such chromium-based catalysts.

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.

General reading
C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7

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