Bis(benzene)chromium

Bis(benzene)chromium is the chemical compound with the formula Cr(η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important historic role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex of two arene ligands.

Preparation
It was first prepared by Hafner and Fischer by the reaction of CrCl3, aluminium and benzene, in the presence of AlCl3. This so-called reductive Friedel-Crafts method was pioneered by E.O. Fischer and his students. The product of the reaction was yellow [Cr(C6H6)2]+, which was then reduced to the neutral complex. Slightly idealized equations for the synthesis are:
 * CrCl3 +  2/3 Al  +  1/3 AlCl3  +  2 C6H6  →  [Cr(C6H6)2]AlCl4 +  2/3 AlCl3
 * [Cr(C6H6)2]AlCl4 +  1/2 Na2S2O4  →  [Cr(C6H6)2]  +  NaAlCl4  +  SO2

Compounds closely related to [Cr(C6H6)2]+ had been prepared many years before Fischer's work by Franz Hein by the reaction of phenylmagnesium bromide and CrCl3. Hein's reaction affords cationic sandwich complexes containing bi- and terphenyl, which baffled chemists until the breakthrough by Fischer and Hafner, and thus Hein had unknowingly discovered sandwich complexes a half century ahead of the work on ferrocene. Fischer and Seus soon prepared Hein's [Cr(C6H5-C6H5)2]+. . Illustrating the rapid pace of this research, the same issue of Chem. Ber. also describes the Mo(0) complex.

Reactions
The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as chromium(II) acetate, which have interesting structures. Oxidation affords [Cr(C6H6)2]+.

The compound finds limited use in organic synthesis.

The substance is air sensitive and is handled using air-free techniques.