Bis(benzene)chromium

Bis(benzene)chromium is the organometallic compound with the formula Cr(η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containing two arene ligands. In the late 1910s, Franz Hein started the investigation of "triphenylchromium" by reacting chromium trichloride with a Grignard reagent, phenyl magnesium bromide. Such a reaction gave a mixture of phenyl chromium and Hein suggested that it contained a Cr(VI) species, "(C6H5)5CrBr", generated via valence disproportionation. 5C6H5MgBr + 4CrCl3 ⟶ (C6H5)5CrBr + 2MgBr2 + 3MgCl2 + 3CrCl2 This event marked an advance in organochromium chemistry at the time and "(C6H5)5CrBr" was described to have salt-like properties. However, the reported workup procedures for "(C6H5)5CrBr" was challenging and the yield was low. Later scrutinization by Zeiss and Tsutsui revealed that Hein's formulation of the chromium-containing products was flawed. The actual discovery of bis(benzene)chromium was largely contributed by Ernst Otto Fischer and Walter Hafner in the 1950s. Ernst Otto Fischer postulated that it might be possible to synthesize a neutral chromium(0) complex with two benzene ligands, which has a sandwich structure, similar to that of ferrocene. In 1954, Walter Hafner, a PhD student of Ernst Ottot Fischer at the time, put the idea into practice. A reaction of chromium trichloride, aluminium trichloride, aluminium powder in m-xylene resulted in the formation of yellow [Cr(C6H6)2]+, which was then reduced by sodium dithionite in aqueous sodium hydroxide. The resulting solid was determined to be the target, bis(benzene)chromium. 6C6H6 + 3CrCl3 + 2Al + xAlCl3 ⟶ 3[(C6H6)2Cr][AlCl4].(x-1)AlCl3 2[(C6H6)2Cr]+ + S2O42− + 4OH− ⟶ 2(C6H6)2Cr + 2SO32− + 2H2O It was noted that excess aluminium trichloride is needed to solubilize the product. The substance is air sensitive and its synthesis requires air-free techniques.