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Bis Benzene Chromium Synthesis Essay


Bis(benzene)chromium is an organometallic compound with the formula Cr(C6H6)2. This compound had a very important role in the development and synthesis of sandwich organometallic substances, being a prototypical complex containing two aromatic ligands. It was first prepared by Hafner and Fischer in 1955, who used CrCl3, alluminium and benzene in the presence of AlCl3 with a catalytic effect. The compund wich they obtained was the yellow [Cr(C6H6)2]+, which was later reduced with Na2S2O4.The molecular structure of it explains the term “sandwich compounds”:


The preparation of bis(benzene)chromium using standard laboratory tools and substances. The compound is air-sensitive, but we don’t have any air-free instalation, so we tried our best and obtained the substance in a small but important quantity. The methode of preparing Na2S2O4 wich is also needed in the experiment is also explained.

Necessary materials

Laboratory tools:  1 crucible, 1 clay triangle, 1 Bunsen burner, 1 Erlenmeyer flask, 1 beaker, a filtering installation.

Chemical substances: S(powder), Zn,solid NaHSO3, benzene, CrCl3 sollution, AlCl3 sollution, Al, Cd(NO3)2.

 Method of working

1.The preparation of Na2S2O4

Sodium dithionite can be prepared by heating a mixture of NaHSO3 and Zn powder in a current of SO2. Instead of blowing SO2, we tried adding sulphur in the mixture so the SO2 formed will be necessary for the reaction.  These are the equations of the reactions that occur while heating the crucible with aproximately 5mg of NaHSO3, 6mg of Zn and enough sulphur to provide the SO2:

S+O2 -> SO2

2 NaHSO3+ SO2+ Zn -> Na2S2O4+ ZnSO3+ H2O

ZnS is a secondary product: Zn+S -> ZnS

After the reaction ends and the crucible cools down, the contents are dilluted into a beaker with 200ml water. ZnSO3 immediately precipitates and the S  anions can also be precipitated with a Cd(NO3)2 sollution.  After adding cca. 100 ml of Cd(NO3)2 sollution, the mixture in the beaker is to be filtered in any kind of filtering installation. The filter paper contains yellow CdS and white ZnSO3, while the beaker contains the Na2S2O4 sollution that we needed.

2. Synthesis of bis(benzene)chromium.

Add in the Erlenmeyer flask 200 ml of C6H6 and 50 ml of water for the inorganic compounds. Then add the AlCl3 sollution in a small quantity, 100ml of CrCl3, 50 mg of Al and 50 ml of the already prepared Na2S2O4 sollution. Heat the mixture at the Bunsen burner and mix energically for the reaction to begin. A small quantity of grey bis(benzene)chromium will form in the benzene layer and then it will dissolve, causing the benzene layer to become slightly yellow and then a nuance more grey because of the reduction of the [Cr(C6H6)2]+ion. Here is how the Erlenmeyer looks after the reaction:

The equations for the reactions that occur in the flask are the following:

3 CrCl3+ 2 Al+ AlCl3+ 6 C6H6 -> 3[(C6H6)2Cr]+[AlCl4]

2[(C6H6)2Cr]+ S2O4-2+ 4 HO -> 2 Cr(C6H6)2 + 2 SO3-2 + 2 H2O

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