Question

In the laboratory preparation of chromium complexes such as [Cr(en)₃](ClO₄)₃, the typical starting material is CrCl₃·6H2O. When combined with excess en (ethylenediamine) in an appropriate solvent, the reaction proceeds unacceptably slowly, however the addition of a small, catalytic piece of Zn rapidly accelerates the reaction, such that it is complete within an hour. Discuss this reaction in terms of the role of the Zn as a catalyst and the relative labilities of the oxidation state(s) of chromium involved.

Answer 1

The procedure for synthesis of involve the addition of a single piece of granulated zinc to green solution of .No product is obtained in the absence of zinc. Other reducing metals such as magnesium ribbon or granulated tin may be used instead. But granulated tin reaction is much slower than with zinc or magnesium.

The zinc catalyst functions by generating liable Cr(II) species. Zn interacts with two equivalent of Cr ³⁺ ions to generate Cr ²⁺ and en ligand intera with this and it gets oxidised to Cr³⁺