Kinetic and structural investigations of [Fe III (edta)]–[edta = ethylenediaminetetra-acetate(4–)] catalysed decomposition of hydrogen peroxide

A detailed kinetic analysis is given of hydrogen peroxide decomposition catalysed by [FeIII(edta)]–[edta = ethylenediaminetetra-acetate(4–)]. Structural investigations have been made using n.m.r. And electronic absorption spectroscopy. It is demonstrated that the monohydroxy complex, [Fe(edta)(OH)]2–, is the primary active catalyst and this reacts with the hydrogenperoxide ion to produce the well known purple complex [Fe(edta)(OH)(HO2)]3–. Decomposition of hydrogen peroxide conforms to Michaelis–Menten kinetics, the rate-determining step involving breakdown of this complex. In contrast to earlier reports, it is shown that the HO2– ion displaces a carboxy-group from [Fe(edta)(OH)]2– rather than the hydroxy group. The dihydroxy complex [Fe(edta)(OH)2]3– is also shown to form a purple peroxy-complex with HO2–, but its breakdown occurs at a much slower rate. The results are consistent with formation of radicals upon breakdown of the peroxy-complex and in subsequent reactions.