Iron chelates: a challenge to chemists and Mössbauer spectroscopists

The speciation of iron in aqueous solutions containing Fe3+ and selectedchelates such as EDTA, EDDA, CDTA and HEDTA has been studied using transmission57Fe Mössbauer spectrometry in frozen solutions. The protonation of variouscomplexes as well as binuclear complex formation could be detected as a function ofpH. Autoreduction of Fe3+ to Fe2+ was observed in several cases. Reaction withhydrogen peroxide proved to be rather different for the four ligands, while thedihapto complex [XFe(?2-O2)]3- had surprisingly identical Mössbauer parametersfor X = EDTA, CDTA or HEDTA. Paramagnetic spin relaxation observed in the Mössbauer spectra was found to be strongly influenced by the identity of thechelating ligand, despite the basically spin-spin origin of the phenomenon