Iron chelates: a challenge to chemists and Mössbauer spectroscopists

The speciation of iron in aqueous solutions containing Fe3 + and selected chelates such as EDTA, EDDA, CDTA and HEDTA has been studied using transmission 57Fe Mössbauer spectrometry in frozen solutions. The protonation of various complexes as well as binuclear complex formation could be detected as a function of pH. Autoreduction of Fe3 +  to Fe2 +  was observed in several cases. Reaction with hydrogen peroxide proved to be rather different for the four ligands, while the dihapto complex [XFe(η2-O2)]3 −  had surprisingly identical Mössbauer parameters for X = EDTA, CDTA or HEDTA. Paramagnetic spin relaxation observed in the Mössbauer spectra was found to be strongly influenced by the identity of the chelating ligand, despite the basically spin-spin origin of the phenomenon.