Variable co-ordination of iron(III) complexes in co-ordinating solvents

The spectroscopic properties of the complexes [FeIII(edtaH)(H2O)], [FeIII(edta)(H2O)]– and [FeIII(hedta)(H2O)2](edta = ethylenediamine tetra-acetate and hedta = hydroxyethylethylenediamine triacetate) have been examined in various solvents and in the solid state. By correlating the known structures of the EDTA complexes with their characteristically different absorption spectra in the solid state, it is possible to indicate the co-ordination in various solvents. All complexes have been found to be seven-co-ordinate in water, methanol, formamide, and glycerol but six-co-ordinate in dimethylformamide (DMF) and dimethyl sulphoxide (DMSO). The results suggest that in solution the complexes contain pentadentate edta or hedta and either two or one co-ordinated solvent molecules the number of which is determined by the steric requirements of the solvent molecule. The Mossbauer parameters of the complexes are discussed.