 Another comparison that can be made between (S,S)-EDDS and EDTA is the structure of the chelated complex.EDTA’s six donor sites form five five-membered chelate rings around the metal ion, four NC2OFe rings and one C2N2Fe ring.The C2N2Fe ring and two of NC2OFe rings define a plane, and two NC2OFe rings are perpendicular to the plane that contains the C2-symmetry axis.