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. The five-membered rings are slightly strained. EDDS’s six donor sites form both five- and six-membered chelate rings around the metal ion: two NC2OFe rings, two NC3OFe rings, and one C2N2Fe ring. Studies of the crystal structure of the Fe[(S,S)-EDDS]− complex show that the two five-membered NC3OFe rings project out of the plane of the complex, reducing the equatorial ring strain that exists in the Fe− complex. The complex also has C2 symmetry.