AbstractSubstituted iminodiacetic acids of the general formula RN(CH2‐COOH)2have been synthesized and their coordination tendency with 12 different metal cations investigated. The groups R of the 13 different complexing agents are tabulated on page 1147–1148 of this paper. Most of them carry donor atoms in such a position, that a third chelate ring may be formed when the anion of the iminodiacetic acid combines with the metal. It could be proved that the oxygen, the sulfur and the nitrogen atom of aliphatically bound HO, CH3O, HS, CH3S,(−)S, NH2,(−)OOC, H2NCO are actually coordinated in this manner, raising thereby the stability of the metal complex considerably. The carbonamide group coordinates probably by its oxygen atom rather than by its nitrogen atom. The negatively charged mercaptide sulfur is a very powerful donor for the heavy metals and there is a linear relation between the logarithm of the solubility product of the metal sulfide and the logarithm of the stability constant of the metal complex of(−)SCH2CH2N(CH2COO(−))2. The trimethylammonium group in of course lowers the complex stability, its effect however is much less important than expected. Also the cyano group in NCCH2‐N(CH2‐COO(−))2weakens the metal binding much less than the addition of the pro