The parent of this family of ligands is the amino acid glycine, HNCHCOOH, in which the amino group, NH, is separated from the carboxyl group, COOH by a single methylene group, CH. When the carboxyl group is deprotonated the glycinate ion can function as a bidentate ligand, binding the metal centre through the nitrogen and one of two carboxylate oxygen atoms, to form chelate complexes of metal ions.
Replacement of a hydrogen atom on the nitrogen of glycine by another acetate residue, –CHCOOH gives iminodiacetic acid, IDA, which is a tridentate ligand. Further substitution gives nitrilotriacetic acid, NTA, which is a tetradentate ligand. These compounds can be described as aminopolycarboxylates. Related ligands can be derived from other amino acids other than glycine, notably aspartic acid.
Higher density is achieved by linking two or more glycinate or IDA units together. EDTA contains two IDA units with the nitrogen atoms linked by two methylene groups and is hexadentate. DTPA has two CHCH bridges linking three nitrogen atoms and is octadentate. TTHA has ten potential donor atoms.