In general, since a chelating agent may bond to a metal ion in more than one
place simultaneously (Figure 3.10), chelates are more stable than complexes involving unidentate ligands. Stability tends to increase with the number of chelating sites
available on the ligand. Structures of metal chelates take a number of different forms,
all characterized by rings in various configurations. The structure of a tetrahedrally
coordinated chelate of nitrilotriacetate ion is shown in Figure 3.10.
The ligands found in natural waters and wastewaters contain a variety of
functional groups which can donate the electrons required to bond the ligand to ametal ion. Among the most common of these groups are