**1. Introduction**

In this chapter, *ligands* have a more focused definition than they typically do. Ligands are molecules having donor atoms with a pair of electrons that form coordinate covalent bonds with metal ions. Unidentate ligands have a single donor atom, while chelating agents have two or more donor atoms, and in attaching to the metal ions form rings that are associated with enhanced stability. Further, two kinds of chelating agents are involved: *molecular* and *supported.*

*Molecular chelating agents*, such as ethylenediaminetetraacetic acid, (HOOCCH2)2NCH2 - CH2N(CH2COOH)2 EDTA react with metal ions, such as magnesium, calcium, or transition metal ions to form soluble molecular entities in aqueous solutions. These chelating agents are said to "sequester" the metal ions, much as a prisoner in jail is sequestered or removed from society. The metal ion is sequestered in the sense that the act of forming the compound changes the properties of the metal ion. For example, EDTA may be added in small quantities to beer to prevent a haze in the solution due to formation of insoluble, dispersed calcium carbonate. Two molecules of iminodiacetic acid, HN( CH2N(CH2COOH)2, H2L may react to form soluble complexes of the typeMg(L)2 <sup>=</sup> . Other ligand-complex entities may be insoluble, but the coordination compounds formed, soluble or insoluble, are molecular entities.
