In Chapter 2, the four basic chemical criteria absolutely essential for chelate formation were discussed.1 These are, ‘rst, the bidentate ligand must possess at least two functional groups, each of which is capable of donating electrons to the metal ion. When this occurs, the metal ion acts as a Lewis acid at the time it accepts electrons from the ligand, and through the process of donating electrons, the ligand acts as a Lewis base. The resulting bond between the metal and ligand is a covalent bond with an ionic character. The second bond between the ligand and metal can be covalent if the metal ion shares an electron with the ligand and the ligand shares its electron with the cation.2