Chelates are complexes in which an organic molecule forms co-ordinate bonds with certain metal ions. Electron pairs, normally from nitrogen and oxygen atoms, bond with empty orbitals of the metal cation. Nitrogen and oxygen atoms are particularly effective as a source of lone pair electrons and feature in many chelating agents. Nature has many examples of chelates, one of the more familiar ones is the green compound in which a magnesium cation is chelated to form chlorophyll as shown in Figure 1.2.

Figure 1.2: Chelated metal ions: (a) chlorophyll, one of nature's chelates, (b) EDTA, a widely used synthetic chelating agent, is effective in sequestering calcium ions

In chemical formulation work it is the synthetic chelating agents that are important although some natural based ones are used and now in increasing amounts. Of the man-made ones it is the aminocarboxylic acids that predominate.

In a solution of EDTA (ethylenediaminetetracetate) ion, a common synthetic chelating agent, six co-ordinate bonds are formed to a calcium ion. Here the lone pairs are from the two nitrogen atoms and four of the oxygen atoms. A stable hexadentate (six chelating bonds) complex is formed and the metal ion is thus firmly grasped, pincer style (Greek: chela = crabs claw). As such the metal ion is no longer free to take part in other reactions; it has been sequestered.

Many other metals can be chelated by EDTA but not always in a hexadentate arrangement. Usually one mole of EDTA will complex with...