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“Chelation is the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom.[1] Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or sequestering agents.

The ligand forms a chelate complex with the substrate. Chelate complexes are contrasted with coordination complexes composed of monodentate ligands, which form only one bond with the central atom.

Chelants, according to ASTM-A-380, are "chemicals that form soluble, complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale."

The word chelation is derived from Greek χηλή, chelè, meaning claw; the ligands lie around the central atom like the claws of a lobster.

The chelate effect describes the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal.

Consider the two equilibria, in aqueous solution, between the copper(II) ion, Cu2+ and ethylenediamine (en) on the one hand and methylamine, MeNH2 on the other.

 Cu2+ + en [Cu(en)]2+ (1) Cu2+ + 2 MeNH2 [Cu(MeNH2)2]2+ (2)

In (1) the bidentate ligand ethylene diamine forms a chelate complex with the copper ion. Chelation results in the formation of a five–membered ring. In (2) the bidentate ligand is replaced by two monodentate methylamine ligands of approximately the same donor power, meaning that the enthalpy of formation of Cu—N bonds is approximately the same in the two reactions. Under conditions of equal copper concentrations and when the concentration of methylamine is twice the concentration of ethylenediamine, the concentration of the complex (1) will be greater than the concentration of the complex (2). The effect increases with the number of chelate rings so the concentration of the EDTA complex, which has six chelate rings, is much much higher than a corresponding complex with two monodentate nitrogen donor ligands and four monodentate carboxylate ligands. Thus, the phenomenon of the chelate effect is a firmly established empirical fact.

The thermodynamic approach to explaining the chelate effect considers the equilibrium constant for the reaction: the larger the equilibrium constant, the higher the concentration of the complex.

 [Cu(en)] =β11[Cu][en] [Cu(MeNH2)2]= β12[Cu][MeNH2]2

(Wikipedia, Chelates, 6/28/2012


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(RDC 6/5/2012)


Roger D. Corneliussen

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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 6/28/2012.