Chelation

Chelation is a type of bonding of

The chelate effect is the greater affinity of chelating ligands for a metal ion than that of similar nonchelating (monodentate) ligands for the same metal.

The thermodynamic principles underpinning the chelate effect are illustrated by the contrasting affinities of copper(II) for ethylenediamine (en) vs. methylamine.

Cu2+ + en ⇌ [Cu(en)]2+

(1)

Cu2+ + 2 MeNH2 ⇌ [Cu(MeNH2)2]2+

(2)

In (1) the ethylenediamine forms a chelate complex with the copper ion. Chelation results in the formation of a five-membered CuC₂N₂ ring. In (2) the bidentate ligand is replaced by two monodentate methylamine ligands of approximately the same donor power, indicating that the Cu–N bonds are approximately the same in the two reactions.

The thermodynamic approach to describing 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]

(3)

[Cu(MeNH2)2] = β12[Cu][MeNH2]2

(4)

Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the

An equilibrium constant, K, is related to the standard

${\displaystyle \Delta G^{\ominus }}$

${\displaystyle \Delta G^{\ominus }=-RT\ln K=\Delta H^{\ominus }-T\Delta S^{\ominus }}$

where R is the gas constant and T is the temperature in kelvins. ${\displaystyle \Delta H^{\ominus }}$ is the standard enthalpy change of the reaction and ${\displaystyle \Delta S^{\ominus }}$ is the standard entropy change.

Since the enthalpy should be approximately the same for the two reactions, the difference between the two stability constants is due to the effects of entropy. In equation (1) there are two particles on the left and one on the right, whereas in equation (2) there are three particles on the left and one on the right. This difference means that less entropy of disorder is lost when the chelate complex is formed with bidentate ligand than when the complex with monodentate ligands is formed. This is one of the factors contributing to the entropy difference. Other factors include solvation changes and ring formation. Some experimental data to illustrate the effect are shown in the following table.^[4]

Equilibrium	log β	${\displaystyle \Delta G^{\ominus }}$	${\displaystyle \Delta H^{\ominus }\mathrm {/kJ\ mol^{-1}} }$	${\displaystyle -T\Delta S^{\ominus }\mathrm {/kJ\ mol^{-1}} }$
Cu2+ + 2 MeNH2 ⇌ Cu(MeNH2)22+	6.55	−37.4	−57.3	19.9
Cu2+ + en ⇌ Cu(en)2+	10.62	−60.67	−56.48	−4.19

These data confirm that the enthalpy changes are approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term, which is much less unfavorable. In general it is difficult to account precisely for thermodynamic values in terms of changes in solution at the molecular level, but it is clear that the chelate effect is predominantly an effect of entropy.

Other explanations, including that of Schwarzenbach,^[5] are discussed in Greenwood and Earnshaw (loc.cit).

In nature

Numerous

Dentin adhesives were first designed and produced in the 1950s based on a co-monomer chelate with calcium on the surface of the tooth and generated very weak water-resistant chemical bonding (2–3 MPa).^[17]

Chelation therapy

Dechelation (or de-chelation) is a reverse process of the chelation in which the chelating agent is recovered by acidifying solution with a mineral acid to form a precipitate.^[35]: 7

See also

• EDDS – chemical compoundPages displaying wikidata descriptions as a fallback

References

 This article incorporates text by Kaana Asemave available under the CC BY 4.0 license.