Chelation

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Chelation (from Greek χηλή, chelè, meaning claw) is the process of reversible binding (complexation) of a ligand - the chelant, chelator, chelating agent, sequestering agent, or complexing agent - to a metal ion, forming a metal complex, the chelate. The term is generally reserved for complexes in which the metal ion is bound to two or more atoms of the chelating agent, although the bonds may be any combination of coordination or ionic bonds.

Image:Metal-EDTA.png

Metal-EDTA chelate

1 History
2 General
3 Examples
4 Uses
5 See also
6 References cited

[edit] History

The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew, who stated: "The adjective chelate, derived from the great claw or chele (Greek) of the lobster or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings."<ref>J. Chem. Soc., 1920, 117, 1456</ref>

[edit] General

Relative to the aqua complexes, e.g. [M(H₂O)₆]²⁺, the increased stability of a chelated complex is called the chelate effect. A chelating agent is a polydentate ligand that bonds to more than one coordination site on an metal atom. Because it is necessary to break all of the bonds to the central atom for the ligand to be displaced, it requires more energy to increase the number of separate molecules. If a chelate were replaced by several monodentate ligands (such as water or ammonia), the total number of molecules would decrease, whereas if several monodentate ligands were replaced by a chelate, the number of free molecules increases. The effect is therefore entropic in that more sites are used by less ligands and this leaves more unbonded molecules: a total increase in the number of molecules in solution and a corresponding increase in entropy.

[edit] Examples

Most metal complexes in the environment and in nature are bound in some form of chelate ring, e.g. with "humic acid" or a protein. Thus, metal chelates are relevant to the mobilization of metals in the soil, the uptake and the accumulation of metals into plants and micro-organisms. Selective chelation of heavy metals is relevant to bioremediation, e.g. removal of ¹³⁷Cs from radioactive waste.<ref>Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001</ref>

Antibiotic drugs of the tetracycline family are chelators of Ca²⁺ and Mg²⁺ ions.

[edit] Uses

Chelators are used in chemical analysis, as water softeners, as preservatives, and in medicine (chelation therapy), where they are employed to safely bind with poisonous metal agents such as mercury, arsenic, or lead to stabilize them and allow them to be excreted without further interaction with the body. Natural chelators include the porphyrin rings in hemoglobin or chlorophyll and the Fe³⁺ chelating siderophores secreted by microorganisms, and are contained in herbs such as cilantro, which has long been used as a treatment for heavy metal poisoning. A commonly used synthetic chelator is EDTA.

The term is used in water treatment programs and specifically in steam engineering, to describe a boiler water treatment system: Chelant Water Treatment system.