Chelation (from Greek χηλή, chelè, meaning claw) is the process of reversible binding of a ligand, the chelant, chelator or chelating agent (also known as sequestering agents), to a metal ion, forming a metal complex, the chelate.

History

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The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew in J. Chem. Soc., 1920, 117, 1456, 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."

General

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A chelator does not have to be organic. In contrast to the simple monodentate ligands like H₂O or NH₃, which are easily broken apart by other chemical processes, the polydentate chelators form multiple bonds with the metal ion, resulting in more stable complexes.

Metal complexes are of widespread interest and studied by inorganic chemists, physical and organic chemists, biochemists, pharmacologists, molecular biologists, and environmentalists.

Antibiotic drugs of the tetracycline family are also chelators of Ca²⁺ and Mg²⁺ ions and are incorporated into bones and teeth.

In ecology chelation compounds are related to the mobilization of metals in the soil, the uptake and the accumulation of metals into plants and micro-organisms and as a mechanism for resistance and hyperaccumulation adaptations. Such chelation of heavy metals can be used in bioremediation.Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001

Uses

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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.

In nutrition, certain amino acids are utilized as chelating agents to replicate the natural mineral forms found in raw fruits, vegetables, and grains. The resultant chelated minerals are absorbed directly into the bloodstream through the intestinal villi in relatively high proportions. Because micromineral levels in farm soils tend to gradually decline over the years, and because modern food processing often breaks the chelate bond and renders any remaining minerals much less bioavailable, humans today receive reduced quantities of minerals from their meals. Dozens of medical studies have shown that dietary supplementation with both vitamins and chelated minerals can have wide-ranging benefits to health, mental acuity, and life expectancy. Chelated minerals are sometimes prescribed by doctors to treat such ailments as anemia, arthritis, diabetes, nervous disorders, and heart attacks.

See also

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• Chelate effect
• Bioremediation
• Chelation therapy
• Electron counting
• Organometallic chemistry

References cited

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Coordination chemistry | Chelating agents

Chelat | Chelatligand | Ligando quelato | Chélation | Kel-ionuro | Chelatas | Chelatie | キレート | Хелаты | 螯合物