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[[Image:Metal-EDTA.png|thumb|right|Metal-[[EDTA]] '''chelate''']]

'''Chelation''' ({{pronEng|kiːˈleɪʃən}}) is the formation or presence of two or more separate bindings between a
[[Ligand#Polydentate_and_polyhapto_ligand_motifs_and_nomenclature|polydentate (multiple bonded)]] [[ligand]] and a single central atom.<ref name="IUPAC"> [http://goldbook.iupac.org/C01012.html IUPAC definition of chelation.]</ref>
Usually these [[ligand]]s are [[organic compound]]s, 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 complex]]es with [[Denticity|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 language|Greek]] χηλή, ''chelè'', meaning claw; the ligands lie around the central atom like the claws of a [[lobster]].<ref>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 language|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."<br />{{cite journal | journal = [[J. Chem. Soc., Trans.]] | year = 1920 | volume = 117 | pages = 1456 | doi = 10.1039/CT9201701456 | title = CLXII.—Researches on residual affinity and co-ordination. Part II. Acetylacetones of selenium and tellurium | author = Morgan, Gilbert T.; Drew, Harry D. K.}} (nonfree access)</ref>

== The chelate effect==
[[File:M-en1.png|thumb|[[Ethylenediamine]] [[ligand]], binding to a central metal ion with two bonds]]
[[Image:Cu chelate.png|thumb|Cu<sup>2+</sup> [[Coordination complex|complexes]] with [[methylamine]] (left) ''and'' [[ethylenediamine]] (right)]]
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, Cu<sup>2+</sup> and [[ethylenediamine]] (en) on the one hand and [[methylamine]], MeNH<sub>2</sub> on the other.
:Cu<sup>2+</sup> + en {{eqm}} [Cu(en)]<sup>2+</sup> (1)
:Cu<sup>2+</sup> + 2 MeNH<sub>2</sub> {{eqm}} [Cu(MeNH<sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (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 [[Denticity|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 [[equilibrium thermodynamics|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)] =β<sub>11</sub>[Cu][en]
:[Cu(MeNH<sub>2</sub>)<sub>2</sub>]= β<sub>12</sub>[Cu][MeNH<sub>2</sub>]<sup>2</sup>
Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the [[stability constant]]s, β, indicate the [[stoichiometry]] of the complex. When the [[analytical concentration]] of methylamine is twice that of ethylenediamine and the concentration of copper is the same in both reactions, the concentration [Cu(en)] is much higher than the concentration [Cu(MeNH<sub>2</sub>)<sub>2</sub>] because β<sub>11</sub> >> β<sub>12</sub>.

An equilibrium constant, ''K'', is related to the standard [[Gibbs free energy]], Δ''G''<sup>[[Image:StrikeO.png]]</sup> by
:Δ[[G]]<sup>[[Image:StrikeO.png]]</sup> = −RT ln ''K'' = Δ''H''<sup>[[Image:StrikeO.png]]</sup> − TΔ''S''<sup>[[Image:StrikeO.png]]</sup>
where ''R'' is the [[gas constant]] and ''T'' is the temperature in [[kelvin]]s. Δ''H''<sup>[[Image:StrikeO.png]]</sup> is the standard [[enthalpy]] change of the reaction and Δ''S''<sup>[[Image:StrikeO.png]]</sup> is the standard [[Entropy (statistical thermodynamics)|entropy]] change. It has already been posited that the enthalpy term should be approximately the same for the two reactions. Therefore the difference between the two stability constants is due to the entropy term. 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 means that less [[Entropy (order and disorder)|entropy of disorder]] is lost when the chelate complex is formed 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.<ref name=GE>{{Greenwood&Earnshaw}} p 910</ref>
:{| class="wikitable" 
! Equilibrium !! log β !! ΔG<sup>[[Image:StrikeO.png]]</sup>!! Δ''H''<sup>[[Image:StrikeO.png]]</sup> /kJ mol<sup>−1</sup>!!  −''T''Δ''S''<sup>[[Image:StrikeO.png]]</sup> /kJ mol<sup>−1</sup>
|-
| Cd<sup>2+</sup> + 4 MeNH<sub>2</sub> {{eqm}} Cd(MeNH<sub>2</sub>)<sub>4</sub><sup>2+</sup>
||6.55|| -37.4 || -57.3||19.9 
|-
| Cd<sup>2+</sup> + 2 en {{eqm}} Cd(en)<sub>2</sub><sup>2+</sup>
||10.62|| -60.67 || -56.48||-4.19
|}
These data show that the standard enthalpy changes are indeed approximately equal for the two reactions and that the main reason why the chelate complex is so much more stable is that the standard entropy term is much less unfavourable, indeed, it is favourable in this instance. 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,<ref>{{cite journal |last=Schwarzenbach |first=G |date=1952 |title=Der Chelateffekt |journal=Helv. Chim. Acta|volume=35|pages=2344–2359|doi=10.1002/hlca.19520350721}}</ref> are discussed in Greenwood and Earnshaw (''loc.cit'').

==In nature==
Virtually all biochemicals exhibit the ability to dissolve certain metal [[cation]]s. Thus, proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions. In addition to these adventitious chelators, several biomolecules are produced to specifically bind certain metals (see next section). [[Histidine]], [[malate]] and [[phytochelatin]] are typical chelators used by plants.<ref>{{cite journal | author=U Krämer, J D Cotter-Howells, J M Charnock, A H J M Baker, J A C Smith | title=Free histidine as a metal chelator in plants that accumulate nickel| journal=Nature| year=1996| volume=379| doi= 10.1038/379635a0| pages=635–638}}</ref><ref>{{cite journal | author=Jurandir Vieira Magalhaes| title=Aluminum tolerance genes are conserved between monocots and dicots| journal=Proc Natl Acad Sci USA| year=2006| volume=103| issue=26| page=9749–9750| url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1502523 | pages=9749 | doi=10.1073/pnas.0603957103| pmid=16785425 | pmc=1502523}}</ref><ref>{{cite journal | author=Suk-Bong Ha, Aaron P. Smith, Ross Howden, Wendy M. Dietrich, Sarah Bugg, Matthew J. O'Connell, Peter B. Goldsbrough, and Christopher S. Cobbett |title=Phytochelatin synthase genes from arabidopsis and the yeast Schizosaccharomyces pombe | journal=Plant Cell| year=1999| volume=11| url=http://www.plantcell.org/cgi/content/full/11/6/1153?ck=nck| pages=1153–1164 |doi=10.1105/tpc.11.6.1153 |pmid=10368185 | issue=6 | pmc=144235}}</ref>

===In biochemistry and microbiology===
Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.<ref>S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994.  ISBN 0-935702-73-3.</ref>  Such chelating agents include the [[porphyrin]] rings in [[hemoglobin]] and [[chlorophyll]].  Many microbial species produce water-soluble pigments that serve as chelating agents, termed [[siderophores]]. For example,  species of ''[[Pseudomonas]]'' are known to secrete [[pycocyanin]] and [[pyoverdin]] that bind iron.  [[Enterobactin]], produced by [[E. coli]], is the strongest chelating agent known.

===In geology===
In earth science, chemical [[weathering]] is attributed to organic chelating agents, ''e.g.'' [[peptide]]s and [[sugar]]s, that extract metal ions from minerals and rocks.<ref>Dr. Michael Pidwirny, University of British Columbia Okanagan, http://www.physicalgeography.net/fundamentals/10r.html</ref>  Most metal complexes in the environment and in nature are bound in some form of chelate ring, ''e.g.'' with a [[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 <sup>137</sup>Cs from radioactive waste.<ref>Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001</ref>

==Applications==
Chelators are used in [[chemical analysis]], as [[water softener]]s, and are ingredients in many commercial products such as [[shampoos]] and food [[preservative]]s. [[Citric acid]] is used to [[Citric_acid#Water_softening|soften water]] in [[soap]]s and laundry [[detergent]]s. A common synthetic chelator is [[EDTA]]. [[Phosphonate]]s are also well known chelating agents. Chelators are used in water treatment programs and specifically in [[steam engineering]], e.g., [[boiler water treatment system]]: ''Chelant Water Treatment system.''

===Heavy metal detoxification===
{{main|Chelation therapy}}
Chelation therapy is the use of chelating agents to detoxify [[poison]]ous metal agents such as [[mercury poisoning|mercury]], [[arsenic]], and [[lead]] by converting them to a chemically inert form that can be excreted without further interaction with the body, and was approved by the [[U.S. Food and Drug Administration]] in 1991. In alternative medicine, chelation is used as a [[Autism therapies#Chelation therapy|treatment]] for [[autism]], though this practice is controversial due to no scientific plausibility, lack of FDA approval, and its potentially deadly side-effects.<ref>{{cite journal|journal=Can J Neurol Sci|date=2006|volume=33|issue=4|pages=341–46|title=Immunizations and autism: a review of the literature|author=Doja A, Roberts W|pmid=17168158}}</ref>

Though they can be beneficial in cases of heavy metal poisoning, chelating agents can also be dangerous.  The U.S. CDC reports that use of disodium EDTA instead of calcium EDTA has resulted in fatalities due to [[hypocalcemia]].<ref>U.S. Centers for Disease Control, "Deaths Associated with Hypocalcemia from Chelation Therapy" (March 3, 2006), http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5508a3.htm</ref>

===Other medical applications===
[[Antibiotic]] [[medication|drug]]s of the [[tetracycline]] family are chelators of [[Calcium|Ca]]<sup>2+</sup> and [[Magnesium|Mg]]<sup>2+</sup> ions.  

EDTA is also used in [[endodontic therapy|root canal treatment]] as a way to irrigate the canal. EDTA softens the dentin facilitating access to the entire canal length and to remove the smear layer formed during instrumentation.

Chelate complexes of [[gadolinium]] are often used as [[contrast medium|contrast agent]]s in [[MRI|MRI scan]]s.

===Chemical applications===
[[Homogeneous catalyst]]s are often chelated complexes. A typical example is the [[ruthenium(II) chloride]] chelated with [[BINAP]] (a bidentate [[phosphine]]) used in e.g. [[Noyori asymmetric hydrogenation]] and asymmetric isomerization. The latter has the practical use of manufacture of synthetic [[Menthol|(–)-menthol]].

Products such as Evapo-Rust are chelating agents sold for the removal of rust from iron and steel.

==References==
{{wiktionary|chelate}}
{{Reflist|2}}

{{Chelating agents}}

[[Category:Coordination chemistry]]
[[Category:Equilibrium chemistry]]
[[Category:Chelating agents|*]]

[[ar:تمخلب]]
[[ca:Quelat]]
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[[es:Agente quelante]]
[[fa:کیلات]]
[[fr:Chélation]]
[[ko:킬레이트]]
[[io:Kel-ionuro]]
[[it:Chelazione]]
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[[ja:キレート]]
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[[ru:Хелаты]]
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