Download Chelating agents are ligands for metals that bind via

Chelating agents are ligands for metals that bind via multiple atoms,
thus taking up several coordination sites on the metal.
LEARNING OBJECTIVE [ edit ]
Describe the origin of the chelate effect.
KEY POINTS [ edit ]
Chelation is the formation or presence of two or more separate coordinate bonds between
a polydentate (multiple bonded) ligand and a single central atom.
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.
Chelation therapy is the use of chelating agents to detoxify poisonous metal agents, such as
mercury, arsenic, and lead, by converting them to a chemically inert form that can be excreted
without further interaction with the body.
TERMS [ edit ]
ligand
An ion, molecule, or functional group that binds to another chemical entity to form a larger
complex.
chelating agent
Any compound that reacts with a metal ion to produce a chelate.
chelate compound
A cyclic compound in which a metal atom is bonded to at least two other atoms.
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Chelation
Chelation is the formation or presence of
two or more separate
coordinate bonds between
a polydentate (multiple
bonded) ligand and a single central atom.
Usually these ligands are
organic compounds and are called
chelants, chelators, chelating agents, or
sequestering agents; the resulting
complexes are called chelate compounds.
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Metal­EDTA chelate
Chemical structure of EDTA chelate.
Chelate complexes are contrasted with coordinationcomplexes composed
of monodentate ligands, which form only one bond with the central atom. Chelating agents,
unlike the other ligands in coordination compounds, bind via multiple atoms in the
ligand molecule, not just one.
The Chelate Effect
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
(CH3NH2 (MeNH2)) on the other.
2+
C u 2+ + en → [Cu(en)]
(1)
2+
C u 2+ + 2M eN H 2 → [Cu(M eN H 2 ) 2 ]
(2)
In (1), the bidentate ligand ethylenediamine forms a chelate complex with the copper ion.
Chelation results in the formation of a five­membered ring. In (2), the two monodentate
methylamine ligands of approximately the same donor power (the enthalpy of formation of
Cu—N bonds is approximately the same in the two reactions) forms a complex. Under
conditions of equal copper concentrationsand when the concentration of methylamine is
twice the concentration of ethylenediamine, the concentration of the complex in (1) will be
greater than the concentration of the complex in (2). The effect increases with the number of
chelate rings, so the concentration of the EDTA complex, which has six chelate rings, is much
higher than a corresponding complex with two monodentate nitrogendonor ligands and four
monodentate carboxylate ligands. Thus, the phenomenon of the chelate effect is a firmly
established empirical fact.
Ethylenediamine chelate
Ethylenediamine serves as a chelating agent by binding via its two nitrogen atoms.
Applications of Chelating Agents
Chelation therapy is the use of chelating agents to detoxify poisonous metal agents, such as
mercury, arsenic, and lead, by converting them to a chemically inert form that can be
excreted without further interaction with the body. This therapy was approved by the U.S.
Food and Drug Administration in 1991. Although they can be beneficial in cases of heavy
metal poisoning, chelating agents also can be dangerous. Use of disodium EDTA instead of
calcium EDTA has resulted in fatalities due to hypocalcemia.
Virtually all biochemicals exhibit the ability to dissolve certain metal cations. Thus, proteins,
polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions.
Organic compounds such as the amino acidsglutamic acid and histidine, organic diacids such
as malate, and polypeptides such as phytochelatin are also typical chelators. In addition to
these adventitious chelators, several biomolecules are specifically produced to bind certain
metals. Virtually all metalloenzymes feature metals that are chelated, usually to peptides
or cofactors and prosthetic groups. Such chelating agents include the porphyrin rings
in hemoglobinand chlorophyll. Many microbial species produce water­soluble pigments that
serve as chelating agents, termed siderophores. For example, species of Pseudomonas are
known to secrete pyocyanin and pyoverdin that bind iron. Enterobactin, produced by E. coli,
is the strongest chelating agent known.