chelate

—chelatable, adj.

/kee"layt/, adj., n., v., chelated, chelating.

adj.

1. Chem.

a. of or noting a heterocyclic compound having a central metallic ion attached by covalent bonds to two or more nonmetallic atoms in the same molecule.

b. of or noting a compound having a cyclic structure resulting from the formation of one or more hydrogen bonds in the same molecule.

2. Zool. having a chela or chelae.

n.

3. Chem. a chelate compound.

v.i. Chem.

4. (of a heterocyclic compound) to react to form a chelate.

5. (of a compound) to form a ring by forming one or more hydrogen bonds.

v.t.

6. Chem. to combine (an organic compound) with a metallic ion to form a chelate.

[1820-30; CHEL(A)¹ + -ATE¹]

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Any of a class of coordination or complex compounds consisting of a central atom of a metal (usually a transition element) attached to a large molecule (ligand).

Any ligand that can bind to the metal at two or more points to form a ring structure, more stable than a nonchelated compound of the same general chemical formula, is a chelating agent. The process of binding to the metal is called chelation. Chelating agents such as EDTA salts are used in medicine to remove toxic metals (e.g., lead, cadmium) from the body. Others are used in analysis as indicators and in industry to extract metals. The iron-binding porphyrin group in hemoglobin, the magnesium-binding porphyrin in chlorophyll, and the cobalt-binding porphyrin in vitamin B₁₂ are natural chelators.

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▪ chemistry

      any of a class of coordination or complex compounds consisting of a central metal atom attached to a large molecule, called a ligand, in a cyclic or ring structure. An example of a chelate ring occurs in the ethylenediamine-cadmium complex:

      The ethylenediamine ligand has two points of attachment to the cadmium ion, thus forming a ring; it is known as a didentate ligand. (Three ethylenediamine ligands can attach to the Cd²⁺ ion, each one forming a ring as depicted above.) Ligands that can attach to the same metal ion at two or more points are known as polydentate ligands. All polydentate ligands are chelating agents.

      Chelates are more stable than nonchelated compounds of comparable composition, and the more extensive the chelation—that is, the larger the number of ring closures to a metal atom—the more stable the compound. This phenomenon is called the chelate effect; it is generally attributed to an increase in the thermodynamic quantity called entropy that accompanies chelation. The stability of a chelate is also related to the number of atoms in the chelate ring. In general, chelates containing five- or six-membered rings are more stable than chelates with four-, seven-, or eight-membered rings.

      In medical practice, chelating agents, particularly salts of EDTA, or edetic (ethylenediaminetetraacetic) acid, are widely used for direct treatment of metal poisoning because they bind the toxic metal ions more strongly than do the vulnerable components of the living organism. Chelating agents are also employed as extractants in industrial and laboratory separation of metals and as metal-ion buffers and indicators in analytical chemistry. Many commercial dyes and a number of biological substances, including chlorophyll and hemoglobin, are chelate compounds.

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