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Chemistry · Glossary

What is Polydentate ligand, chelate?

Also known as: polydentate ligand · chelate

Definition 12.2 University Chemistry — Year 1 · Chapter 12 — Complexation

A ligand that binds the same central ion through several of its atoms at once is a polydentate ligand; the complex it forms, in which the ligand closes rings around the metal ion, is a chelate. Ethylenediamine HX2N−CHX2−CHX2−NHX2\ce{H2N-CH2-CH2-NH2} binds through two N atoms, the oxalate ion CX2OX4X2−\ce{C2O4^2-} through two O atoms, and the ethylenediaminetetraacetate ion (edta, written YX4−\ce{Y^4-}) through two N and four O atoms.

Left: the edta ion Y4-, with its two N atoms and four carboxylate groups. Right: the calcium–edta chelate [ CaY]2-, schematic: the six donor atoms (blue bonds) sit at the corners of an octahedron around the ion, the two N atoms side by side, and the carbon chains of the ligand (orange) close five rings of five atoms each, the chelate rings. Left: the edta ion Y4-, with its two N atoms and four carboxylate groups. Right: the calcium–edta chelate [ CaY]2-, schematic: the six donor atoms (blue bonds) sit at the corners of an octahedron around the ion, the two N atoms side by side, and the carbon chains of the ligand (orange) close five rings of five atoms each, the chelate rings.
Left: the edta ion YX4−\ce{Y^4-}, with its two N atoms and four carboxylate groups. Right: the calcium–edta chelate [CaY]2−[\ce{CaY}]^{2-}, schematic: the six donor atoms (blue bonds) sit at the corners of an octahedron around the ion, the two N atoms side by side, and the carbon chains of the ligand (orange) close five rings of five atoms each, the chelate rings.
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