Binding of a polydentate ligand to a metal through multiple donor atoms, forming stable ring structures like EDTA complexes.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Chelation is the binding of a polydentate ligand to a metal ion through two or more donor atoms, forming one or more rings that include the metal. These chelate complexes are much more stable than comparable complexes with monodentate ligands, an effect called the chelate effect. The extra stability arises mainly from entropy, because one polydentate ligand displaces several monodentate ligands and so increases the number of free particles in solution. EDTA, a hexadentate ligand, forms highly stable complexes with many metal ions. Chelation is used in water softening, analytical titration and the treatment of metal poisoning.
Key ideas: Compare stabilities by formation constants: [Ni(NH3)6]2+ has log K about 8.6, and [Ni(en)3]2+ is about 18.3. Five- and six-membered rings are the most stable. Entropy is the driving force: delta S positive because one en replaces two NH3 giving more free molecules, with delta H similar. Used in EDTA titrations, detergents, food preservation and treatment of lead poisoning.
Common mistakes and connections: Students attribute the chelate effect to a stronger bond, whereas bond enthalpies are similar and the difference is mainly entropic. Another error is thinking that any polydentate ligand forms chelates, when ring size matters: large rings are strained or have low stability. Links to ligands, entropy, complexometric titrations and biological chelators like haem.
Chelates are much more stable than complexes of monodentate ligands: ring formation raises log Kf by about ten units (the chelate effect). The interactive lesson shows this as a live animation or chart.
EDTA binds calcium ions in hard water, forming a stable complex that prevents soap from precipitating.
Binding of a polydentate ligand to a metal through multiple donor atoms, forming stable ring structures like EDTA complexes.
Chelation is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Coordination Compounds, Ligands, Coordination Number.
The next topics in Bonding & Molecular Structure are Crystal Field Theory and Spectrochemical Series.
Yes. The interactive lesson includes a self-check question on Chelation, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Chelation is topic 91 of 104 in Bonding & Molecular Structure.