L

Ligand

L: → M; monodentate (1 donor), bidentate (2 donors), chelate (polydentate)
Quick Reference
Formula / NotationL: → M; monodentate (1 donor), bidentate (2 donors), chelate (polydentate)
Also Known AsLewis base (in coordination context), donor molecule, coordinating ligand, chelating agent

What is Ligand?

An ion or molecule that donates a pair of electrons to a central metal atom or ion to form a coordination complex. Monodentate ligands have one donor atom; polydentate (chelate) ligands have multiple. Common ligands include water, ammonia, CN⁻, and EDTA.

Formula & Notation

L: → M; monodentate (1 donor), bidentate (2 donors), chelate (polydentate)

Other Names / Synonyms: Lewis base (in coordination context), donor molecule, coordinating ligand, chelating agent

Properties & Characteristics

A ligand is a molecule or ion that donates one or more electron pairs to a central metal atom in a coordination compound. Monodentate ligands (NH₃, H₂O, Cl⁻) have one donor atom. Polydentate ligands (chelates) have multiple donor atoms: ethylenediamine (en, bidentate), EDTA (hexadentate). Chelate effect: polydentate ligands form more stable complexes than equivalent monodentate ligands due to entropy gain. Ligand field strength follows the spectrochemical series.

Uses & Applications

Ligands are central to coordination chemistry: EDTA (analytical reagent, chelation therapy), CO (in metal carbonyl catalysts: Fe(CO)₅, Rh(CO)(PPh₃)₂Cl), phosphines (in homogeneous catalysts: Wilkinson's), biological ligands (porphyrin in haemoglobin, corrin in vitamin B₁₂), and pharmaceutical ligands (DOTA for Gd MRI contrast agents).

Safety Information

Many ligands are toxic or reactive. CO is acutely toxic (see carbon monoxide). Cyanide ligands release HCN on acidification. Phosphine ligands are toxic and flammable. Understanding ligand exchange lability is key to predicting whether protective complexes in chelation therapy will remain intact in the body.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Ligand
Formula L: → M; monodentate (1 donor), bidentate (2 donors), chelate (polydentate)
Synonyms Lewis base (in coordination context), donor molecule, coordinating ligand, chelating agent

Frequently Asked Questions

An ion or molecule that donates a pair of electrons to a central metal atom or ion to form a coordination complex. Monodentate ligands have one donor atom; polydentate (chelate) ligands have multiple. Common ligands include water, ammonia, CN⁻, and EDTA.

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