Chemistry Definition of Ligand

A microscopic view of a ligand binding to a receptor
A microscopic view of a ligand binding to a receptor. Stocktrek Images/Getty Images

A ligand is an atom, ion, or molecule that donates or shares one or more of its electrons through a covalent bond with a central atom or ion. It is a complexing group in coordination chemistry that stabilizes the central atom and determines its reactivity.

Ligand Examples

Monodentate ligands have one atom that can bind to a central atom or ion. Water (H2O) and ammonia (NH3) are examples of neutral monodentate ligands.

A polydentate ligand has more than one donor site. Bidentate ligands have two donor sites. Tridentate ligands have three binding sites. ​1,4,7-triazaheptane (diethylenetriamine) is an example of a tridentate ligandTetradentate ligands have four binding atoms. A complex with a polydentate ligand is called a chelate.

An ambidentate ligand is a monodentate ligand that can bind in two possible places. For example, The thiocyanate ion, SCN-, can bind to the central metal at either the sulfur or the nitrogen.