LIgand

A ligand is an ion or molecule that binds to a central atom or receptor to form a larger complex. In chemistry the term is most often used for species that bind to a metal center to give coordination complexes, while in biochemistry it describes a molecule that binds to a protein, enzyme, or receptor to modulate its function.

In coordination chemistry, ligands donate electron pairs to the metal through donor atoms such as nitrogen, oxygen, sulfur, or phosphorus. Ligands are classified by denticity, or how many donor atoms they use: monodentate ligands bind through a single site, while polydentate (multidentate) ligands use two or more sites to form chelates. The overall number of ligands and their denticity determine the coordination number of the metal center. Common examples include water, ammonia, chloride, cyanide, and carbon monoxide. Multidentate ligands such as ethylenediamine or EDTA can significantly increase complex stability, a phenomenon known as the chelate effect. Ligands can also bridge between two or more metal centers, influencing the geometry and reactivity of the complex.

In biology and medicine, ligands encompass substrates, hormones, neurotransmitters, and drugs that bind to proteins such as enzymes or receptors. Binding affinity and specificity govern processes from enzyme catalysis to signal transduction. Concepts such as agonists, antagonists, inhibitors, and allosteric modulators describe functional outcomes of ligand binding. Examples include oxygen binding to iron in hemoglobin and neurotransmitter binding to neuronal receptors, which underpin many physiological responses and therapeutic interventions.

Etymology traces the word to ligare, meaning to bind. The term has been used in chemistry since the 19th century to describe species that bind to a central atom, and it remains central in both inorganic and biological contexts.