In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. An array of such ligands around a centre is termed a complex.
The central atom usually has a positive charge which is stabilised by donation of negative charge from the ligands. Neutral or negatively charged centres are also known, usually stabilised by donating electron density back to the ligand in a process known as "back-bonding". The charge on the central atom constrains the number of ligands that may be bonded, since each type of ligand donates a characteristic number of electrons and there is a requirement for overall neutrality. If the directly-bonded ligands (the "inner-sphere" ligands) do not balance the charge, this may be done by purely ionic interaction with another set of ions (the "outer-sphere" ligands).
The inner-sphere ligands arrange themselves in a certain geometry, fixed for a given complex but in some cases mutable by reaction to another stable isomer. Ligands which bind to the central atom through more than one site of their own are termed polydentate; a ligand binding through two sites, for example, is bidentate.
The ligand geometries are named and described as if the central atom were in the middle of a polyhedron, and the corners of that shape were the locations of the ligands. For example, a complex with four regularly distributed ligands would be described as tetrahedral, while one with six would be octahedral.
The polyhedra need not be regular: there are other possible geometries, such as square pyramidal (four ligands equally distributed in a plane, and one ligand normal to this plane).
Ligands which only bond to the central atom through one site are termed monodentate. Some ligand molecules are able to bind to the metal ion through multiple sites, since they have free lone pairs on more than one atom; these are called polydentate. EDTA is a classic example of a polydentate ligand - it is able to bond through six sites, completely surrounding the central atom. Complexes of polydentate ligands are called chelated complexes; they tend to be more stable than monodentate complexes as it is necessary to break all of the bonds to the central atom for the ligand to be displaced.
In biochemistry, ligand refers to a small molecule that binds to a larger macromolecule, whether or not the ligand actually binds at a metal site. This is probably a carryover from the large number of binding studies on oxygen transport proteins, such as hemoglobin, where the ligand does indeed bind at a metal site, an expansion of the term to a more general case of binding.
Protein ligands are studied in structural biology and metabolomics.
Radioactive ligands (radioligands) are used together with positron emission tomography to study the receptor systems of the brain.