Ligand

In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central (usually metal) ion(s) forming a complex.

The central atom or group of atoms will have a positive charge, and the ligands will bond by compensating that charge with their own negative charge or characteristics. As the central atom has a specific charge, it has a maximum potential number of ligands (as each must donate at least one electron into the charge compensation equation) that it can bond to. These ligands will also arrange themselves into a certain arrangement around the central atom. If a single ligand bonds to more than one of these bonding sites on the central atom, then it is said to be bidentate (three=tridentate, etc)

Ligand arrangements

These are named and described as if the central atom is in the middle of a polyhedron, and the corners of that shape are the locations of the ligands. For example, a complex with four, regularly distributed ligands would be described as tetrahedral.

Aside from the regular polyhedra, there are special descriptions, such as pyramidal (four ligands equally distributed in a plane, and one ligand normal to this plane).

Polydentate ligands

Ligands which only bond to the central metal atom/ion through one site are termed monodentate. Some ligand molecules are able to bind to the metal ion through multiple sites, due to having free lone pairs on more than one atom, these are called polydentate. EDTA is an example of a polydentate ligand - it is able to bond through 6 sites, completely surrounding the ion. Polydentate ligands tend to be very stable, as it is necessary to break all of their bonds to the central atom for them to be displaced.

Common ligands

F^-
Cl^-
Br^-
I^-
CO
RC:CR - compounds with double carbon bonds
Benzene
Cryptates
Crown ethers
OH^-

In biochemistry, a ligand will refer to a small molecule that binds to a larger macromolecule, whether or not the ligand actually binds at a metal site or not. This is probably a carryover from the large number of binding studies on oxygen transport proteins, such as hemoglobin, where the ligand indeed did bind at a metal site, an expansion of the term to a more general case of binding.