An ion or molecule that donates a pair of electrons to a metal atom or ion in forming a coordination complex. Molecules that function as ligands are acting as Lewis bases (see acid). For example, in the complex hexaquocopper(II) ion [Cu(H₂O)₆]²⁺ six water molecules coordinate to a central Cu²⁺ ion. In the tetrachloroplatinate(II) ion [PtCl₄]²⁻, four Cl⁻ ions are coordinated to a central Pt²⁺ ion. A feature of such ligands is that they have lone pairs of electrons, which they donate to empty metal orbitals. A certain class of ligands also have empty p- or d-orbitals in addition to their lone pair of electrons and can produce complexes in which the metal has low oxidation state. A double bond is formed between the metal and the ligand: a sigma bond by donation of the lone pair from ligand to metal, and a pi bond by back donation of electrons on the metal to empty d-orbitals on the ligand. Carbon monoxide is the most important such ligand, forming metal carbonyls (e.g. Ni(CO)₄).

The examples given above are examples of monodentate ligands (literally: ‘having one tooth’), in which there is only one point on each ligand at which coordination can occur. Some ligands are polydentate; i.e. they have two or more possible coordination points. For instance, 1,2-diaminoethane, H₂NC₂H₄NH₂, is a bidentate ligand, having two coordination points. An interesting type of tridentate ligand is a scorpionate, in which two of the three coordination sites are positioned like the pincers of a scorpion, with the ligand bending back and over the plane formed by the metal and the first two sites to provide the scorpion’s tail; this gives rise to a third coordination site. Certain polydentate ligands can form chelates.