ligand

If the metal is the center of a flower, the ligands are the petals attached all around it. The word comes from the Latin ligare, 'to bind' — a ligand is simply a molecule or ion that binds to a central metal. Water, ammonia, chloride, and carbon monoxide are everyday examples; each clings to the metal and helps define the complex.

Precisely, a ligand is a species that donates one or more lone pairs of electrons to a metal, forming coordinate bonds. To do this it needs at least one atom carrying a lone pair — the donor atom — such as the oxygen of water, the nitrogen of ammonia, or the carbon of carbon monoxide. Ligands can be neutral (H2O, NH3, CO) or anionic (Cl-, CN-, OH-); a few are even cations. They differ in how many points they grip the metal (their denticity): a monodentate ligand binds through one atom, a bidentate ligand such as ethylenediamine through two, and a polydentate ligand like EDTA through several at once, wrapping the metal like a claw. Beyond just donating, some ligands also accept electron density back from the metal (CO and CN- are famous pi acceptors), which strongly shapes the bonding.

Ligands are the adjustable part of a complex: keep the metal but change the ligands and you change the color, the geometry, the stability, and the reactivity. Ranking ligands by how strongly they split the metal's d orbitals gives the spectrochemical series — from weak-field ligands like iodide and water to strong-field ligands like cyanide and CO — which the ligand-field part of this subject uses to predict color and magnetism.