In complexation reactions, metal atoms or cations interact with ligands to form donor-acceptor adducts called metal complexes. Ligands that bind through one donor site are monodentate, ligands with two donor sites are bidentate, and those with more than two donor sites are polydentate ligands. For example, ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms, forming a five-membered ring. EDTA is a polydentate ligand that binds through four oxygen and two nitrogen atoms.

Bidentate and polydentate ligands are also called chelating agents, and the corresponding complexes are called chelates. Chelate is a Greek word that means "claw-like." Metal complexes formed by the chelating agents are more stable than those formed by their monodentate counterparts, as the reaction for their formation is entropically favored. This property is known as the chelate effect or the entropy effect.