Polydentate ligands form ring structures with metal ions called chelate rings. The enhanced stability of chelated complexes over comparable monodentate complexes is the chelate effect. Thermodynamic basis: replacing monodentate ligands with a chelating ligand releases more free molecules, increasing entropy. [Ni(en)3]2+ is far more stable than [Ni(NH3)6]2+ even though both have 6 Ni-N bonds. Five-membered chelate rings (like those from en) are most stable. EDTA forms 5 chelate rings simultaneously, giving extremely high stability constants. Chelation is exploited in analytical chemistry, medicine (chelation therapy for heavy metal poisoning), and water softening.