2,2':6',2''-terpyridine is a tridentate heterocyclic ligand that binds metals at three meridional sites giving two adjacent 5-membered MN2C2 chelate rings.
Related images
Metal-binding conformation of the terdentate ligands (a) terpyridine (Terpy) and (b) 2,6-bisbenzimidazolylpyridine (Bip) with labeled functional sites: R, R0, X, and Y. The gray spheres represent metal ions.