Tridentate ligand
A tridentate ligand (or terdentate ligand) is a ligand that has three atoms that can function as acceptor atoms in a coordination complex. Well-known tridentate ligands include diethylenetriamine with three nitrogen donor atoms, and the iminodiacetate anion which consists of one deprotonated amine nitrogen and a pair of carboxylate groups. An octahedrally coordinated atom has six positions around it. Two tridentate ligands may form a complex with such an atom. There are two possible arrangements for such a complex: "fac" where the coordination is in a triangle on one face of the octahedron, and "mer" where the  coordinating atoms are in an arc around the central atom, with two atoms of the ligand opposite each other. "Fac" tridentate ligands are termed scorpionate ligands, especially in reference to polypyrazolylborates. If the tridentate ligand is not symmetrical, then in the "fac" complexes in octahedral coordination there are three possible isomers. In the "mer" complexes there are two enantiomers, mirror images of each other.