A compound of: is disclosed. In Formula IV, ring B and ring C are each independently a 5 or 6-membered carbocyclic or heterocyclic ring; A-B represents a bonded pair of carbocyclic or heterocyclic rings coordinated to a metal M via a nitrogen atom in ring A and an sp2 hybridized atom in ring B; A-C represents a bonded pair of carbocyclic or heterocyclic rings; M is a metal having an atomic number greater than 40; L′ is a monoanionic bidentate ligand; m is the oxidation state of the metal M; and n is at least 1.