A compound including a ligand L1 of Formula I, is described. In Formula I: rings A and B each independently represent a 5-membered or 6-membered carbocyclic or heterocyclic ring; X, Y, Z, and W are either carbon or nitrogen; R1 to R6 are selected from a variety of substituents, where adjacent substituents can be joined to form a fused ring; ligand L1 is coordinated to a metal M by X-M and Y-M bonds; and ligand L1 is optionally linked with other ligands to produce a tridentate, tetradentate, pentadentate or hexadentate ligand. Formulations and devices, such as an OLEDs, that include the compound that include a ligand L1 of Formula I are also described.