A compound comprising a first ligand LA of Formula I: is described. In Formula I, Z is selected from O, S, and Se; at least one of X1, X2, X3, and X4 is nitrogen, while the remainder are carbon; Y1 and Y2 are independently selected from NR, O, and S; R, RA, RB, and RC are each independently selected from a variety of substituents, and adjacent substitutions in R, RA, RB, and RC are optionally joined to form a fused ring; ligand LA is at least a tridentate ligand coordinated to a metal M; M is coordinated to ring A and to ring B through metal-carbene bonds; and the ligand LA is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand. Formulations and devices, such as an OLEDs, that include the compound with a ligand LA of Formula I are also described.