Organic electroluminescent materials and devices

A compound comprising a ligand LA having a structure of

• Formula I
• wherein:
• each of X1, X2, X3, X4, X5, X6, X7, and X8 is independently C or N;
• each of Y1′ and Y2′ is independently selected from the group consisting of O, S, Se, CRR′, and SiRR′;
• each of RA, RB, and RC is independently represent mono to the maximum allowable substitution, or no substitution;
• each of R, R′, RA, RB, and RC is independently a hydrogen or a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, boryl, selenyl, arylalkyl, alkoxy, aryloxy, amino, silyl, germyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof;
• LA is coordinated to metal M through the indicated dashed lines to form a 5-membered chelate ring;
• metal M is selected from the group consisting of Ru, Os, Ir, Pd, Pt, Cu, Ag, and Au;
• Ring B is coordinated to M by a C-M bond;
• M may be coordinated to other ligands;
• LA can be joined with other ligands to form a tridentate, tetradentate, pentadentate, or hexadentate ligand;
• and
• any two adjacent R, R′, RA, RB, and RC can be joined or fused to form a ring.