Disclosed are emissive materials of formula (I) or (II), comprising two bidentate NO-type ligands, or a tetradentate NOON-type ligand, and a transition metal. The emissive materials are useful as electrophosphorescent emitters in organic light-emitting devices. Also disclosed are methods for preparing organic light-emitting diodes comprising these emissive materials, and the use of such diodes as white and yellow organic light-emitting devices. ##STR00001##