Phosphorescent iridium complexes having Formula I or Formula II: ##STR00001## wherein X is chosen from a monoanionic bidentate ligand; Z is chosen from an atomic group wherein Z together with the buta-1,3-diene to which Z is attached form an aryl group or heteroaryl group; R, R.sub.1, and R.sub.2 are independently chosen from a hydrogen atom or a substituent; m is an integer from 0 to 4; and n is an integer from 0 to the maximum number of possible substituents on Z, are disclosed. Light emitting devices using the phosphorescent iridium complexes are also disclosed.