A compound of formula (I): wherein M is a transition metal; x is at least 1; z is 0 or a positive integer; L1 is a ligand selected from one of ligands (II-A), (II-B); L2 is a ligand selected from another of ligands of formulae (II-A), (II-B) and (II-C); and L3 is a ligand other than a ligand of formulae (II-A), (II-B) or (II-C): (II-A) (II-B) (II-C) in which each of R1-R6 is substituent and Ar1-Ar3 are each an unsubstituted or substituted aryl or heteroaryl group. The compound of formula (I) may be used as a phosphorescent light-emitting material of an organic light-emitting device.