A process for forming a tris-cyclometallated iridium or rhodium complex comprises reacting in the presence of a solvent:

• a) a bis-cyclometallated complex comprising an Ir (III) or Rh (III) metal, two bidentate ligands, two monodentate ligands and a counterion, and
• b) a heterocyclic compound capable of forming an organometallic cyclometallated complex.