Process for preparing cyclometallated transition metal-carbene complexes comprising at least one carbene ligand, which comprises reacting a ligand precursor with a base, an auxiliary reagent and a metal complex comprising at least one metal M1 (route A) or reacting the ligand precursor with a basic auxiliary reagent and a metal complex comprising at least one metal M1 (route B). The present invention further relates to the use of an auxiliary reagent selected from among salts comprising at least one metal selected from the group consisting of Ag, Hg, Sb, Mg, B and Al together with a base in a process for preparing cyclometallated metal complexes.