Cationic ruthenium complexes of the formula A or B or mixtures containing them, where B may be stabilized by a further ligand L4 and X is an anion which does not coordinate or coordinates weakly to the metal center, Y is a monodentate or multidentate anionic ligand, R and R′ are each, independently of one another, hydrogen or a substituted or unsubstituted C1-C20-alkyl, C6-C20-aryl or C7-C20-alkylaryl radical and L1, L2, L3 and L4 are, independently of one another, uncharged electron donor ligands, are described.