The coordinating ability of aromatic diamines is conditioned both by the low basicity of the nitrogen atoms and their situation in ortho, meta or para positions on the aromatic ring. Important aspects of the aromatic diamines taking into account their possible applications are: processes based on electron transfer forming Würster type radical colored cations; trace analysis of metal ions and other species based on catalysis of the oxidative coloration system; the development of biosensors coated with poly(o-phenylenediamine) and their mutagenic and toxic character. o-Phenylenediamine ligands coordinate in a monodentate, chelating bidentate and bridging bidentate fashion in dinuclear complexes; p-phenylenediamine is mainly a bridging bidentate ligand as expected.