The present invention provides an improved redox couple for electrochemical and optoelectronic devices. The redox couple is based on a complex of a first row transition metal, said complex containing at least one mono-, bi-, or tridentate ligand comprising a substituted or unsubstituted ring or ring system comprising a five-membered N-containing heteroring and/or a six-membered ring comprising at least two heteroatoms, at least one of which being a nitrogen atom, said five- or six-membered heteroring, respectively, comprising at least one double bond. The invention also relates to electrolytes and to the devices containing the complex, and to the use of the complex as a redox couple. The invention further provides electrochemical and/or optoelectronic devices comprising a first and a second electrode and, between said first and second electrode, a charge transport layer, said a charge transport layer comprising tetracyanoborate ([B(CN)4]−) and a cationic metal complex functioning as redox-couple.