Most of the chemical reactions in industry are two-electron processes, and these two-electron redox-reactions are exclusively carried out by the precious metals such as platinum, palladium, iridium, etc. There is a continuous quest for the development of catalysts based on less toxic, more abundant, and hence cheaper transition metals such as iron, cobalt, nickel. These base metals can often only change their oxidation states by one electron. For this reason, these metals are unable to perform the above mentioned two-electron redox-reactions. In order to overcome this problem, redox-active ligands are studied for the development of catalysts based on base metals. Redox-active ligands are those ligands which are actively participating in the...