Polyoxometalates (POMs) are an inorganic class of compounds constituted only from transition metal(s) (M) and oxygen (O) of the general chemical formula [MmOo]k- with m ≥ 3, o ≥ 9, k > 0, i.e., they are larger, anionic metal-oxygen coordination complexes. The structural connections between the coresponding transition metal oxides can be quite close or only very loose, depending on the transitions metal's characteristics.