Inorganic ligands, such as polyoxovanadates, may offer a new way to further expand the boundary of coordination chemistry for inorganic complexes featuring a part of metal oxide structures. Many industrially important applications require metal oxides that support modern electronics, magnets, sensors, and catalysts. However, because of the complexity of the oxides, it is difficult to elucidate the structure and property relationships. Investigating the reactivity and physical properties by modifying the oxide structures in a systematic manner is challenging to test the hypothesis of a postulated mechanism. For these reasons, designing a model complex to replicate those properties and reactivity is important by utilizing polyoxometalates as all-inorganic ligands. In this article, the fundamental chemistry of polyoxovanadates is described from the point of inorganic ligand system by the classification based on their structural building units.