Early studies by Streb and co-workers have used a placeholder strategy for the predictable metal-functionalization of POVs. This approach uses the dodecanuclear species (NMe2H2)2[V12O32Cl] 3-, (= (DMA)2{V12}, DMA = dimethyl ammonium), where two vacant metal coordination sites are blocked by hydrogen-bonded dimethyl ammonium cations. In-situ exchange of these cations with a variety of metals is possible, leading to the mono-or (more rarely) di-metal-functionalized {V12} species, and applications ranging from (light-driven) catalysis to energy storage. Note that for most of the di-metal-functionalized {V12} species were only accessible as mixed V IV/V oxidation state clusters. Recently, a similar synthetic approach to metal-functionalized molybdates has been pioneered by Yamaguchi, Suzuki and co-workers: the group used pyridine moieties to coordinatively stabilize and functionalize lacunary polyoxomolybdates which are otherwise difficult to access. Most often, temperature, solvent, solution acidity and type of metal salt are the key synthetic parameters varied to facilitate metal functionalization of POVs. In contrast, the use of light, i.e. photons with energies in the visible spectral range has rarely been discussed as a facile control parameter to trigger POM and POV functionalization. This is surprising, as the (visible) light photoactivity of POMs is well-documented, and the lightinduced excitation of O→M ligand-to metal charge-transfer transitions is an easy tool to selectively access reduced POM species. The concept has been pioneered by Yamase and co-workers who demonstrated that previously unknown mixed-valent POMs and POVs can be accessed photochemically in the presence of suitable electron donors, e.g., organic amines. POV chemistry is particularly sensitive to photoinduced reactions triggered by visible light, as POVs typically show higher visible light absorption compared with tungstates and molybdates. This approach has recently been explored by Liu and co-workers who demonstrated the visible-light-assisted synthesis of mixed-valent POVs.