UV-Vis spectroscopy of {Mg2V12} confirms the mixed valent (V IV/V ) character of the species, as indicated by the characteristic, broad intervalence charge-transfer (IVCT) band between ~600-1200 nm. Further support of the mixed-valent nature of {Mg2V12} is given by continuous wave electron paramagnetic resonance (EPR) spectroscopy which unambiguously shows the presence of one V IV species with S = ½ (ESI, Figure ), while a pure V V cluster would be EPR silent. Furthermore, EPR-based spin counting is in good agreement with one V IV centre per {Mg2V12}. The change of the UV-Vis-NIR signals over time can be used to monitor the rate of {Mg2V12} formation, see Figure . This system also provides ideal model to explore the fundamentals of the light-induced formation mechanism of metal-functionalized vanadates. First, we were interested in comparing the photoreduction of {V12} in the presence and absence of Mg 2+ (Figure ). Strikingly, {V12} reduction is only observed in the presence of Mg 2+ , while in the absence of Mg 2+ , no formation of V IV centres and no IVCT signal is detected. We hypothesized that this finding indicates that Mg 2+ interacts with {V12} in the reaction solution to give a photoactive reactive intermediate. Based on our understanding of the system, we hypothesized that this intermediate could be the mono-Mg-functionalized species {MgV12} (= (DMA)[(MgCl)V12O32Cl] 3-). The formation of an intermediate species is indicated by UV-Vis spectroscopy which shows distinct spectral changes upon addition of Mg 2+ to the {V12} reaction solution (SI, Figure ). In addition, 51 V NMR spectroscopy shows that immediately after addition of Mg 2+ to a solution of {V12}, a four-signal spectrum is observed which is characteristic for the mono-metal-functionalized {MV12} species (Figure ). Also, high-resolution electrospray ionization mass spectrometry (HR ESI MS) allowed us to identify a series of signals corresponding to the mono-Mg-functionalized species, e.g., [HMgV12O32Cl] 2-(observed: 591.590 m/z, calculated: 591.852 m/z, see ESI, Figure and Table for further peak assignments). Based on Le Chatelier's principle, we hypothesized that addition of DMA to the reaction solution should therefore shift the equilibrium to the reagent side (see Figure ), thus preventing the formation of the photoactive {MgV12}. This behaviour is indeed observed. When an excess of DMA . HCl is added to the standard {V12} reaction solution and the sample is irradiated, no reduction is observed by UV-Vis spectroscopy (SI, Figure ). This indicates that the reactive intermediate which enables photoreduction is not present under these conditions and lends further support to {MgV12} being the photoactive intermediate. Also, when the {V12} photoreduction is performed in the presence of Mg 2+ and air, virtually no vanadate reduction is observed (as indicated by the absence of IVCT bands in the UV/Vis spectrum, see SI, Figure ). This provides further support that a light-induced electron transfer to {V12} is a key process in the formation of {Mg2V12}. In the presence of O2, this step is inhibited as O2 is easier to reduce than {V12}-based species (E1/2 (O2/H2O) ca. +0.6 V (vs Fc + /Fc in DMF), E1/2({V12} 3-/ {V12} 4-) ca. -0.7 V (vs Fc + /Fc in DMF). In sum, these data suggest that upon reaction of {V12} and Mg 2+ an equilibrium is formed to give the mono-functionalized {MgV12} as photoactive, reactive intermediate (Figure ), which can then be converted to the di-functionalized {Mg2V12} upon irradiation with visible light. The resulting coupled reaction equilibria are shown in Figure .