In the chemistry of transition metals, three elements stand out distinctly: tungsten (W), molybdenum (Mo), and vanadium (V). Located in group 6 in the periodic table, under the right conditions, they can self-assemble into polynuclear metal-oxo clusters, or polyoxometalates (POMs), comprised of early transition-metals (addenda) and oxide atoms. In POMs, the transition metals, V, Mo and W are fully oxidized to d 0 and form diverse structures with {MOx} as the principal building block. Those polyoxo clusters are formed from the acidification of aqueous transition metals salts. POMs are typically formed by transition metals in group 6, but can be formed by transition metals in groups 3 through 12 of the periodic table. These metals often have multiple oxidation states, which allows them to form complex structures with oxygen atoms. The big three are: tungsten (W), molybdenum (Mo), and vanadium (V), that together with niobium (Nb), tantalum (Ta), and chromium (Cr), are the most common POM transition metals building-blocks. POM chemists have classified the clusters into two big groups that involve the presence or absence of heteroatom: iso-polyoxometalates (or isopolyanions, IPAs) have the formula [MnOx} q-, and hetero-polyoxometalates (or heteropolyanions, HPAs) with the formula [XnMnOx} q-being X=P, Si, Ge, As, Se, Te, etc. Classic structures of polyoxometalates, such as the Keggin type (typically [XM12O40] n-) and the Wells-Dawson type (typically [X2M18O62] n-), are characterized by their highly symmetrical clusters which exhibit remarkable versatility in terms of composition, shape, and potential applications. As a rule of thumb, tungsten clusters are the most structurally robust in the POM family; hence, partial hydrolysis of tungsten derived poly-oxo clusters, achieved through the controlled addition of base produces lacunary clusters. This straightforward synthetic method forms one or several metal voids in the structure of the POM molecule. The defective region, named lacuna, can react with other transition-metal cations forming a new structure, for example M=Mn III , Co III , Fe III , Ti IV , Ni II , Cr III and Zr IV , amongst others. Mono-transition-metal-substituted polyoxotungstates, explored in this work have the formula [M(L)XW11O39] q-(X = P(V) or Si(IV), L = H2O, DMSO, and M= Mn, Co, Fe. Once the new hexacoordinated transition-metals are introduced, and in aqueous solution, their sixth coordination site is occupied by solvent (H2O); or with other ligands from the local environment, such a DMSO. Given their modularity and stability, TM-substituted polyoxometalates, known as TM-POMs for short, have attracted significant interest as single atom catalysts (SACs) in fields including water oxidation, carbon dioxide reduction, and nitrogen activation. Since POMs are anions their counter cations have often been overlooked, until recently. Initial theoretical work by Lopez, et al. explored the relative stability of rotational isomers of Keggin heteropolyanions, α/β-[XM12O40] q-. Those calculations, back in the early 2000s, used the local-density approximation (LDA) functional coupled with Vosko-Wilk-Nusair (VWN) parametrization, in the gas phase, given the computational tools available. In their conlcusions, the stability of the α-[XM12O40] q- isomer was attributed to the higher energy of the lowest occupied molecular orbital (LUMO). When looking into four-times-reduced clusters: [PW12O40] 7-and [SiMo12O40] 8-there was a slight preference (ca. 0.4 eV) towards the β-isomer, in the gas phase. Following on this initial trend, Zhang et al. in a systematic study highlighted the importance of including implicit solvation (COSMO) for reproducing experimental geometries, by comparing gas-phase and aqueous geometry optimizations for the α/β-[XM12O40] q-Keggin structures. Computational chemistry results helped to envisage a mechanism in the single-site ruthenium catalysts with general formula a-[Ru II/III (DMSO)XW11O39] n-(X=P, Si; n=4-6 depending on the heteroatom X and on Ru oxidation state. In their work, the teams of Bo and Bonchio shed light on the mechanism of ruthenium-based dioxygenase catalysis, and presented POMs as a reliable material to replicate bio-inspired pathways bridging the gap between heterogenous and homogeneous catalysts. Understanding the role of the heteroatom in self-sorting architectures of polyoxotungstates is also very important and requires the efforts of computational and experimental chemists, in their work Zheng et al., presented a new family of cross-shaped polyanions [(XYW15O54)4(WO2)4] 32-/36-(X = H-P, Y = Se, Te, As). In a recent study, Poller and co-workers, have also combined experimental and computational tools to investigate the influence of the group 15 elements nitrogen, phosphorus, arsenic, and antimony on the formation of POM structures. The redox properties of polyoxometalates are also affected by the counteraction as is the case of the lithium salts of the Wells-Dawson polyoxoanion [P2W18O62] 6-can be reversibly reduced by 18 electrons per anion in aqueous solution. It took over four years and an international team or experimentalists and theoretical chemists to understand the mechanism behind that unconventional redox activity of Li6[P2W18O62], that is also transferable to other tungsten salts such as of [P5W30O110] 15-and [P8W48O184] anions, which can be charged to 23 and 27 electrons per cluster, respectively. The results, comparing Li, Na and K pointed out that lithium facilitated the protonation and agglomeration of reduced POM clusters in solution via cation-mediated contacts are complementary factors behind the formation of super-reduced species. Counter cations have an important role in POM chemistry. In 2023, we explored the accuracy of various exchange-correlation functionals reproducing experimental literature potentials, U 0 Red, in anionic [M(H2O)PW11O39] q-compounds where M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II). The x-c functionals employed in our study were chosen to explore the influence of Hartree-Fock (HF) exchange (0% PBE, 15% B3LYP*, 20% B3LYP, 25% PBE0, and 50% BH&H) for reproducing potentials. Our work involved explicitly located K + counterions, see Figure , which induced a positive shift of potentials by > 500 mV which improved the accuracy of computed potentials where M = Mn(III/II)/Co(III/II); however, this was not the case where M= Fe(III/II)/Ru(III/II)