Here we report on the use of a silanol-decorated polyoxotungstate, [SbW9O33('BuSiOH)(3)](3-) (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)(3).thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW9O33('BuSiO)(3)V(thf)](3-) (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d(2)-V-III high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal.mol(-1) more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(equivalent to Si-O)(3)V-III(OH2)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW9O33('BuSiO)(3)VO](3-) (3). In compound 2, the d(2)-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW9O33(tBuSiO)(3)VO](5-), 3.(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d(1)-V(IV) and d(1)-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (sigma-donor) vs oxo (pi-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O3VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3.(2e).