• Polyoxometalates;
  • Keggin compounds;
  • Redox chemistry;
  • Tungsten;
  • Density functional calculations;
  • ­Quantum chemistry


In the 1960s, Pope and co-workers found a linear dependence between the redox potentials of a family of isostructural α-[XW12O40]n Keggin anions and their molecular charge with a slope of –0.18 V per unit charge. That finding was restricted to variations in the internal atom or heteroatom of the molecule (X = PV, SiIV, AlIII, etc.). However, other ways to change the charge of Keggin anions are commonplace, such as previous reduction processes or chemical changes in the external positions of the W12O36 cage, and also lead to variations in their oxidising power. In the present theoretical work, we analyse the oxidising power, computed as reduction energies, for a family of differently charged α-[XMW11O40]n Keggin derivatives (X = P, Si, Al and M = W, Mo, V, Nb, Ti). The nature of the unitary charge increment affects the reduction energy differently; the mildest change occurs upon variations in the internal position (heteroatom substitution), and the strongest change (by a factor of ca. 1.85 per additional charge unit compared to the previous one) occurs in the presence of a delocalised (or “blue”) electron. The replacement of a metal atom (M) from an external position by another one with different oxidation state has an intermediate effect.