Difference in voltammetric properties between the Keggin-type [XW12O40]n− and [XMo12O40]n− complexes

The voltammetric properties of the Keggin-type [XW12O40]n− complexes were compared with those of the corresponding Mo-analogues, [XMo12O40]n−, where X=P, Ge; n=3, 4. The one-electron waves for the Keggin anions are converted into two-electron waves by the presence of small cations such as H+, Li+ and Na+, and the conversion occurs much more easily for [XMo12O40]n−, as compared with [XW12O40]n− with an identical ionic charge. A comparison of the redox potentials for the V(V)/V(IV) couple of the mono-vanadium derivatives made it possible to estimate the ionic radius of [XMo12O40]n− as 5.4 Å, with the ionic radius of [XW12O40]n−=5.6 Å as reference. The big difference in voltammetric behaviors between the Mo- and W-complexes was interpreted in terms of the smaller ionic radius and consequently greater basicity of [XMo12O40]3/4−, as compared with [XW12O40]3/4−.