Effect of the Central Oxoanion Size on the Voltammetric Properties of Keggin-Type [XW12O40]n− (n = 2−6) Complexes

An alpha-Keggin-type [ZnW(12)O(40)](6-) complex was prepared and structurally characterized. Unlike [XW(12)O(40)](n-) (n = 3-5) complexes that undergo one- and two-electron reductions in neutral and acidic media, respectively, [ZnW(12)O(40)](6-) showed a four-electron redox wave in acidified CH(3)CN. The present study demonstrated that the voltammetric properties of the Keggin anions were governed by the bond valence of the mu(4)-O-W bond as well as the ionic charge, and the four-electron behavior was ascribed to the increase of the bond valence (the decrease of the mu(4)-O-W distance), owing to the greater X-mu(4)-O distances. For the Keggin anions with identical ionic charge, the first one-electron redox wave was situated at more positive potentials with an increase of the bond valence. It turned out that the electron density on the W atom was decreased with an increase of the bond valence, because the positive shift of the one-electron wave parallels the downfield shift of the (183)W NMR chemical shift value.