Redox behavior of cerium in heteropolyoxotungstate complexes

In situ XAFS spectroelectrochemistry is used to characterize the oxidation state and coordination environment of cerium in two different heteropolytun gstates, the Wells-Dawson [Ce(alpha-2-P2W17O61)(2)](17-) anion, and the Pre yssler [CeP5W30O110](12-) anion. Ce(III), coordinated to the Wells-Dawson f ramework, is oxidized at an applied potential of +0.37 V vs. Ag/AgCl. This is 1.15 V less than the Ce(IV)/Ce(III) standard reduction potential of +1.5 2 V vs. Ag/AgCl, demonstrating that Ce(IV) is stabilized in this heteropoly anion. In contrast, Ce(III) when encapsulated in the Preyssler anion is not oxidized at potentials higher than would be required for its oxidation in a non-complexing medium, indicating a stabilization of Ce(III). This is des pite the 12- charge on the anion. The different redox behavior of Ce is und erstood in terms of the different coordination environments afforded by the two heteropolyanions. The relative importance of saturative coordination v ersus electrostatic stabilization in the redox behavior of these two comple xes is discussed.