ELECTRON-EXCHANGE REACTIONS BETWEEN HETEROPOLY ANIONS - COMPARISON OF EXPERIMENTAL RATE CONSTANTS WITH THEORETICALLY PREDICTED VALUES

This paper presents the first variable ionic strength study of the rates of intercomplex electron transfers between species wherein the electron to be exchanged is delocalized over numerous equivalent metal atoms. The complexes used were heteropoly tungstophosphates and their isomorphous heteropoly blue reduction products, the latter being class II mixed-valence species wherein the added “blue” electrons are delocalized over a dozen or more WO6units. Heteropoly complexes characteristically have extremely low interactions with solvent. Electron-exchange rates in solution were evaluated from 13P NMR data. In the case of the nearly spherical α-[PW12O40]3-exchanging with its heteropoly blue le reduction product, the experimental rates coincide very closely with those calculated from current theory of electron transfer in solution. These results lead to the general conclusion that extensive electron delocalization (over all 12 WO6octahedra) does not significantly contribute to activation energy for intercomplex electron exchange. In the case of the paramagnetic le blue reduction product α-[PW12O40]4-exchanging with its diamagnetic 2e-reduced isomorph, there is intrusion of an energy consideration for unpairing the delocalized electron to be exchanged. This significantly lowers the electron-exchange rate. Other cases, wherein geometric factors are expected to lower the rates, gave deviations in the expected direction and in the expected order. The exchanging pairs considered arc as follows; (1) α-[PW12O40]3-and its le heteropoly blue reduction product, (2) α-[PW12O40]4-and the corresponding 2c reduction product α-[PW12O40]5-, (3) α-[P2W18062]6-and its le blue reduction product, and (4) α-[PMo3W15O62]6-and its le blue reduction product. © 1990, American Chemical Society. All rights reserved.