Crystal structures of α-[CoIIW12O40]6- and its heteropoly blue 2e reduction product, α-[CoIIW12O40]8-. Structural, electronic, and chemical consequences of electron delocalization in a multiatom mixed-valence system

Abstract

This paper reports for the first time X-ray crystal structures for a parent heteropoly complex and for its heteropoly blue reduction product. The potassium salt of the former complex, α-[CoIIW12O40]6-, is ordered in space group P6,22; and the potassium salt of its two-electron heteropoly blue reduction product, α-[CoIIW12O40]8-, is disordered in space group Pm3m. The structures were refined to R(on F) = 0.034 and R(on F) = 0.044, respectively. Except for a shortening of each central CO-O,, distance, by 0.03 A, and a consequent corresponding increase in W-0,,, distances, the reduction caused remarkably little change in interatomic distances within the complex. Very slight lengthening of W-W distances between edge-sharing W06 octahedra, upon reduction, and a corresponding contribution toward slight shortening of the W-W distances through corner-sharing between WO6 groups, as suggested by previous data, were near the limits of statistical significance. On the other hand, the reduction apparently creates sizable new energy barriers to atomic displacements for both W's and Os. The displacement parameters for all of those atoms in the heteropoly complex decrease markedly, upon its reduction, while such parameters remain unchanged for all of the atoms not in the complex (H20s and K+s). The attendant significantly enhanced rigidity of the reduced complex is consistent with increased kinetic stability of heteropoly blue products to substitution and to degradation by base. The increased resistance to atom displacements in the heteropoly blue species may be seen as a consequence of the additional energy factor involved in maintaining favorable orbital overlaps for the delocalization of the added electrons. While the delocalization of the >blue> electrons presumably does involve thermal hopping of those electrons among W atoms, the greatly decreased displacement parameters, observed for the 0's as well as the Ws, suggest the importance of a ground-state delocalization mechanism involving partial >blue> electron residency in molecular orbitals that involve oxygen atoms. The increase in negative charge on bridging 0 atoms is consistent with their increased nucleophilicity and with the above-cited changes in CO-O,, and W-0,,, distances. Those changes imply that the central Co>04 tetrahedron is slightly more isolated from the rest of the W-0 framework in the reduced species. This is consistent with a pronounced decrease, upon reduction, in the magnitude of the large isotropic shift for the IE3W NMR signal.