Keggin-like anions

Earlier electrochemical investigations had shown that reductions can proceed beyond the two-electron stage a recent example is the behavior of [S2Mo18062]4- which, like other molybdates, can be reduced by up to six electrons.141,427-430 Density functional theory calculations have been reported for Keggin anions treated as clathrates of X04 - in a neutral M12036 shell these provide satisfactory explanations for the relative stabilities of the a- and /3-structures (see Table 8) and for the first reduction steps of 2, 2, and 2 electrons.109,431 Several structure determinations of heteropoly blues have now been reported but, as expected, none can unambiguously locate the... 

Dilfuse reflectance spectra reported for a few compounds, generally, are similar to those obtained from aqueous solutions. Different spectra may be observed, as in the case of some anions with M = Co , in compounds with large cations, such as Et3NH+ and others. " These distinct spectra may be due to a particular arrangement of the structural features in the solid, like the structure with chain-linked Keggin anions found for the compounds with X = P,... 

The metal substituted polyoxotungstates are, like the lacunary and the parent Keggin anions, able to reversibly accept a certain number of electrons (up to six) on the tungsten atoms. Besides that, oxidation or reduction of the metal M may occur. Electrochemical techniques, like polarography " - and cyclic voltammetry, " have proved to be quite useful for the identification and characterisation of these anions. 

The layered double hydroxides (LDHs) architecture was also used to design 3D frameworks with large channels hosting the plenary Keggin HPA [142]. The material exhibited a 3D Ce(III) hydrotalcite-like structure constructed from 2D cationic [ Ce(H20)5 2 Ce(pdc)2(H20)4 Ce(pdc)3 ] " layers, where pdc = pyridine-2,6-dicarboxylate, pillared by [PWi204ol anions (see Fig. 3.22). 

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