A systematic study of the cation-size effect on the vibrational spectra of a-and ß-^ ^"" polyoxoanions has been performed (X = Bnl, SiIV, GeIV, Pv, Asv; M = MoVI, WVI). The M-Od stretching frequencies decrease as the cation size increases. This is attributed to a weakening of anion-anion interactions of the electrostatic type. These interactions vanish for only interanionic oxygen-oxygen distances as long as 6 Á (tetrabutylammonium (TBA) salts). For these polyanions, the approximation for the isolated anion strictly holds with large counterions such as TBA. In addition, some preparations of polyanions are described for the first time.

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