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.

Functionalization via covalent grafting of organic functions allows to tune the redox and acid-base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.

A convenient method for the systematic introduction of a variety of organoimido ligands at terminal oxo sites in the hexamolybdate cluster [Bu 4 N] 2 [Mo 6 O 19 ] is described, consisting of reaction with the appropriate organic isocyanate RNCO (R ) n-butyl, cyclohexyl, 2,6-diisopropylphenyl) in pyridine solution. Singly functionalized imido-hexamolybdates [Mo 6 O 18 (NR)] 2incorporating each of the above substituents are described. In the case of the 2,6-(diisopropyl)phenylimido ligand (NAr), multiple functionalization has also been achieved: hexamolybdate derivatives [Mo 6 O (19-x) (NAr) x ] 2incorporating two, three, four, and five NAr ligands at terminal sites have been prepared. The complexes have been characterized by cyclic voltammetry, multinuclear ( 1 H, 14 N, 17 O, and 95 Mo) NMR and electronic spectroscopy, and single-crystal X-ray diffraction studies.

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