A—Keggin structures

Several studies have addressed the cubic NLO properties of salts of heteropolymolybdate/ tungstate cluster anions.360,557-561 The tetraanion in a-H4SiW12O40,4HMPA,2H2O (HMPA= hexamethylphosphoramide) has a Keggin structure which has idealized Td point symmetry... 

Mo-heteropolyanions with a Keggin structure are readily reduced to give heteropoly blue in solution. A trend exists toward anions with X of higher charge (small value of n) being more easily reduced, while the reduction potential decreases in the order V > Mo > W. In the case of XMnWn04oH2", the Mn + in the anion is oxidized by O2. 

Figure 3 Top The Al,304(0H)24(H20)i2 ( q) tridecamer consists of AlO octahedra that share edges and a single, central, AIO4 tetrahedron in a Keggin structure (3). At the apices of the AlOg octahedra are twelve -OHj sites (t -0H2) that can undergo Bronsted acid-base reactions with the aqueous phase. There are two types of P2-0H sites and one type of ILI4-O site.

Figure 1. The a-Keggin structure. One face of the central XO4 tetrahedron is partly visible. (Image from Crystal Impact GbR, Bonn, Germany).

Figure 19-9. (A) Keggin structure of the tungstophosphoric anion a-[PWi2O40p shown in a bond filling representation. (B)-(D) represent ammonium tungstophoric acid salts precipitated under fixed temperatures and background salt concentrations (B) precipitated at 298 K with 1.0 M NaCl (C) precipitated at 368 K with no background salt (D) precipitated at 368 K with 1.0 M NaCl. By precipitating the salt under different temperatures and background salt concentrations, different morphologies were achieved (Shah, 2001).

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