About the Keggin isomers: Crystal structure of [N(C4H9)(4)](4)-gamma-[SiW12O40], the gamma-isomer of the Keggin ion. Synthesis and W-183 NMR characterization of the mixed gamma-[SiMo2W10O40](n-) (n=4 or 6)

The tetrabutylammonium gamma -dodecatungstosilicate has been crystallized i n a 6/1 acetonitrile/water solvent. An X-ray single-crystal analysis was ca rried out on [N(C4H9)(4)]-gamma-[SiW12O40] which crystallizes in the orthor hombic system, space group P2(1)2(1)2(1), with a 19.0861(3) Angstrom, b = 2 1.4435(3) Angstrom, c 26.0799(1) Angstrom, V = 10674.9(2) Angstrom (3), Z = 4, and rho (caled) = 2.392 g/cm(3). The idealized C-2 nu arrangement of th e anion results from the rotation of 60 degrees of two trigonal {W3O13} gro ups in the Keggin anion, Taking as reference the geometrical characteristic s of the Keggin anion, it appears that the bond lengths and bonds angles wi thin the four {W3O13} groups are not significantly modified while the mu -o xo junctions between the two rotated groups and those between the two unrot ated groups involve more acute and opened W-O-W angles, respectively. The s yntheses and W-183 NMR characterizations of the mixed gamma-[SiW10Mo2O40](n -) compounds corresponding to he oxidized (Mo-VI; n = 4) and to the two ele ctron-reduced (Mo-V; n = 6) anions are reported. Structural analysis by W-1 83 NMR has proved unambiguously that the C-2 nu structure Of the gamma-[SiW 10O36](8-) subunit is retained in both the compounds. The electronic behavi or of the series gamma-[SiW10M2E2O36](6-) (M = Mo or W; E = O or S) is exam ined, compared and related to 183W NMR data.