A REINVESTIGATION OF ISOMERISM IN THE DAWSON STRUCTURE - SYNTHESES AND W-183 NMR STRUCTURAL CHARACTERIZATION OF 3 NEW POLYOXOTUNGSTATES [X2W18O62]6- (X=P-V, AS-V)

Three new Dawson-type heteropolyoxotungstates beta- and gamma-[As2W18O62]6- and gamma-[P2W18O62]6- have been prepared and identified. A comparative study of these three compounds and of the four other already known [X2W18O62]6-anions (alpha and beta for X=P(V), and alpha and gamma* for X=As(V)) has been performed by spectroscopic (IR, Raman, UV-Vis, W-183 and, eventually, P-31 NMR), polarographic and kinetic measurements, and some erroneous literature assignments have been rectified. The stabilities of the different isomers are discussed in relation with their conditions of formation: the isomerization proceeds along the path gamma-->beta-->alpha; the relatively high stability of the gamma* anion is also discussed on the basis of the geometrical features of the alpha- and beta-XW9 half-anion. A rational nomenclature is proposed for the staggered belt junction with respect to the eclipsed one by adding a star, as in gamma* - [As2W18O62]6-. W-183 solution NMR easily distinguishes alpha-XW9 (DELTAdelta about 40 ppm for X = P and 20 ppm for X = As; 2J(W-W) greater than 21 Hz) and beta-XW9 (DELTAdelta about 80 ppm for X = P and 55 ppm for X = As; 2J(W-W less than 21 Hz).