Compared Behaviours Of Dawson-Type Tungstodiarsenates And -Diphosphates

The lacunary alpha(2)-[As2W17O61](10-) derived from the Dawson molecule alpha-[As2W18O62](6-) was used as a ligand for the following metal cations: Mn2+, Fe3+, CO2+, Ni2+, Cu2+ and Zn2+. The cyclic voltammograms of this lacunary precursor species, as well as those of the substituted complexes were run in a medium at pH = 3. A systematic comparison was carried out with the electrochemical characteristics of the corresponding complexes derived from alpha-[P2W18O62](6-), with the aim of highlighting the differences in their behaviour, which could be ascribed to the identity of the central atoms. The study was also extended to the Fe3+ species of the alpha(1) series. The comparisons reveal that whatever the series, the first several voltammetric waves are driven in the positive potential direction by the presence of As as the central atom in the heteropolyanions. These results agree with those from crystallographic evaluations, and with the knowledge of the stability constants of the complexes, indirectly reveal a coherent picture of the behaviour of the vacant sites in the diarsenates and diphosphates. In particular, the influence of the central heteroatom on the redox and/or acid-base properties could be assessed and compared. Steric effects, expected on geometrical grounds, and already reflected in the values of the stability constants of the metal ion substituted compounds, also influence the cyclic voltammograms.