A CYCLIC VOLTAMMETRIC STUDY OF MANGANO(II) UNDECATUNGSTOSILICATE - ANILLUSTRATIVE EXAMPLE OF THE REDUCTION, PROTONATION AND DISPROPORTIONATION PATHWAYS OF TRANSITION-METAL SUBSTITUTED HETEROPOLYTUNGSTATES IN AQUEOUS-SOLUTION

Mangano(II) undecatungstosilicate [(H2O)MnSiW11O39](6-) was studied by cyclic voltammetry in aqueous solution at various pH values. The pi-I -dependent wave patterns are influenced by additional processes and we re further investigated by variation of scan rates, substance concentr ation and buffer concentration, focussing on the first 2-electron redu ction. Polarographic data and results obtained from [(H2O)CoSiW11O39]( 6-) and [(H2O)(2)Mn-4(PW9O34)](10-) were included for comparison. A su itable pH range of 2.5-9 was determined from monitoring the decay of [ (H2O)MnSiW11O39](6-) by electronic spectroscopy over a period of sever al weeks. Around pH 5, the cyclic voltammetric wave patterns appear mo re distorted than at higher and lower pH values. Conditions were speci fied under which the reoxidation scans exhibit two species with differ ent degrees of protonation. The results were interpreted in terms of p rotonation and successive disproportionation of reduced species and su ggest that a complex network of reversible reaction pathways takes pla ce during the initial reduction processes of [(H2O)MnSiW11O39](6-) and related compounds in aqueous solution.