STRONG MEDIUM AND COUNTERION EFFECTS UPON THE REDOX POTENTIAL OF THE 12-TUNGSTOCOBALTATE(III/II) COUPLE

The 12-tungstocobaltate(III/II) redox couple, both oxidation states being highly negatively charged (5-/6-), is strongly influenced by the nature of the counterion present. In particular, tetraalkylammonium ions decrease E-degrees[Co(III)/(II)] significantly, relative to that measured in the presence of alkali-metal ions in an organic solvent such as acetonitrile, thus causing large decreases in redox reactivity with respect to oxidation. An increasing proportion of water in the solvent increases E-degrees. The redox properties of the hexachloroiridate(IV)/(III) couple are affected by cations in a similar way. A new high-potential couple at ca. 1.8 V vs. the Ag/AgCl electrode was detected in the 12-tungstocobaltate system, presumably due to the 12-tungstocobaltate(IV/III) couple.