Tc(IV) chemistry in mixed chloride/sulphate acidic media. Formation of polyoxopolymetallic species

The substitution of the Cl- ions of TcCl62- in various media (Cl-, SO42-, TFMS-) at various pH or the electroreduction of TcO4- in the same media led to a common Tc(IV) species characterized by an UV-visible absorption band at 500 nm. This band was present in the spectra of Tc(IV) in all the media investigated, except in chloride at low pH. The analysis of the spectroscopic data showed that it can be attributed to a Tc(IV) oxopolymeric species. The data fits better with a trimeric species Tc3O44+ which reminds the Mo3O4.9H(2)O(4+) clusters known for Mo(IV). The stability of 3 x 10(-5) M Tc3O44+ solutions has been investigated as a function of pH with respect to the formation of Cl- complexes at low pH, and precipitation of TcO2 at pH, higher than 3. The anions of the media interact with Tc3O44+ clusters as shown by small changes in their UV-visible spectra. We observed that the affinity of Tc3O44+ for these ions is in the order: SO42- > Cl- > TFMS-. Neutral species Tc3O4(SO4)(2) are observed in 0.1 M sulphate solutions.