Studies with dithizone : the extraction of thallium (III)
Thallium(III) reacts with dithizone(I) in a complicated way through a number of intermediate products which depend on the relative proportions of reactants, the pH of the aqueous phase, the order of mixing of reactants, the time of phase equilibrium and the nature of the organic solvent used. The reaction between thallium(III) and dithizone is essentially oxidative in character. In the present work the complete reaction scheme is proposed: Thallium(III) reacts with dithizone to give a thallic-dithizonate complex, [TlIII(HDz)₃], which spontaneously and oxidatively disproportionates to the disulphide (III) and a thallium(I)-dithizonate complex, [Tl(HDz)]. The disulphide(III) decomposes by first-order heterolytic fission to give equimolar amounts of dithizone and the mesoionic compound (IV) . The Tl(HDz) complex undergoes reversion to give Tl+ in the aqueous phase and dithizone in the organic phase. In the presence of excess thallium(III) the regenerated dithizone is oxidized to the mesoionic compound (IV): the mesoionic compound (IV), partitions between the two phases and appears to react with excess thallium(III) to give possibly a mesoionic compound (IV)-thallium(III) complex and an unspecified oxidation product. The molecular structure of the 1 : 1 adduct formed between diphenylthallium( III) and dithizone has been shown by single crystal X-ray diffraction, to be a five co-ordinate diphenylthallium(III) complex: the dithizone acts as a bidentate chelating ligand co-ordinating through the nitrogen and sulphur atoms. Ethanol (solvent of recrystallisation) co-ordinates in the fifth position.
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