Dithionic acid, H₂S₂O₆

   Dithionic acid is another sulfur oxoacid that is only known in aqueous solution (in which it behaves as a strong acid) or in the form of salts containing the dithionate, [S₂O₆]^2-, ion. Such salts can be isolated as crystalline solids and Figure 15.17a shows the presence of a long S^_S bond; the anion possesses a staggered conformation in the solid state. The dithionate ion can be prepared by controlled oxidation of [SO₃]^2- (equations 1.1 and 1.2), but not by the reduction of [SO₄]^2- (equation 1.3).

    The [S₂O₆]^2- can be isolated as the soluble salt BaS₂O₆, which is easily converted into salts of other cations.

                          (1.1)

                  (1.2)

Fig. 1.1(a) The structure of [S₂O₆]^2- showing the staggered conformation; from the salt [Zn{H₂NNHC)O(Me}₃][S₂O₆]. 2.5H₂O  [I.A. Krol et al. (1981) Koord. Khim., vol. 7, p. 800]; (b) the C₂ structure of gas-phase H₂SO₄. Colour code: S, yellow; O, red; H, white.

         (1.3)

    The [S₂O₆]^2_ ion is not easily oxidized or reduced, but in acidic solution it slowly decomposes according to equation 1.4, consistent with there being a weak S^_S bond.

                                                                  (1.4)