Increasing H2SO4 contents in disulfates cresting in the new hydrogenium-bis-hydrogendisulfate anion [H(HS2O7)2]−: Li2[S2O7]·H2SO4, Li[HS2O7], and Li[H(HS2O7)2]

In mixtures of H₂SO₄ and SO₃ or even in neat SO₃ a number of acidic disulfates could be prepared displaying unique hydrogen bond systems.

Hydrogenium-bis-hydrogensulfate anions adjacent to [S2O7]2− in Rb3[S2O7][H(HSO4)2]: a structural evidence of the increasing acidity of polysulfuric acids with growing chain length

Rb3[H(HSO4)2][S2O7] (monoclinic, C2/c (no. 15), a=1638.75(6), b=768.52(3), c=1187.88(4) pm, β=106.805(2)°, V=1432.14(9)·106 pm, Z=4) was obtained from the reaction of Rb2[CO3] (99.9%, not dried, 150 mg, 0.6 mmol) with neat SO3 in a torch-sealed glass ampoule. The remaining quantity of water in the starting material was obviously sufficient to lead to an H-containing substance. The structure shows the unprotonated disulfate anion [S2O7]2− adjacent to the hydrogenium-bis-hydrogensulfate anion [H(HSO4)2]− with the central hydrogen atom located close to a center of inversion.