Does the compound hexaaqua-zinc(ii)bis(hydrogensulfate)dihydrate, [Zn(H2O)6](HSO4·H2O)2, really exist?

A careful examination of the crystal structure of the hydrogensulfate compound [Zn(H2O)6](HSO4·H2O)2 reported in this journal shows that the sample used for X-ray diffraction was almost certainly the Tutton salt [Zn(H2O)6](SO4·NH4)2, isoelectronic with the former elusive compound (F 000 = 416, P21/c space group). Indeed, any chemistry involving ammonium and sulfate moieties in an aqueous medium containing a transition metal cation should afford the corresponding Tutton salt as a by-product. We redetermined the structure of [Zn(H2O)6](SO4·NH4)2, on the basis of high-resolution X-ray data (d = 0.47 Å), with the purpose of illustrating that at such resolution, difference Fourier maps may be used to unambiguously differentiate between a sulfate and a hydrogensulfate ion. On the other hand, regardless of the data resolution, geometrical considerations may be enough to avoid misassignment of such small ions in crystal structures, providing that some knowledge about the average shape of these ions is available from curated crystallographic databases.