Group 2 metal salts of pyromellitic acid: [Mg(H2O)6](C10H4O8) and [Ba(C10H4O8)(H2O)5].

Structural determinations of the magnesium(II) and barium(II) salts of pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid) are presented. Hexaaquamagnesium(II) benzene-1,2,4,5-tetracarboxylate(2-), [Mg(H(2)O)(6)](C(10)H(4)O(8)), (I), and pentaaqua[benzene-1,2,4,5-tetracarboxylato(2-)]barium(II), [Ba(C(10)H(4)O(8))(H(2)O)(5)], (II), are both centrosymmetric and both possess a 1:1 metal-ligand ratio, but the two structures are found to differ in that the magnesium salt contains a hexaaqua cation and possesses only hydrogen-bonding interactions between cations and anions, while the barium salt exhibits coordination of the carboxylate ligand to the nine-coordinate metal centre. In (I), both ions sit on a 2/m site symmetry, and in (II), the cation and anion are located on m and i site symmetries, respectively.