In the title hydrate, (C4H12N2)[Co{B6O7(OH)6}2]·6H2O, both the dication and dianion are generated by crystallographic inversion symmetry. The Co2+ ion in the dianion adopts a fairly regular CoO6 octa­hedral coordination geometry arising from the two O,O′,O′′-tridentate ligands. In the crystal, the dianions and water mol­ecules are linked by O—H⋯O hydrogen bonds, generating a framework with large [100] channels, which are occupied by the organic dications. N—H⋯O and C—H⋯O hydrogen bonds consolidate the structure.

In the title organic–inorganic hybrid compound, (C5H7N2)2[ReVW4O19]·7H2O, the Lindqvist-type polyoxido anion has crystallographically imposed mm2 symmetry and is built up by six MO6 (M = W, V, Re) edge-sharing distorted octa­hedra. The Re and V atoms share the same crystallographic site in a 0.5:0.5 ratio. The 4-amino­pyridinium cations lie on a mirror plane. Three of the four independent water O atoms in the asymmetric unit are located on a mirror plane whereas the remaining O atom has mm2 site symmetry. In the crystal, the cations, anions and water mol­ecules are linked into a three-dimensional network through O—H⋯O and N—H⋯O hydrogen-bonding inter­actions.

In the title hydrated salt, [Co(C12H8N2)3]2[PVW11O40]·2H2O, the complete Kegggin ion is generated by crystallographic inversion symmetry, which imposes statistical disorder on the O atoms of its central PO4 group. The V atom is statistically disordered over all the metal sites of the anion. In the cation, the Co2+ ion is coordinated by three bidentate 1,10-phenanthroline (phen) ligands, generating a distorted CoN6 octa­hedron. Possible very weak intra­molecular C—H⋯π inter­actions occur in the cation. In the crystal, the components are linked by O—H⋯O and C—H⋯O inter­actions, building a three-dimensional network featuring one-dimensional voids along the c-axis direction.

In the title compound, (C9H8N)4[Te2V8O28]·8H2O, the com­plete heteropolyanion is generated by a crystallographic inversion centre. One of the two quniolinium ions forms an N—H⋯Op (p = polyoxidometallate) hydrogen bond and the other an N—H⋯Ow (w = water) hydrogen bond. The water mol­ecules further link the components by O—H⋯Op and O—H⋯Ow hydrogen bonds. A number of C—H⋯O inter­actions and aromatic π–π stacking inter­actions [shortest centroid–centroid separation = 3.541 (7) Å] are also observed. Together, these generate a three-dimensional network.

The asymmetric unit of the title salt, 2C 6 H 5 CH 2 NH 3 + ·H 2 P 2 O 7 2− , contains two independent benzyl­ammonium cations and a di­hydrogen diphosphate dianion. In the crystal, O—H⋯O and N—H⋯O hydrogen bonds link the cations and anions, forming a two-dimensional network parallel to (010). Within this network, weak C—H⋯O hydrogen bonds are observed.