The title compound, C(13)H(10)N(+)·C(7)H(4)NO(4) (-)·H(2)O or (acrH)(+)(pydcH)(-)·H(2)O, is a monohydrate of acridinium cations and a mono-deprotonated pyridine-2,6-dicarb-oxy-lic acid. The structure contains a range of non-covalent inter-actions, such as O-H⋯O, O-H⋯N and N-H⋯O hydrogen bonds, as well as π-π stacking [range of centroid-centroid distances = 3.4783 (5)-3.8059 (5) Å]. The N-H⋯O hydrogen bond between the donor acridinium cation and the carboxyl-ate acceptor is particularly strong. The average separation between the π-stacked acridinium planes is 3.42 (3) Å.

The title compound, (C(4)H(8)N(3)O)[Cr(C(7)H(3)NO(4))(2)]·C(7)H(5)NO(4)·6H(2)O, was obtained by the reaction of Cr(NO(3))(3)·9H(2)O with pyridine-2,6-dicarboxylic acid (pydcH(2)) and creatinine (creat) in aqueous solution (molar ratio 1:2:2). The cation is a protonated creatinine (creatH(+)) while the anion is a bis-pydc(2-) Cr(III) complex. The Cr(III) is coordinated by four oxygen and two nitro-gen atoms of two (pydc)(2-) groups and has a disorted octa-hedral coordination environment. The structure also contains a neutral mol-ecule of pydcH(2) that is hydrogen bonded to the creatH(+) and six mol-ecules of water. Extensive inter-molecular inter-actions, including seventeen classical hydrogen bonds, two weak C-H⋯O bonds, and C-O⋯π stacking inter-actions, with O⋯centroid distances of 3.211 (13) and 3.300 (12) Å, connect the various components in the crystal structure.

The title compound, (C(4)H(12)N(2))[Ce(2)(C(4)H(4)O(4)S)(4)(H(2)O)(2)]·3H(2)O, features a polymeric anion with a centrosymmetric Ce(2)O(2) core and a Ce⋯Ce distance of 4.3625 (4) Å. The anions form ribbons {[Ce(2)(C(4)H(4)O(4)S)(4)(H(2)O)(2)](2-)}(n) extending along [100]. The doubly protonated piperazinium cations reside on centers of inversion and link the polymeric ribbons via N-H⋯O hydrogen bonding. Each Ce(III) cation is ten-coordinated by an O(2)S donor set from two tridentate sulfanediyldiacetate (tda) ligands, one water mol-ecule and three other tda O donors from adjacent {Ce(tda)(2)(H(2)O)} units in a distorted bicapped cubic environment. Additional O-H⋯O hydrogen bonding involving the coordinated and solvent water mol-ecules is also present. H atoms of the crystal water molecules could not be located and were not included in the refinement.

The reaction of copper(II) nitrate hexa-hydrate with pyridine-2,6-dicarboxylic acid (pydcH(2)) and 2,4,6-triamino-1,3,5-triazine (melamine) in aqueous solution in a 1:2:2 molar ratio gave the title compound, (H(5)O(2))(C(3)H(7)N(6))(3)[Cu(C(7)H(3)NO(4))(2)](2)·C(7)H(5)NO(4)·6H(2)O. The hydroxonium hydrate (H(5)O(2))(+), also known as the Zundel cation, resides on a twofold rotation axis. The O-H distance is 1.274 (14) Å, the O⋯O distance is 2.518 (5) Å, and the O-H-O angle is 162 (8)°. One of the melamine H(+) cations, the uncoordinated pydcH(2), and two water mol-ecules also reside on crystallographic twofold axes. The Cu(II) atom has a tetra-gonally distorted octa-hedral coordination environment. The structure features extensive hydrogen bonding, with 21 distinct inter-actions. There is also a centrosymmetric C=O⋯π inter-action with an O⋯centroid distance of 3.288 (3) Å. The structure is similar to a mixed-valence manganese(II/III) structure but shows inter-esting differences in the metal-atom coordination. One of the water molecules is equally disordered with respect to a twofold axis.