In the centrosymmetric dinuclear complex anion of the title compound, (C5H8N3)2[Bi2(C7H3NO4)4(H2O)2]·4H2O, the BiIII atom is eight-coordinated in an N2O6 environment and has a distorted bicapped trigonal–prismatic coordination environment. Extensive inter­molecular O—H⋯O, N—H⋯O and weak C—H⋯O hydrogen bonds lead to the stability of the crystal structure. Inter­actions between one C—H group of the 2,3-diamino­pyridinium [(2,3-dapyH)+] cation and the aromatic ring of the pyridine-2,6-dicarboxyl­ate (pydc) ligand (C—H⋯centroid distance = 2.78 Å) and π–π inter­actions between the (2,3-dapyH)+ cations and between the (2,3-dapyH)+ cation and the pydc ligand [centroid–centroid distances = 3.489 (5) and 3.694 (5) Å] are observed.

Key indicators: single-crystal X-ray study; T = 293 K; mean (C-C) = 0.003 Å; R factor = 0.017; wR factor = 0.041; data-to-parameter ratio = 19.9.In the title compound, (C 3 H 12 N 2 )[Hg(C 7 H 3 NO 4 ) 2 ]Á2H 2 O, the Hg II ion is coordinated by four O and two N atoms of two pyridine-2,6-dicarboxylate (pydc) ligands in a distorted octahedral environment. The structure contains two uncoordinated water molecules. In the crystal, N-HÁ Á ÁO, O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds and -stacking interactions between the pyridine rings of the pydc ligands, with a centroid-centroid distance of 3.4582 (18) Å , stabilize the structure. Related literature

Key indicators: single-crystal X-ray study; T = 120 K; mean (C-C) = 0.003 Å; R factor = 0.028; wR factor = 0.075; data-to-parameter ratio = 14.4.In the title compound, (C 3 H 12 N 2 )[Cu(C 7 H 3 NO 4 ) 2 ]Á4H 2 O, the Cu II atom is six-coordinated in a distorted octahedral geometry by two tridentate pyridine-2,6-dicarboxylate (

Key indicators: single-crystal X-ray study; T = 298 K; mean (C-C) = 0.003 Å; R factor = 0.030; wR factor = 0.063; data-to-parameter ratio = 16.8.The reaction of cadmium nitrate dihydrate, propane-1,2-diamine and pyridine-2,6-dicarboxylic acid in a 1:1:2 molar ratio in an aqueous solution resulted in the formation of the title compound, (C 3 2 O (where p-1,2-da is propane-1,2-diamine and pydcH 2 is pyridine-2,6-dicarboxylic acid). The Cd II ion is coordinated by four O and two N atoms of two pydc ligands in a distorted octahedral environment. The structure also contains two uncoordinated water molecules. In the crystal, there are several intermolecular N-HÁ Á ÁO, O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds. There are also -stacking interactions between the pyridine rings of pydc units, with centroid-centroid distances of 3.4708 (16) Å . Related literature