Hexaaquacobalt(II) and hexaaquanickel(II) bis(μ-pyridine-2,6-dicarboxylato)bis[(pyridine-2,6-dicarboxylato)bismuthate(III)] dihydrate.

The title complexes, hexa­aqua­cobalt(II) bis­(μ-pyridine-2,6-dicarboxyl­ato)bis­[(pyridine-2,6-dicarboxyl­ato)bismuthate(III)] dihydrate, [Co(H2O)6][Bi2(C7H4NO4)4]·2H2O, (I), and hexa­aqua­nickel(II) bis­(μ-pyridine-2,6-dicarboxyl­ato)bis[(pyridine-2,6-dicarboxyl­ato)bismuthate(III)] dihydrate, [Ni(H2O)6][Bi2(C7H4NO4)4]·2H2O, (II), are isomorphous and crystallize in the triclinic space group P\overline{1}. The transition metal ions are located on the inversion centre and adopt slightly distorted MO6 (M = Co or Ni) octa­hedral geometries. Two [Bi(pydc)2]− units (pydc is pyridine-2,6-dicarboxyl­ate) are linked via bridging carboxyl­ate groups into centrosymmetric [Bi2(pydc)4]2− dianions. The crystal packing reveals that the [M(H2O)6]2+ cations, [Bi2(pydc)4]2− anions and solvent water mol­ecules form multiple hydrogen bonds to generate a supra­molecular three-dimensional network. The formation of secondary Bi⋯O bonds between adjacent [Bi2(pydc)4]2− dimers provides an additional supra­molecular synthon that directs and facilitates the crystal packing of both (I) and (II).