Poly[[diaqua-bis-[μ(4)-5-nitro-isophthalato-κ(4)O(1):O(1):O(3):O(3')]bis-[μ(3)-pyridine-4-carboxyl-ato-κ(3)O:O':N]tricobalt(II)] tetra-hydrate].

The title compound, {[Co(3)(C(6)H(4)NO(2))(2)(C(8)H(3)NO(6))(2)(H(2)O)(2)]·4H(2)O}(n), exhibits a two-dimensional layer-like structure in which the Co(II) ions exhibit two kinds of coordination geometries. One nearly octa-hedral Co(II) ion with crystallographic inversion symmetry is coordinated to six carboxyl-ate O atoms from four bridging 5-nitro-isophthalate (NIPH) ligands and two isonicotinate (IN) anions, while the other type of Co(II) ion binds with one N atom and one carboxyl-ate O atom from two IN anions, two carboxyl-ate O atoms from two different NIPH anions and one ligated water mol-ecule, displaying a distorted square-pyramidal coordination geometry. Three adjacent Co(II) ions are bridged by six carboxyl-ate groups from four NIPH ligands and two IN anions to form a linear trinuclear secondary building unit (SBU). Every trinuclear SBU is linked to its nearest neighbours in the ab plane, resulting in a two-dimensional layer-like structure perpendicular to the c axis. Along the a-axis direction neighbouring mol-ecules are connected through carboxyl-ate and pyridyl units of the IN anions, along the b axis through carboxyl-ate groups of the NIPH ligands. The H atoms of one free water mol-ecule are disordered in the crystal in a 1:1 ratio. Typical O-H⋯O hydrogen bonds are observed in the lattice, which include the following contacts: (a) between coordinated water mol-ecules and carboxyl-ate O atoms of the NIPH anions, (b) between lattice water mol-ecules and carboxyl-ate O atoms of the NIPH anions, and (c) between coordinated and lattice water mol-ecules. These inter-molecular hydrogen bonds connect the two-dimensional layers to form a three-dimensional supra-molecular structure.