Poly[[diaqua­bis­[μ(4)-5-nitro­isophthalato-κ(4) O (1):O (1):O (3):O (3′)]bis­[μ(3)-pyridine-4-carboxyl­ato-κ(3) O:O′:N]tricobalt(II)] tetra­hydrate]

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 octa­hedral Co(II) ion with crystallographic inversion symmetry is coordinated to six carboxyl­ate O atoms from four bridging 5-nitro­isophthalate (NIPH) ligands and two isonicotinate (IN) anions, while the other type of Co(II) ion binds with one N atom and one carboxyl­ate O atom from two IN anions, two carboxyl­ate O atoms from two different NIPH anions and one ligated water mol­ecule, displaying a distorted square-pyramidal coordination geometry. Three adjacent Co(II) ions are bridged by six carboxyl­ate 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 mol­ecules are connected through carboxyl­ate and pyridyl units of the IN anions, along the b axis through carboxyl­ate groups of the NIPH ligands. The H atoms of one free water mol­ecule 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 mol­ecules and carboxyl­ate O atoms of the NIPH anions, (b) between lattice water mol­ecules and carboxyl­ate O atoms of the NIPH anions, and (c) between coordinated and lattice water mol­ecules. These inter­molecular hydrogen bonds connect the two-dimensional layers to form a three-dimensional supra­molecular structure.