Bis(6′-carb­oxy-2,2′-bipyridine-6-carboxyl­ato-κ(3) N,N′,O (6))nickel(II) tetra­hydrate

In the title compound, [Ni(C(12)H(7)N(2)O(4))(2)]·4H(2)O, the Ni atom is located at the centre of a distorted octa­hedron, formed by four N atoms and two O atoms from the same two tridentating chelated 6-carb­oxy-2,2′-bipyridine-6′-carboxyl­ate (L) ligands. Face-to-face π-stacking inter­actions between inversion-related pyridine rings with centroid–centroid distances of 3.548 (3) and 3.662 (3) Å (perpendicular distances between the respective rings are 3.314 and 3.438 Å) are found. Inter­molecular O—H⋯O hydrogen bonds between water mol­ecules and L ligands form R (5) (3)(10), R (6) (5)(14) and R (5) (5)(12) rings and also a centrosymmetric cage-like unit of water mol­ecules, which link eight adjacent Ni(II) centers, forming a three-dimensional framework.