catena-Poly[[[di-μ(2)-hydroxido-bis-[(di-2-pyridylamine)nickel(II)]]-μ-fumarato] dihydrate].

The Ni(II) ion in the one-dimensional title complex, {[Ni(2)(C(4)H(2)O(4))(OH)(2)(C(10)H(9)N(3))(2)]·2H(2)O}(n), has a distorted square-pyramidal coordination environment formed by three O atoms from two bridging hydroxide groups and one carboxyl-ate group of the fumarate ligand and two pyridine N atoms from a di-2-pyridylamine (dpa) ligand. Two hydroxide groups link adjacent metal centers, forming a centrosymmetric four-membered [Ni(2)(OH)(2)] ring. In the crystal structure, the H atoms of the bridging hydroxide groups form inter-molecular hydrogen bonds to both water mol-ecules. These are further linked to the uncoordinated O atoms of the carboxyl-ate groups and the NH group of a dpa ligand to generate a three-dimensional network from the chains of the coordination polymer.