Poly[[di-mu2-aqua-di-mu5-croconato(2-)-nickel(II)dipotassium(I)] tetrahydrate].

In the title compound, [[K2Ni(C5O5)2(H2O)2].4H2O]n, the Ni atom lies on an inversion centre. Two inversion-related croconate [4,5-dihydroxy-4-cyclopentene-1,2,3-trionate(2-)] ligands and an Ni(II) ion form a near-planar symmetrical [Ni(C5O5)2]2- moiety. The near-square coordination centre of the moiety is then extended to an octahedral core by vertically bonding two water molecules in the [Ni(C5O5)2(H2O)2]2- coordination anion. The crystal structure is characterized by a three-dimensional network, involving strong K...O...K binding, K...O-Ni binding and hydrogen bonding.