Bis(2,2'-bipyridine-κN,N')(maleato-κO,O)nickel(II) 7.34-hydrate.

The title compound, [Ni(C(4)H(2)O(4))(C(10)H(8)N(2))(2)]·7.34H(2)O, was obtained by crystallization from an aqueous ethano-lic reaction mixture containing nickel(II) acetate, maleic acid, bipyridine, sodium hydroxide and ammonia. The asymmetric unit contains one independent complex mol-ecule and 7.34 water mol-ecules occupying eight crystallographically independent positions. Two of these water molecules are disordered. The nickel(II) atom is coordinated in a distorted octa-hedral geometry by two O atoms from one carboxyl-ate group of the maleato ligand and by four N atoms from two 2,2'-bipyridine (bipy) ligands. The water mol-ecules, along with the O atoms of the uncoordinated carboxyl-ate group, form an extended hydro-philic three-dimensional hydrogen-bonded system with large cavities in which the hydro-phobic bipy ligands are located. One H atom of the maleate ligand is involved in a weak hydrogen bond of the C-H⋯O type. Stacking inter-actions between the pyridyl rings of the bipy ligands [centroid-centroid distance = 3.549 (15) Å] lead to the formation of pairs of complex mol-ecules.