Zoledronate complexes. II. catena-Poly[[tetraaquabis[hemihydrogen mu3-1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonato-kappa3O:O':O'']bis[mu3-1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonato-kappa4O:O,O':O'']trisodium] dihydrate].

The title compound, {[Na(3)(C(5)H(9)N(2)O(7)P(2))(2)(C(5)H(9.5)N(2)O(7)P(2))(2)(H(2)O)(4)].2H(2)O}(n), (II), is polymeric and consists of undulating chains parallel to [011] interconnected by hydrogen-bonding and pi-pi interactions. There are two independent Na(+) cations in the asymmetric unit (one lying on an inversion centre), two zoledronate anions and three water molecules, two of which are coordinated and one of which is a free solvate. Each cation is surrounded in an octahedral fashion by O atoms from four different zoledronate units and two/one coordinated water molecules. The zoledronate groups present their usual zwitterionic character, with negative charges in the protonated phosphonates and a positive charge at the protonated imidazole N atom. Two symmetry-related phosphonate groups share (in the form of a very strong linear hydrogen bond) an H atom lying on a symmetry centre, midway between the O atoms involved. The zoledronate binding modes present in (II) are both unreported for bisphosphonate anions. Intra- and inter-chain interactions are enhanced by a variety of hydrogen bonds where all the available O-H and N-H donors are involved, in addition to a strong imidazole-phosphonate C-H...O interaction, typical in these kinds of structures.