Chains, planes, and tunnels of metal diphosphonates:: synthesis, structure, and characterization of Na3Co(O3PCH2PO3)(OH), Na3Mg(O3PCH2PO3)F•H2O, Na2Co(O3PCH2PO3)•H2O, NaCo2(O3PCH2CH2CH2PO3)(OH), and CO2(O3PCH2PO3)(H2O)

Four cobalt(II) and one magnesium diphosphonates with one-, two-, and three-dimensional structures have been synthesized and characterized structurally, thermally, magnetically and spectroscopically (IR and W-VIS). Isolated chains of corner-sharing octahedra (trans OH or F groups) are observed in the colorless Na3Co(O3PCH2PO3)(mu-OH) 1 and in the nearly isostructural Na3Mg(O3PCH2PO3)(mu-F) . H2O 2. Planes characterize the structures of the layered Na2Co(O3PCH2PO3) . H2O 3 with square-pyramidal cobalt, and of NaCo2(O3PCH2CH2CH2PO3)(mu(3)-OH) 4 with octahedral and trigonal-bipyramidal cobalt. Hydrophobic tunnels are found in Co-2(O3PCH2PO3)(H2O) 5. The diphosphonates co-ordinate as tetradentate ligands in 1 and 2, pentadentate ligands in 3, and hexadentate ligands in 4 and 5. Four of the six Co-O distances of the CoO6 octahedra in 1 are anomalously long. Remarkably, the compound is colorless, a quite unusual phenomenon for a cobalt(II) compound. Magnetic measurements showed Curie-Weiss behavior for 1, 3, 4, and 5, but also some unusual transitions at low temperatures for compound 1.