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 UV-VIS). Isolated chains of corner-sharing octahedra (trans OH or F groups) are observed in the colorless Na₃Co(O₃PCH₂PO₃)(µ-OH) 1 and in the nearly isostructural Na₃Mg(O₃PCH₂PO₃)(µ-F)·H₂O 2. Planes characterize the structures of the layered Na₂Co(O₃PCH₂PO₃)·H₂O 3 with square-pyramidal cobalt, and of NaCo₂(O₃PCH₂CH₂CH₂PO₃)(µ₃-OH) 4 with octahedral and trigonal-bipyramidal cobalt. Hydrophobic tunnels are found in Co₂(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.