Reactions of N-methyliminobis(methylenephosphonic acid), CH3N(CH2PO3H2)(2) (H4L), with divalent metal acetates under different conditions result in metal diphosphonates with different structures. Mn(H3L)(2).2H(2)O (complex 1) with a layer structure was prepared by a layering technique. It is triclinic, P(1) over bar, with a = 9.224(3) Angstrom, b = 9.780(3) Angstrom, c = 10.554(3) Angstrom, alpha = 82.009(6)degrees, beta = 74.356(6)degrees, gamma = 89.853(6)degrees, Z = 2, The Mn(II) ion is octahedrally coordinated by six phosphonate oxygen atoms from four ligands, two of them in a bidentate and two in a unidentate fashion, Each MnO6 octahedron is further linked to four neighboring MnO6 octahedra through four bridging phosphonate groups, resulting in a two-dimensional metal phosphonate (002) layer. These layers are held together by strong hydrogen bonds between uncoordinated phosphonate oxygen atoms. The zinc complex Zn-3(HL)(2) (complex 2) was synthesized by hydrothermal reactions (4 days, 438 K, autogenous pressure). It is monoclinic, P2(1)/n with a = 7.7788(g) Angstrom, b = 17.025(2) Angstrom, c = 13.041(2) Angstrom, beta = 94.597(2)degrees, Z = 4. The structure of complex 2 features a 3D network built from ZnO4 tetrahedra linked together by bridging phosphonate groups. Each zinc cation is tetrahedrally coordinated by four phosphonate oxygen atoms from four ligands, each of which connects with six zinc atoms, resulting in voids of various sizes. Magnetic measurements for the manganese complex shows an antiferromagnetic interaction at low temperature. The effect of the extent of deprotonation of phosphonic acids on the type of complex formed is discussed.