The supramolecular chemistry and crystal structures of five bis(imidazolium 2,6-pyridinedicarboxylate)M(II) trihydrate complexes, where M = Mn2+, Co2+, Ni2+ Cu2+, Or Zn2+ (1-5, respectively), are reported. These complexes serve as supramolecular building blocks that self-assemble when crystallized to generate a single, well-defined, predictable structure in the solid state. 2,6-Pyridinedicarboxylate anions and imidazolium cations form strong ionic hydrogen bonds that dominate crystal packing in compounds 1-5 by forming two-dimensional networks. or layers of molecules. This layer motif serves as a platform with which to control and predict molecular packing by design for engineering the structures of crystals. Moreover, compounds 1-5 create a robust organic host lattice that accommodates five different transition metals without significantly altering molecular packing. Growth of crystals from solutions that contain two or more different metal complexes produces mixed crystals in which mixtures of the different metal complexes are incorporated in the same relative molar ratio present in solution. Epitaxial growth of crystals from one metal complex on the surface of a seed crystal that contains a second metal complex generates composite crystals in which the different metal complexes are segregated into different regions of the crystals. Compounds 1-5 form crystalline solids that represent. a new class of modular materials in which the organic ligands serve as a structural component that defines a single packing arrangement that persists over a range of structures, and in which the metal serves as an interchangeable component with which to vary the physical properties of the material.