Two new copper(I)-rhenate(VII) hybrid solids, Cu(bpy)ReO4(I) and Cu(bpy)(2)ReO4 center dot 0.5H(2)O (II) (bpy = 4,4'-bipyridine), with 2-fold and 4-fold interpenetrating networks, respectively, were prepared from hydrothermal reactions, and their structures characterized by single-crystal X-ray diffraction [I, Pbca (No. 61), Z=8, a = 10.8513(3) angstrom, b = 12.9419(4) angstrom, c = 15.6976(5) angstrom; II, P (1) over bar (No. 2), Z= 2, a = 11.8190(4) angstrom, b = 12.6741(4) angstrom, c = 13.7585(5) angstrom, alpha = 85.8653(13)degrees, beta = 81.6197(13)degrees, gamma = 84.0945(11)degrees]. The structure of I contains 6(3) nets of neutral CuReO4 layers that are pillared via bpy ligands on the Cu sites {CuO3N2] to yield a 2-fold interpenetrating pillared-layered network. Conversely, the structure of II consists of a 4-fold interpenetrating diamond-type network with tetrahedral {CuN4} coordination nodes that are bridged by bpy ligands, with both H2O and ReO4- within the pores. A surprising reversible structural interconversion between these two interpenetrating structures is possible via the insertion and removal of a single bpy ligand and 1/2H(2)O per copper atom. The structural interconversion is accompanied by a change in color from yellow to red for I and II, respectively. Measured UV-vis diffuse reflectance spectra exhibit a significant red-shift in the absorption edge of similar to 0.3 eV, with the optical bandgap size decreasing from similar to 2.5 eV to similar to 2.2 eV for I and II, respectively. X-ray photoelectron spectra and electronic structure calculations indicate that the valence band derived from the Cu 3d and N 2p orbitals in II are pushed higher in energy compared to those in I because of the coordination of the additional bpy ligand. There is a much smaller change in the energy of the conduction band that is derived from the Re 5d orbitals. These results demonstrate that the ligand-mediated structural transformations of (d(0)/d(10))-hybrid solids represent a new and convenient low-temperature approach to modulate their optical bandgap sizes toward the visible wavelengths for use with solar energy.