Photochromic compounds have gained intense interest owing to their novel photochemical and photophysical reactions as well as their various potential applications such as in sunglasses and optical memories.[1] An appreciable number of photochromic families have been reported to date; [2] however, those based on an electron-transfer (redox) chemical process, especially for metal-organic compounds, are rare.[3] We present herein a hydrothermally synthesized 3D complex [Cd 2 (ic)(mc)(4,4'-bipy) 3 ] n ·4n H 2 O (1Y; ic= itaconate, mc = mesaconate) which exhibits reversible radicalbased redox photochromic behavior. This compound also merits attention for its unusual five-connected Archimedeantype topological structure and unexpected "capture" of isomeric ic and mc ligands that are generated in situ from trans-propene-1,2,3-tricarboxylic acid (H 3 L, Scheme 1).Yellow prismatic crystals of 1Y accompanied by a few colorless block crystals of [Cd 3 L 2 (H 2 O) 6 ] n (2; Figure S1 in the Supporting Information) were obtained by the hydrothermal reaction of CdCO 3 , 4,4'-bipy, H 3 L, and deionized water in a molar ratio of 1:1:1:1111 at 120 8C for 12 h. Powder X-ray diffraction (PXRD) verified the purity of a mechanically separated sample of 1Y ( Figure S2 in the Supporting Information). Thermal analysis and PXRD data show that compound 1Y still keeps its metal-organic framework at 200 8C after the elimination of lattice water molecules ( Figure S2 and S3 in the Supporting Information).A single-crystal X-ray diffraction study of 1Y revealed a 3D structure based on a network of seven-coordinated Cd II centers linked by three kinds of bridging ligands. As shown in Figure 1, the central Cd1 atom is located in a pentagonalbipyramidal coordination environment with N41 and N42 atoms of crystallographically independent 4,4'-bipy ligands acting as the two pyramidal apexes. An important feature of 1Y is the simultaneous "capture" of isomeric ic and mc ligands that originate from the in situ pyrolysis of H 3 L. [4]