1A. Pictures of single crystals that were measured crystallographically:Compound 2: [Fe 3 ((µ 3 -O)(µ 2 -CH 3 COO) 6 (C 5 H 5 NO) 2 (H 2 O)] ClO 4 • 3H 2 O (C 5 H 5 NO) 2 (CH 3 OH)] ClO 4 • 3H 2 O Regenerated 1: [Fe 3 (µ 3 -O)(µ 2 -CH 3 COO) 6 (C 5 H 5 NO) 2 (H 2 O)] ClO 4 • 3H 2 OFigure S1. Photographs of the single crystals of compounds 1, 2 and Regenerated 1 S1 1B. Single crystal data collection: Crystal data for compounds 1, 2 and regenerated 1 were measured on a Bruker SMART APEX CCD area detector system [λ(Mo-Kα) = 0.71073 Å] at liquid nitrogen temperature, graphite monochromator with an ω scan width of 0.3 o , crystal-detector distance 60 mm, collimator 0.5 mm. Data reduction by SAINTPLUS (Software for the CCD Detector System, Bruker Analytical X-Ray Systems Inc., Madison, WI, 1998), an empirical absorption correction using equivalent reflections was performed with the program SADABS ( Sheldrick, G.

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When an aqueous solution of sodium molybdate is added to an aqueous solution of ferric chloride, acidified with acetic acid, a giant {MoFe} cluster is instantaneously formed as the amorphous substance Na[MoFeO(CHCOO)(OH)(HO)]·180 HO (1). Compound 1 consists of aggregated nanovesicles of {MoFe} clusters, as confirmed by field-emission scanning electron microscopy and transmission electron microscopy images of 1. An aqueous suspension of 1 upon moderate heating results in the formation of crystalline nanoferric molybdate, which gives insight into understanding the formation of a yellow coating mineral, ferrimolybdite, frequently found on the ores of molybdenum.

The colourless crystals of (PPh ) [PW O ]⋅3 C H NO (1) are converted to the dark blue crystals of {(PPh ) [PW O ]⋅3C H NO} {(PPh ) (C H NO) [PW W O ] ⋅2C H NO)} (2) upon irradiation with visible light in an interesting single crystal to single crystal transformation. This photochromic conversion is accompanied by the reduction of concerned Keggin anion from {PW } to {PW W }. This redox conversion is characterized by various spectroscopic techniques including single crystal X-ray diffraction studies. The photochromic properties of compound 1 can be controlled reversibly through the dimethylformamide (DMF) molecule as a function of temperature and proton exposure in a gas-solid reaction. The present work can be described as a new concept of programmable photochromism with the formation of photochromic pockets in crystalline 1 host (solid state), wherein a solvent can be plugged at a time to show light induced coloration.