Dehydration of Octacyanido-Bridged NiII-WIV Framework toward Negative Thermal Expansion and Magneto-Colorimetric Switching

An inorganic three-dimensional [Ni-II(H2O)(2)](2) [W-IV(CN)8]center dot 4H(2)O (1) framework undergoes a single-crystal-to-single-crystal transformation upon thermal dehydration, producing a fully anhydrous phase Ni-2(II)[W-IV(CN)(8)] (1d). The dehydration process induces changes in optical, magnetic, and thermal expansion properties. While 1 reveals typical positive thermal expansion of the crystal lattice, greenish-yellow color, and paramagnetic behavior, id is the first ever reported octacyanido-based solid revealing negative thermal expansion, also exhibiting a deep red color and diamagnetism. Such drastic shift in the physical properties is explained by the removal of water molecules, leaving the exclusively cyanido-bridged bimetallic network, which is accompanied by the transformation of the octahedral paramagnetic [Ni-II(H2O)(2)(NC)(4)](2-) to the square-planar diamagnetic [NiII(NC)(4)](2-) moieties.