Photochromic Molecular Material Based on Anderson–Evans Polyoxometalate $$\hbox{[}\hbox{Cr}\hbox{(OH)}_{6}\hbox{Mo}_{6}\hbox{O}_{18}\hbox{]}^{3-}$$ and Ruthenium Mononitrosyl Complexes $$\hbox{[}\hbox{RuNO}\hbox{(}\hbox{NH}_{3}\hbox{)}_{4}\hbox{(X)}\hbox{]}^{n+}\hbox{(}\hbox{X}=\hbox{NH}_{3}, \hbox{OH}; n=3, 2\hbox{)}$$

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The synthesis and characterization of two novel complexes formed by the photochromic mononitrosyl cations \([\hbox{RuNO}(\hbox{NH}_{3})_{5}]^{3+}\) and \([\hbox{RuNO}(\hbox{NH}_{3})_{4}\hbox{OH}]^{2+}\), and the paramagnetic polyoxometalate (POM) anion \([\hbox{Cr}(\hbox{OH})_{6}\hbox{Mo}_{6}\hbox{O}_{18}]^{3-}\) are reported. Compound \([\hbox{RuNO}(\hbox{NH}_{3})_{4}\hbox{OH}]_{3}[\hbox{Cr}(\hbox{OH})_{6} \hbox{Mo}_{6}\hbox{O}_{18}]_{2}\cdot15\hbox{H}_{2}\hbox{O}\) (1) was characterized by X-ray diffraction, it crystallizes in the triclinic space group, P −1, a = 10.5858(2), b = 11.0407(2), c = 16.8077(4) A, \(\alpha=76.9192(7), \beta = 73.7736(7), \gamma = 84.090(1)^{\circ}\), R=0.0636 for 16046 reflections with I \( > 2 \sigma\) (I). Compound 1 and compound \([\hbox{RuNO}(\hbox{NH}_{3})_{5}][\hbox{Cr}(\hbox{OH})_{6}\hbox{Mo} _{6}\hbox{O}_{18}]\cdot8\hbox{H}_{2}\hbox{O}\), (2) are paramagnetic in the whole temperature range 2–300 K. The metastable state (MSI) was detected in 1 by differential scanning calorimetry (DSC). The decay curve shows two peaks in the heat release at about 245 and 265 K with activation energies and frequencies values equal to 0.72 eV and 3 × 1011 s−1 and 0.67 eV and 7 × 1011 s−1 for the high- and low-temperature peak, respectively [see F. Bottomley (J. Chem. Soc. Dalton Trans. 1600, 1997)].