Mercury(II) nitroprusside: A framework with an unusual topology

The titled compound was prepared by the precipitation method from diluted aqueous solution of sodium nitroprusside andmercury(II) nitrate. The orange solid formed, with formula unit Hg[Fe(CN)₅NO], crystallizes with an orthorhombic unit cell in the Pmna space group with cell parameters: a = 11.2788(3), b = 6.1965(3), and c = 12.3786(6) A˚. The unit cell accommodates four formula of the compound (Z = 4). Its crystal structure was solved from X-ray powder patterns and then refined by the Rietveld method. The material framework is formed by tetrahedral coordination of Hg atoms at the N end of the equatorial CN groups of the [Fe(CN)₅NO] building block. That framework results from the interpenetration of two identical sub-frameworks with a relative shift of (a/2, b/2, c/2). The sub-framework has two types of cavities, ellipsoidal and rhombohedral, with transversal section of ca. 4.5 x 9.2 A˚ and ca. 8.5 A˚ transversal section, respectively. That system of cavities results eclipsed by the relative shift of neighboring sub-frameworks. No transport of H₂ and N₂ molecules through the material framework was observed. The thermal decomposition also reveals limitation for the decomposition products diffusion through the practically compact structure. The structural study was complemented with TG, IR, UV-vis and N₂ and H₂ adsorption data. Neighboring Hg atoms are distant 4.54 (3) A˚, a relatively large distance to suppose the existence of metal-metal interaction. No previous study on the crystal structure and related properties of mercury(II) nitroprusside has been reported.