Silver nitroprusside: Atypical coordination within the metal nitroprussides series

The nitroprusside complex anion forms an insoluble salt, of formula unit Ag₂[Fe(CN)₅NO], when it reacts with an aqueous solution of Ag⁺. Related to the high insolubility of silver halides, silver nitroprusside is a good intermediate to obtain soluble nitroprussides of alkaline, alkaline earth and trivalent transition metals. No other applications for that solid have been reported, probably because its crystal structure and related properties are unknown. This contribution reports the crystal structure for the titled compound from X-ray powder diffraction data and its refinement using the Rietveld method. It crystallizes with a monoclinic unit cell, in the Pc space group, with cell parameters: a = 7.4254(3), b = 6.4121(2), c = 11.8729(2) Å, β = 115.16(1)°. The crystal structure has two sites for the silver atom, corresponding to its coordination two and three to the N ends of CN groups. Calculation of the radial distribution function and IR and UV-Vis spectra provided the a priori structural information required to support the selected structural model to be refined. Such information was then used to validate the refined structure. The structure of this compound is atypical within the metal nitroprussides series. The thermal decomposition in air and nitrogen atmosphere of the title compound was studied from thermogravimetry and IR combined techniques. In the solid residues metallic silver and iron oxide (hematite) were identified.