Crystal structure of orthorhombic ferrous nitroprusside: Fe[Fe(CN) ₅NO].2H₂O

Ferrous nitroprusside can be obtained in three structural modifications: two different unstable phases, monoclinic trihydrate and cubic pentahydrate, and the stable one, an orthorhombic dihydrate. This contribution reports the crystal structure of the last one. Cell parameters are: a=13.9734(2), b=7.4274(1), and c=10.4697(1)Å; with four formula units per cell (Z=4). The crystal structure was refined from the corresponding XRD powder pattern using the Rietveld method. Final agreement factors of the refinement process were Rwp=8.46, Rp=6.54, and S=1.38. The crystal structure is formed by a tridimensional assembling of the [Fe(CN)5NO] molecular block through iron atoms bounded at the N end of the CN ligands. The NO group remains unlinked at its O atom. The octahedral coordination of the assembling metal is completed with a coordinated water molecule which stabilizes a second water through a strong hydrogen bond interaction. The tridimensional structure appears as piled up rippled sheets leading to a system of interconnected small cavities which increase their available volume on the material dehydration. This complex loses its crystal water below 100°C and then remains stable up to above 160°C when the decomposition process begins with the loss of the NO ligand.