Unique coordination in metal nitroprussides: The structure of Cu[Fe(CN)₅NO]·2H₂O and Cu[Fe(CN)₅NO]

The structures of copper pentacyanonitrosylferrate dihydrate, Cu[Fe(CN)₅NO]·2H₂O, and anhydrous, Cu[Fe(CN) ₅NO], have been determined from their XRD powder patterns and refined using the Rietveld method. The dihydrate structure was solved by direct methods and the anhydrous one was elucidated by chemical intuition. Cu[Fe(CN) ₅NO]·2H₂O is orthorhombic, space group Amm2 (38) and Z = 2. In this structure the iron is coordinated to five CN and a NO ligand, while the copper atom is coordinated, in a unique fashion, to four equatorial CN groups at N ends and two water molecules. Cu[Fe(CN)₅NO] is tetragonal, space group 14 mm (107) and Z = 2. This structure is obtained from the dehydration of the orthorhombic one; reordering allows the Cu atom to coordinate not only to the four equatorial cyanides but also to the axial cyanide. The coordination of the iron atom remains as in the orthorhombic structure. Agreement factor obtained from the final refinement were as follows: R_wp = 5.10 and R_B = 4.57 for the orthorhombic structure and R_wp = 6.52 and R_B = 7.82 for the tetragonal one. These crystal structures are also supported by IR and Mössbauer spectroscopic data and thermogravimetry.