Crystal structure of guanidinium nitroprusside, [CN₃H₆]₂[Fe(CN)₅NO], and barium nitroprusside trihydrate, Ba[Fe(CN)₅NO] · 3H₂O

The crystal structure of [CN 3 H 6 ] 2 [Fe(CN) 5 NO] and Ba[Fe(CN) 5 -NO] · 3H 2 O has been determined at 293 K by X-ray diffraction methods. Guanidinium nitroprusside crystallizes in space group Pnma with a 1 = 8.943(1), a 2 = 10.240(2), a 3 = 16.143(2) Å, Z = 4, D m = 1.511(2), D c = 1.512 g/cm 3 . The space group of barium nitroprusside trihydrate is Pbcm with a 1 = 7.610(1), a 2 = 19.389(6), a 3 = 8.628(1) Å, Z = 4, D m = 2.121(2), D c = 2.13 g/cm 3 . Both structures have been solved by direct methods and refined by least-squares calculations to a final R -value of 0.036 with 1055 reflections for [CN 3 H 6 ] 2 [Fe(CN) 5 NO] and R = 0.017 with 1518 reflections for Ba[Fe(CN) 5 NO] · 3H 2 O. In both structures the bonding is mainly effectuated by dominant ionic interactions between nitroprusside anions and the corresponding cations. The barium ion is eightfold coordinated by 5 nitrogen atoms of CN-groups and 3 water molecules. There are no other contacts which range within the usually observed van der Waals distances. The Fe-NO-axes are aligned almost parallel and antiparallel to the a 3 -axis (guanidinium nitroprusside) and a 2 -axis (barium nitroprusside trihydrate).