Crystal structure analysis, pseudosymmetry and spectroscopic data of K4[Fe(CN)5NO2] • KNO2 • H2O

K-4[Fe(CN)(5)NO2] . KNO2 . H2O crystallizes in the non-centrosymmetric orthorhombic space group Bb2(1)m (no. 36) with a = 2698.2(3), b = 1415.7(1), c = 849.6(1) pm and Z = 8 at 291 K. Due to a high degree of pseudosymmetry the deviations from centrosymmetry are small. The gross arrangement of the iron and potassium atoms is that of the AlB2-type, discernible from the distorted hexagons of the boron-analogous part with the composition FeK3 while further potassium atoms are replacing Al in agreement with the formulation K-2[FeK3]. The disorder of the free nitrite ions affiliated to KNO2 is characterized by means of additional data sets collected at temperatures of 173 K and 363 K. The results of the structure analysis are confirmed by infrared spectroscopy. Especially the deformation modes delta(O-N-O) of the free nitrite and those of the nitro ligand show the expected frequency splitting. Mossbauer data serve as a check on chemical composition and give an independent estimate for the mean square displacement of the iron (Fe-57) atoms comparable to the results from X-ray diffraction. The loss of crystal water above 400 K is registered using thermogravimetry and differential scanning calorimetry.