Neutron Diffraction Study on Single Crystals of Strontium Pentacyanonitrosyl Ferrate(II) Tetrahydrate (Strontium Nitroprusside Tetrahydrate)

The crystal structure of strontium nitroprusside tetrahydrate, Sr[Fe(CN)5NO] · 4H2O, has been redetermined by neutron diffraction measurements on single crystals at room temperature (monoclinic, space groupC2/m, (No. 12)a= 20.07(2),b= 7.591(8),c= 8.397(9) Å, β = 98.7(5)°,V= 1260(4) Å3). Refinements of the structure using 867 observed structure factors converged to the finalR= 0.064. Results confirm the oxygen–water disorder found by the X-ray diffraction study; i.e., the oxygen atom of one water molecule is located on two different sites, corresponding to general positions within the asymmetric unit, with occupation factors of 2/3 and 1/3. An infinie hydrogen bond network links the strontium coordination polyhedra along the [010] direction. All sites occupied by hydrogen atoms of the only water molecule not coordinated to a strontium ion, and acceptors of hydrogen bonds in this network, could not be determined. This fact suggests that this water molecule is affected by structural disorder and that it has free unabridged hydrogens. The water molecule, which does not belong to the hydrogen-bond network, is disordered on the symmetry planem. Previous assignments of water IR absorption bands have been revised and improved on the basis of the new structural data.