Study on the boundary structures in a series of lanthanide hexacyanoferrate(III) n-hydrates

Abstract

The properties of a series of lanthanide hexacyanoferrate(III) n-hydrates were studied by means of thermal analysis, IR spectroscopy, Raman spectroscopy and X-ray crystallography. Thermal analyses showed that there were two kinds of complexes in this series, Ln[Fe(CN)6]·5H2O (Ln=La–Nd) and Ln′[Fe(CN)6]·4H2O (Ln′=Sm–Lu). The boundary complex between them was Nd[Fe(CN)6]·5H2O. The IR spectra of the two kinds of complexes were obviously different. For the pentahydrates, there were two sharp CN stretching bands at 2050 and 2140 cm−1, and one band at 1600 cm−1 assigned to the HOH bending. On the other hand, for the tetrahydrates besides the two CN stretching bands at 2050 and 2140 cm−1, a new band was observed at 1940 cm−1, and the HOH bending band split into three bands around 1600 cm−1. From the X-ray crystal analysis, the structure of the boundary complex Nd[Fe(CN)6]·5H2O was determined. It belonged to hexagonal, P63/m, with a=7.467(2) A, c=13.793(3) A and Z=2 (R=0.082, Rw=0.126). Neodymium was nine-coordinated in the form of the NdN6(H2O)3 group. The three coordinated water molecules of the 5H2O complex with Nd have a large value for the equivalent isotropic thermal parameter. One of the three water molecules was dissociated easily and the 5H2O complex changed into the stable 4H2O complex with Nd. The crystal of the 4H2O complex is orthorhombic, and belongs to the space group Cmcm as well as the other Ln[Fe(CN)6]·4H2O (Ln=Sm–Lu). Therefore, the structure of Nd[Fe(CN)6]·5H2O is regarded as the boundary structure.