A reinvestigation of Wilm's salt, (NH4)4[RhCl6]NO3: structure, spectroscopy and thermal analysis

The redetermination of the crystal structure of Wilm's salt, tetraammonium hexachlororhodate(III) nitrate, (NH 4 ) 4 [RhCl 6 ]NO 3 from single crystal data, necessitated a change in the space group symmetry description, as a consequence of comparative structure refinements, from the polar space group R3m to the chiral space group R32 for the room temperature structure. In contrast to the former structure determination it is shown, that in the correct space group a structure model can be derived, in which all parts of the structure - including the H atom positions of the ammonium ions - appear completely ordered on their sites. The ammonium cations and the hexachlororhodate as well as the nitrate anions form a three-dimensional framework via weak N-H...Cl and N-H...O hydrogen bonds. Reproducing the original synthesis conditions and analysing the products by powder diffraction methods, the byproducts have been identified as ammonium nitrate and diammonium aquapentachlororhodate(III). Doing DTA experiments on the title compound, two endothermic effects at 54 °C and 128 °C have been detected that may be related to phase transitions from the ordered room temperature structure to disordered high temperature sub structures. According to the results of thermo gravimetry the title compound decomposes at temperatures above 140 °C. The temperature dependant IR spectroscopy in the range of RT-60 °C it is probable that the above mentioned thermal effect at 54 °C is associated with an order disorder phase transition introduced by the ammonium cations. The UV/Vis spectroscopy shows that at 8 K a significant distortion of the [RhCl 6 ] 3- octahedron from highest 32 point symmetry is plausible.