Infrared spectra of the ammonium ion in crystals. Part 11.—Existence of low-temperature transitions in the cubic hexahalogenometallates(IV)(NH4)2MX6

Abstract

Infrared spectra of the NH3D+ probe ion in ten ammonium hexahalogenometallates(IV), (NH4)2MX6(MX = ReCl, OsCl, PtCl; PdBr, OsBr, PtBr, SnBr, TeBr; PtI, TeI) were studied at temperatures between 10 and 293 K. The temperature variation of the isotopically isolated ν1(NH3D+) and ν4bc(NH3D+) frequencies has revealed the existence of structural transitions in all the hexabromo and hexaiodo compounds. The hexachloro compounds are cubic over the entire 10–293 K range; the others are cubic above their (upper) transition temperatures with the exception of (NH4)2 TeI6, for which Ttr < 293 K. The probe-ion spectra at 10 K of the compounds undergoing transitions fall into two classes. The class containing the Pd, Os and Pt compounds is characterized by a 1:1:2 ν1 triplet indicative of an effective symmetry E=Cs of the ammonium ion, corresponding to site symmetries S=C2h or Cs. The class containing the Sn and Te compounds is characterized by a 1:1:1:1 ν1 quadruplet indicative of E=C1 and S=Ci or C1. The strength of hydrogen bonding increases on cooling through Ttr, but one of the highly-bent hydrogen bonds (which are symmetrically trifurcated in the cubic phases) appears to be retained while the others become normal (i.e. essentially straight) N—H…X hydrogen bonds, as shown by the values of the ν1 as well as the signs of the temperature coefficients of the ν1 and ν4bc frequencies.