Far-infrared and Raman spectra of crystalline tetra-alkylammonium trihalogenomercurate(II) salts. Evidence for monomeric or halogen-bridged associated anions depending on the cation

Abstract

A study has been made of the far-infrared (20–400 cm^–1) and Raman spectra of eight compounds of type [NR₄][HgX₃](R = Me, Et, Prⁿ, or Buⁿ; X = Cl, Br, or I) and of [SMe₃][HgI₃] in the solid state. Solution-phase Raman data have also been obtained. For R = Et and X = Cl two crystalline modifications have been identified. The low-temperature α form has vibrational spectra explicable in terms of an essentially monomeric anionic structure, whereas the β form, stable above 71 °C, clearly contains strong halide-bridged groups. The structures of the other compounds have been deduced by comparison with the data on α- and β-[NEt₄][HgCl₃]. Thus associated anionic structures are present also in [NEt₄][HgX₃](X = Br or I), [NPrⁿ ₄][HgCl₃], and [NBuⁿ ₄][HgI₃]. Evidently the weak halogen bridging found in the crystal structures of [SMe₃][HgI₃] and [NMe₄][HgCl₃] may be disregarded in qualitative interpretation of the vibrational data; the spectra of [NMe₄][HgX₃](X = Br or I) may similarly be described in terms of monomeric anionic structures.