Preparation, structural and spectroscopic characterisation of the salts M3X3AF6(A = As or Sb, M = S or Se, X = Cl or Br) containing the novel sulfur– and selenium–halogen cations (X2MMMX)+

Abstract

The salts X₂MMMX(AF₆)(A = As or Sb, M = S or Se, X = Cl or Br) were prepared quantitatively by the reaction of stoichiometric amounts of MX₃AF₆ and M or from stoichiometric amounts of M, X₂ and AsF₅(M = S or Se; X = Br) in liquid SO₂. They have been characterised by elemental analysis, single-crystal X-ray diffraction, Fourier-transform (FT)-Raman and ⁷⁷Se FT-NMR spectroscopy. The crystal structures of X₂MMMX(AsF₆) consist of (X₂MMMX)⁺ cations and AsF₆^– anions. The structure of the (X₂MMMX)⁺ cation is dominated by an intracationic halogen–chalcogen contact and M–M bond alternation giving rise to a short M–M bond distance indicative of thermodynamically stable np_π–np_π(n= 3 or 4) bonds. Since the structure of these cations is different from those of (YM)₂MY⁺(Y = Me or C₆F₅), theoretical calculations were performed to understand these differences and the bonding in these cations. In the X₂SSSX(AsF₆) salts (X = Cl or Br) the structures of the cations are disordered and therefore exact bond distances could be not obtained. However, bond distances were estimated from their FT-Raman spectra and supported by molecular orbital calculations. The FT-Raman spectrum of Se₂Br₅AsF₆ is reported.