Preparation, structural and spectroscopic characterisation of the salts M(3)X(3)AF(6) (A=As or Sb, M=S or Se, X=Cl or Br) containing the novel sulfur- and selenium-halogen cations (X(2)MMMX)

The salts X(2)MMMX(AF(6)) (A = As or Sb, M = S or Se, X = Cl or Br) were prepared quantitatively by the reaction of stoichiometric amounts of MX,AF, and M or from stoichiometric amounts of M, X(2) and AsF5 (M = S or Se; X = Br) in liquid SO,. They have been characterised by elemental analysis, single-crystal X-ray diffraction, Fourier-transform (FT)-Raman and Se-77 FT-NMR spectroscopy. The crystal structures of X(2)MMMX(AsF6) consist of (X(2)MMMX)(+) cations and AsF6- anions. The structure of the (X(2)MMMX)(+) 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(pi)-np(pi) (n = 3 or 4) bonds. Since the structure of these cations is different from those of (YM)(2)MY(+) (Y = Me or C6F5), theoretical calculations were performed to understand these differences and the bonding in these cations. In the X(2)SSSX(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.