X-RAY CRYSTAL-STRUCTURE AND RAMAN-SPECTRUM OF TRIBROMINE(1+) HEXAFLUOROARSENATE(V), BR3+ASF6-, AND RAMAN-SPECTRUM OF PENTABROMINE(1+) HEXAFLUOROARSENATE(V), BR5+ASF6-

A single crystal of Br3+AsF6- was isolated from a sample of BrF2+AsF6- which had been stored for 20 years. It was characterized by x-ray diffraction and Raman spectroscopy. It is shown that Br3+AsF6- (triclinic, a = 7.644(7) angstrom, b = 5.641(6) angstrom, c = 9.810(9) angstrom, alpha = 99.16(8)-degrees, beta = 86.61(6)-degrees, gamma = 100.11(7)-degrees, space group P1BAR R(F) = 0.0608) is isomorphous with I3+AsF6-. The structure consists of discrete Br3+ and AsF6- ions with some cation-anion interaction causing distortion of the AsF6- octahedron. The Br3+ cation is symmetric with a bond distance of 2.270(5) angstrom and a bond angle of 102.5(2)-degrees. The three fundamental vibrations of Br3+ were observed at 297 (nu-3), 293 (nu-1), and 124 cm-1 (nu-2). The Raman spectra of Cl3+AsF6- and I3+AsF6- were reinvestigated and nu-3(B1) of I3+ was reassigned. General valence force fields are given for the series Cl3+, Br3+, and I3+. Reactions of excess Br2 with either BrF2+AsF6- or O2+AsF6- produce mixtures of Br3+AsF6- and Br5+AsF6-. Based on its Raman spectra, the Br5+ cation possesses a planar, centrosymmetric structure of C2h symmetry with three simi-ionically bound, collinear, central Br atoms and two more covalently, perpendicularly bound, terminal Br atoms.