Tetrachloro- and Tetrabromoarsonium(V) Cations:  Raman and 75As, 19F NMR Spectroscopic Characterization and X-ray Crystal Structures of [AsCl4][As(OTeF5)6] and [AsBr4][AsF(OTeF5)5]

The salts [AsX4][As(OTeF5)6] and [AsBr4][AsF(OTeF5)5] (X = Cl, Br) have been prepared by oxidation of AsX3 with XOTeF5 in the presence of the OTeF5 acceptors As(OTeF5)5 and AsF(OTeF5)4. The mixed salts [AsCl4][Sb(OTeF5)6 - n Cl n - 2] and [AsCl4][Sb(OTeF5)6 - n Cl n ] (n ≥ 2) have also been prepared. The AsBr4 + cation has been fully structurally characterized for the first time in SO2ClF solution by 75As NMR spectroscopy and in the solid state by a single-crystal X-ray diffraction study of [AsBr4][AsF(OTeF5)5]: P1̄, a = 9.778(4) Å, b = 17.731(7) Å, c = 18.870(8) Å, α = 103.53(4)°, β = 103.53(4)°, γ = 105.10(4)°, V = 2915(2) Å3, Z = 4, and R 1 = 0.0368 at −183 °C. The crystal structure determination and solution 75As NMR study of the related [AsCl4][As(OTeF5)6] salt have also been carried out: [AsCl4][As(OTeF5)6], R3̄, a = 9.8741(14) Å, c = 55.301(11) Å, V = 4669(1) Å3, Z = 6, and R 1 = 0.0438 at −123 °C; and R3̄, a = 19.688(3) Å, c = 55.264(11) Å, V = 18552(5) Å3, Z = 24, and R 1 = 0.1341 at −183 °C. The crystal structure of the As(OTeF5)6 - salt reveals weaker interactions between the anion and cation than in the previously known AsF6 - salt. The AsF(OTeF5)5 - anion is reported for the first time and is also weakly coordinating with respect to the AsBr4 + cation. Both cations are undistorted tetrahedra with bond lengths of 2.041(5)−2.056(3) Å for AsCl4 + and 2.225(2)−2.236(2) Å for AsBr4 +. The Raman spectra are consistent with undistorted AsX4 + tetrahedra and have been assigned under T d point symmetry. The 35Cl/37Cl isotope shifts have been observed and assigned for AsCl4 +, and the geometrical parameters and vibrational frequencies of all known and presently unknown PnX4 + (Pn = P, As, Sb, Bi; X = F, Cl, Br, I) cations have been calculated using density functional theory methods.