X-ray Crystal Structures of [XF₆][Sb₂F₁₁] (X = Cl, Br, I); ^35,37Cl, ^79,81Br, and ¹²⁷I NMR Studies and Electronic Structure Calculations of the XF₆⁺ Cations

The single-crystal X-ray structures of [XF6][Sb2F11] (X = Cl, Br, I) have been determined and represent the first detailed crystallographic study of salts containing the XF6+ cations. The three salts are isomorphous and crystallize in the monoclinic space group P21/n with Z = 4: [ClF6][Sb2F11], a = 11.824(2) Å, b = 8.434(2) Å, c = 12.088(2) Å, β = 97.783(6)°, V = 1194.3(4) Å3, R1 = 0.0488 at −130 °C; [BrF6][Sb2F11], a = 11.931(2) Å, b = 8.492(2) Å, c = 12.103(2) Å, β = 97.558(4)°, V = 1215.5(4) Å3, R1 = 0.0707 at −130 °C; [IF6][Sb2F11], a = 11.844(1) Å, b = 8.617(1) Å, c = 11.979(2) Å, β = 98.915(2)°, V = 1207.8(3) Å3, R1 = 0.0219 at −173 °C. The crystal structure of [IF6][Sb2F11] was also determined at −100 °C and was found to crystallize in the monoclinic space group P21/m with Z = 4, a = 11.885(1) Å, b = 8.626(1) Å, c = 12.000(1) Å, β = 98.44(1), V = 1216.9(2) Å3, R1 = 0.0635. The XF6+ cations have octahedral geometries with average Cl−F, Br−F, and I−F bond lengths of 1.550(4), 1.666(11) and 1.779(6) [−173 °C]/1.774(8) [−100 °C] Å, respectively. The chemical shifts of the central quadrupolar nuclei, 35,37Cl, 79,81Br, and 127I, were determined for [ClF6][AsF6] (814 ppm), [BrF6][AsF6] (2080 ppm), and [IF6][Sb3F16] (3381 ppm) in anhydrous HF solution at 27 °C, and spin-inversion-recovery experiments were used to determine the T1-relaxation times of 35Cl (1.32(3) s), 37Cl (2.58(6) s), 79Br (24.6(4) ms), 81Br (35.4(5) ms), and 127I (6.53(1) ms). Trends among the central halogen chemical shifts and T1-relaxation times of XF6+, XO4-, and X- are discussed. The isotropic 1J-coupling constants and reduced coupling constants for the XF6+ cations and isoelectronic hexafluoro species of rows 3−6 are empirically assessed in terms of the relative contributions of the Fermi-contact, spin−dipolar, and spin−orbit mechanisms. Electronic structure calculations using Hartree−Fock, MP2, and local density functional methods were used to determine the energy-minimized gas-phase geometries, atomic charges, and Mayer bond orders of the XF6+ cations. The calculated vibrational frequencies are in accord with the previously published assignments and experimental vibrational frequencies of the XF6+ cations. Bonding trends within the XF6+ cation series have been discussed in terms of natural bond orbital (NBO) analyses, the ligand close-packed (LCP) model, and the electron localization function (ELF).