[TeX₃]⁺ Cations Stabilized by the Weakly Coordinating [Al(OR^F)₄]⁻ Anion: FIR Spectra, Raman Spectra, and Evaluation of an Abnormal Halogen Dependence of the ¹²⁵Te NMR Chemical Shifts

Abstract

TeX₃[Al(OR^F)₄] (X=Cl, Br, I; R^F=C(CF₃)₃) were synthesized by the reaction of Ag[Al(OR^F)₄] and TeX₄ or the reaction of AuX, Ag[Al(OR^F)₄], and elemental tellurium in liquid SO₂. The compounds were characterized by ¹²⁵Te NMR in solution and by X-ray diffraction, Raman, and IR spectroscopy in the solid state. The vibrational spectra and the crystal structure show very weak secondary interactions, indicating “pseudo gas phase conditions” in the condensed phase. The observed trend of the ¹²⁵Te NMR chemical shifts along the [TeX₃]⁺ series follows neither the monotonous decrease known as “normal halogen dependence” nor the increase known as “inverse halogen dependence”. By relativistic two-component calculations based on the ZORA approach, we find that this “abnormal halogen dependence” results from an interplay of relativistic and solvent effects, where non-negligible scalar relativistic effects and intermediate-sized spin-orbit effects compensate to some extent. The reasons for these trends are evaluated in the context of the Te s-orbital character of the TeX bonds and compared with the halogen dependence(s) within the isoelectronic [SeX₃]⁺ and PX₃ series and related trihalomethyl [CX₃]⁺ cations.