Lewis-Acid Properties of Technetium(VII) Dioxide Trifluoride, TcO₂F₃:  Characterization by ¹⁹F, ¹⁷O, and ⁹⁹Tc NMR Spectroscopy and Raman Spectroscopy, Density Functional Theory Calculations of TcO₂F₃, M⁺TcO₂F₄⁻ [M = Li, Cs, N(CH₃)₄], and TcO₂F₃·CH₃CN, and X-ray Crystal Structure of Li⁺TcO₂F₄⁻

Technetium(VII) dioxide trifluoride, TcO 2 F 3, behaves as a Lewis acid toward the fluoride ion and acetonitrile forming M + TcO 2 F 4 - [M = Li, Cs, N(CH 3 ) 4 ] salts and TcO 2 F 3 ·CH 3 CN. Fluorine-19 NMR spectroscopy established that the TcO 2 F 4 - anion has a cis- dioxo geometry in CH 3 CN solution. Variable-temperature 19 F NMR studies of TcO 2 F 4 - and TcO 2 F 3 ·CH 3 CN in CH 3 CN revealed that the one-bond couplings between 99 Tc and the two fluorine environments exhibit widely different degrees of quadrupolar collapse. The 17 O NMR spectra of 17 O-enriched TcO 2 F 4 - and TcO 2 F 3 ·CH 3 CN and the 1 H and 13 C NMR spectra of TcO 2 F 3 ·CH 3 CN indicated that chemical exchange occurs between TcO 2 F 3 ·CH 3 CN and CH 3 CN solvent. The TcO 2 F 4 - anion was characterized by X-ray crystallography as its lithium salt crystallizing in the tetragonal system, space group P 4̄2 1 m, with a = 4.706(1) Å, c = 8.797(2) Å, V = 194.8(1) Å 3, and Z = 2, at 20 °C. Refinement converged with R = 0.0339 ( R w = 0.0320). The anion geometry is a distorted octahedron with the two oxygen ligands cis to each other and is closely related to the [TcO 2 F 4 ] units in polymeric TcO 2 F 3 . The Raman spectra of M + TcO 2 F 4 - [M = Li, Cs, N(CH 3 ) 4 ] and TcO 2 F 3 ·CH 3 CN were assigned under C 2 v and C s point symmetries, respectively. Density functional theory calculations at the local and nonlocal levels predict that monomeric TcO 2 F 3 has a trigonal bipyramidal geometry ( C 2 v point symmetry) and confirm that the cis -dioxo isomers of TcO 2 F 4 - and TcO 2 F 3 ·CH 3 CN (CH 3 CN bonded trans to an oxygen) are the energy-minimized structures.