Technetium(VII) dioxide trifluoride, TcO2F3, behaves as a Lewis acid toward the fluoride ion and acetonitrile forming M+TcO2F4- [M = Li, Cs, N(CH3)(4)] salts and TcO2F3 . CH3CN. Fluorine-19 NMR spectroscopy established that the TcO2F4- anion has a cis-dioxo geometry in CH3CN solution. Variable-temperature F-19 NMR studies of TcO2F4- and TcO2F3 . CH3CN in CH3CN revealed that the one-bond couplings between Tc-99 and the two fluorine environments exhibit widely different degrees of quadrupolar collapse. The O-17 NMR spectra of O-17-enriched TcO2F4- and TcO2F3 . CH2CN and the H-1 and C-13 NMR spectra of TcO2F3 . CH3CN indicated that chemical exchange occurs between TcO2F3 . CH3CN and CH3CN solvent. The TcO2F4- anion was characterized by X-rag crystallography as its lithium salt crystallizing in the tetragonal system, space group P (4) over bar 2(1)m, with a =14.706(1) Angstrom, c = 8.797(2) Angstrom, V = 194.8(1) Angstrom(3). and Z = 2. at 20 degrees 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 [TcO2F4] units in polymeric TcO2F3. The Raman spectra of M+TcO2F4- [M = Li, Cs, N(CH3)(4)] and TcO2F3 . CH3CN were assigned under C-2v and C-5 point symmetries, respectively. Density functional theory calculations at the local and nonlocal levels predict that monomeric TcO2F3 has a trigonal bipyramidal geometry (C-2v point symmetry) and confirm that the cis-dioxo isomers of TcO2F4- and TcO2F3 . CH3CN (CH3CN bonded trans to an oxygen) are the energy-minimized structures.