The existence of the trifluoroxenate( I) anion, XeF3, had been postulated in a prior NMR study of the exchange between fluoride ion and XeF2 in CH3CN solution. The enthalpy of activation for this exchange, Delta H double dagger, has now been determined by use of single selective inversion F-19 NMR spectroscopy to be 74.1 +/- 5.0 kJ mol(-1) (0.18 M) and 56.9 +/- 6.7 kJ mol(-1) (0.36 M) for equimolar amounts of [N(CH3)(4)][F] and XeF2 in CH3CN solvent. Although the XeF3 anion has been observed in the gas phase, attempts to prepare the Cs+ and N(CH3)4(+) salts of XeF3- have been unsuccessful, and are attributed to the low fluoride ion affinity of XeF2 and fluoride ion solvation in CH3CN solution. The XeF3 anion would represent the first example of an AX(3)E(3) valence shell electron pair repulsion (VSEPR) arrangement of electron lone pair and bond pair domains. Fluotine-19 exchange between XeF2 and the F anion has also been probed computationally using coupled-duster singles and doubles (CCSD) and density functional theory (DFT; PBE1PBE) methods. The energy-minimized geometry of the ground state shows that the F anion is only weakly coordinated to XeF2 (F2Xe-F-; a distorted Y-shape possessing C, symmetry), while the XeF3- anion exists as a first-order transition state in the fluoride ion exchange mechanism, and is planar and Y-shaped (C-2 symmetry). The molecular geometry and bonding of the XeF3- anion has been described and rationalized in terms of electron localization function (ELF) calculations, as well as the VSEPR model of molecular geometry. Quantum-chemical calculations, using the CCSD method and a continuum solvent model for CH3CN, accurately reproduced the transition-state enthalpy observed by F-19 NMR spectroscopy, and showed a negative but negligible enthalpy for the formation of the F2Xe-F- adduct in this medium.