The new [XeOF5](-) salt, N(CH3)(4)XeOF5, was prepared. This highly explosive compound was characterized by infrared, Raman, and multinuclear NMR spectroscopy. The electronic structure, vibrational frequencies, and force field of the free [XeOF5](-) anion in C-4v and C-5v symmetry were calculated at the LDFT/PP/DZVP, NLDFT/PP/DZVP, HF/ECP/DZP, and MP2/ECP/DZP levels of theory. Except at the Hartree-Fock level, all of the calculations predict for free [XeOF5](-) a pseudooctahedral C-4v structure with a sterically inactive free valence electron pair on Xe to be lower in energy than a C-5v structure in which the oxygen and the sterically active free valence electron pair on Xe occupy the two axial positions of a pseudopentagonal-bipyramid. The vibrational spectra which were experimentally observed for solid [XeOF5]- salts agree only with the spectra predicted from the C-5v model. The C-5v structure of [XeOF5](-) is analogous to those found for IF7, [IOF6](-), and [XeF5](-), but differs from the distorted octahedral C-s structures found for XeF6 and [IF6](-) and previously also proposed for [XeOF5](-). The preferences of heptacoordinated molecules for either pentagonal-bipyramidal or distorted octahedral structures and the fluxionality of some of these structures can be rationalized by the presence or absence of unequal ligand repulsion effects.