The salt, [OsO2F3][Sb2F11], has been synthesized by dissolution of cis-OsO2F4 in liquid SbF5, followed by removal of excess SbF5 at 0 degrees C to yield orange, crystalline [OsO2F3][Sb2F11]. The X-ray crystal structure (-173 degrees C) consists of an OsO2F3+ cation fluorine bridged to an Sb2F11- anion. The light atoms of OsO2F3+ and the bridging fluorine atom form a distorted octahedron around osmium in which the osmium atom is displaced from its center toward an oxygen atom and away from the trans-fluorine bridge atom. As in other transition metal dioxofluorides, the oxygen ligands are cis to one another and the fluorine bridge atom is trans to an oxygen ligand and cis to the remaining oxygen ligand. The Raman spectrum (-150 degrees C) of solid [OsO2F3][Sb2F11] was assigned on the basis of the ion pair observed in the low-temperature crystal structure. Under dynamic vacuum, [OsO2F3][Sb2F11] loses SbF5, yielding the known [mu-F(OsO2F3)(2)][Sb2F11] salt with no evidence for [OsO2F3][SbF6] formation. Attempts to synthesize [OsO2F3][SbF6] by the reaction of [OsO2F3][Sb2F11] with an equimolar amount of cis-OsO2F4 or by a 1:1 stoichiometric reaction of cis-OsO2F4 with SbF5 in anhydrous HF yielded only [mu-F(OsO2F3)(2)][Sb2F11]. Quantum-chemical calculations at the SVWN and B3LYP levels of theory and natural bond orbital analyses were used to calculate the gas-phase geometries, vibrational frequencies, natural population analysis charges, bond orders, and valencies of OsO2F3+, [OsO2F3][Sb2F11], [OsO2F3][SbF6], and Sb2F11-. The relative thermochemical stabilities of [OsO2F3][SbF6], [OsO2F3][SbF6], [OsO2F3][AsF6], [mu-F(OsO2F3)(2)][SbF6], [mu-F(OsO2F3)(2)][Sb2F11], and [mu-F(OsO2F3)(2)][AsF6] were assessed using the appropriate Born-Haber cycles to account for the preference for [mu-F(OsO2F3)(2)][Sb2F11] formation over [OsO2F3][SbF6] formation and for the inability to synthesize [OsO2F3][SbF6].