The new IOF52- dianion was prepared in the form of its cesium salt by heating stoichiometric amounts of CsF, I2O5, and IFS to 162 degrees C for 14 days. The white stable solid was characterized by vibrational spectroscopy. A normal coordinate analysis was carried out with the help of ab initio calculations at the HF/ECP/DZP level of theory and resulted in an excellent agreement between observed and calculated frequencies. The structure of IOF52- iS that of a pentagonal bipyramid with five equatorial fluorine ligands, and an oxygen atom and one sterically active free valence electron pair occupying the two axial positions. The structure is very similar to that previously established by vibrational spectroscopy and X-ray diffraction for isoelectronic XeOF5- and represents only the second example of a heptacoordinated XOF5E (E = free valence electron pair) main group species. The possible existence of the IOF63- trianion is briefly discussed.