The combination of CH3CN solutions of [N(CH3)(4)][F] and a mixture of cis- and trans-[N(CH3)(4)][IO2F4] produces the novel trans-IO2F52- anion. Under the given conditions, only the trans-IO2F4- anion acts as a fluoride ion acceptor, thus allowing the separation of isomerically pure, soluble cis-IO2F4- from insoluble trans-IO2F52-. The trans-IO(2)F(5)(2-)and cis-IO2F4- anions were characterized by infrared and Raman spectroscopy and theoretical calculations at the LDFT and HF levels of theory. The trans-IO2F52- anion has a pentagonal-bipyramidal geometry with the two oxygen atoms occupying the axial positions. It represents the first example of a heptacoordinated main group AO(2)X(5) species and completes the series of pentagonal-bipyramidal iodine fluoride and oxide fluoride species. The geometries of the pentagonal-bipyramidal series IO2F52-, IOF52-, IF52-, IOF6-, IF6-, and IF7 and the corresponding octahedral series IO2F4-, IOF4-, IF4-, IOF5, IF5, and IF6+ were calculated by identical methods. It is shown how the ionic charge, the oxidation state of the iodine atom, the coordination number, and the replacement of fluorine ligands by either an oxygen ligand or a free valence electron pair influence the stuctures and bonding of these species.