The crystal structure of the 1:1 adduct ClF5. SbF5 was determined and contains discrete ClF4+ and SbF6- ions. The ClF4+ cation has a pseudotrigonal bipyramidal structure with two longer and more ionic axial bonds and two shorter and more covalent equatorial bonds. The third equatorial position is occupied by a sterically active free valence electron pair of chlorine. The coordination about the chlorine atom is completed by two longer fluorine contacts in the equatorial plane, resulting in the formation of infinite zigzag chains of alternating ClF4+ and cis-fluorine bridged SbF6- ions. Electronic structure calculations were carried out for the isoelectronic series ClF4+, BrF4+, IF4+ and SF4, SeF4, TeF4 at the B3LYP, MP2, and CCSD(T) levels of theory and used to revise the previous vibrational assignments and force fields. The discrepancies between the vibrational spectra observed for ClF4+ in ClF4+SbF6- and those calculated for free ClF4+ are largely due to the fluorine bridging that compresses the equatorial F-Cl-F bond angle and increases. the barrier toward equatorial-axial fluorine exchange by the Berry mechanism. A computationally simple model, involving ClF4+ and two fluorine-bridged HF molecules at a fixed distance as additional equatorial ligands, was used to simulate the bridging in the infinite chain structure and greatly improved the fit between observed and calculated spectra.