Crystal structure of ClF4+SbF6-, normal coordinate analyses of ClF4+, BrF4+, IF4+, SF4, SeF4, and TeF4, and simple method for calculating the effectsof fluorine bridging on the structure and vibrational spectra of ions in astrongly interacting ionic solid

The crystal structure of the 1:1 adduct ClF5. SbF5 was determined and conta ins discrete ClF4+ and SbF6- ions. The ClF4+ cation has a pseudotrigonal bi pyramidal structure with two longer and more ionic axial bonds and two shor ter and more covalent equatorial bonds. The third equatorial position is oc cupied by a sterically active free valence electron pair of chlorine. The c oordination about the chlorine atom is completed by two longer fluorine con tacts in the equatorial plane, resulting in the formation of infinite zigza g chains of alternating ClF4+ and cis-fluorine bridged SbF6- ions. Electron ic structure calculations were carried out for the isoelectronic series ClF 4+, BrF4+, IF4+ and SF4, SeF4, TeF4 at the B3LYP, MP2, and CCSD(T) levels o f 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 incr eases. the barrier toward equatorial-axial fluorine exchange by the Berry m echanism. 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 great ly improved the fit between observed and calculated spectra.