The pentafluoroxenate(IV) anion, XeF5-: the first example of a pentagonal planar AX5 species

The results from the crystal structure determination and a normal coordinate analysis show that the XeF 5 plane of XeF 5 − is considerably more rigid than that in the fluxional IF 7 molecule due to the increased repulsion from the xenon free valence electron pairs. Local density functional calculations were carried out for XeF 5 − and XeF 4 with a double-numerical basis set augmented by polarization functions and confirm the experimentally observed geometries and vibrational spectra. It is shown that the bonding in XeF 5 − closely resembles that in XeF 4 . In a valence bond description, it can be visualized as the two axial positions being occupied by two sp-hybridized free valence electron pairs and the equatorial fluorines being bound by two Xe 5p electron pairs through semiionic multicenter four-electron bonds