THE PENTAFLUOROXENATE(IV) ANION, XEF5- - THE 1ST EXAMPLE OF A PENTAGONAL PLANAR AX5 SPECIES

Xenon tetrafluoride forms stable 1:1 adducts with N(CH3)4F, CsF, RbF,KF, and NaF and an unstable 1:1 adduct with FNO. All these adducts are ionic salts containing pentagonal planar XeF5- anions as shown by a crystal structure determination of N(CH3)4+XeF5-, Raman and infrared spectra, and F-19 and Xe-129 NMR spectroscopy. The X-ray crystal structure of N(CH3)4+XeF5- was determined at -86-degrees-C. This compound crystallizes in the orthorhombic system, space group Pmcn, with four molecules in a unit cell of dimensions a = 6.340 (2) angstrom, b = 10.244 (3) angstrom, and c = 13.896 (4) angstrom with R = 0.0435 for 638 observed [I > 3-sigma(I)] reflections. In addition to four N(CH3)4+ cations, the structure contains four pentagonal planar XeF5- anions per unit cell with D5h symmetry. The Xe-F distances are 1.979 (2)-2.034 (2) angstrom with F-Xe-F angles of 71.5 (4)-72.3 (4)degrees. The D5h structure of the XeF5- anion is highly unusual and represents the first example of an AX5E2 (E = valence electron lone pair) species in which all six atoms are coplanar. The results from the crystal structure determination and a normal coordinate analysis show that the XeF5 plane of XeF5- is considerably more rigid than that in the fluxional IF7 molecule due to the increased repulsion from the xenon free valence electron pairs. Local density functional calculations were carried out for XeF5- and XeF4 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 XeF5- closely resembles that in XeF4. 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.