XeF5 anion, structure

The XeF5 anion has an unusual pentagonal planar structure that is much more rigid than the planar equatorial fluorines in the fluxional IF7 molecule. The rigidity is attributed to increased repulsions from the Xe electron pairs.16... 

TTie XeFs GeFj" Structure. A stereoview of the XeF5 GeF5 structure is shown in Figure I. The anion consists of infinite... 

General Discussion. Onyszchuk and co-workers had previously established that the monomeric GeFj", of symmetry, is stablized by large monocations. It seems that such cations cannot make the close anion-cation contacts necessary to sustain a clustered anion arrangement. It is pertinent to examine why such large cations do not sustain relatives of the [XeF5] [GeF5]" structure. 

Only one representative of this type is known, namely the XeF5 anion. The structure of this anion was established by l F and 1 9xe NMR and infrared and Raman spectroscopy and an X-ray crystal structure determination (16). The structure of this unique anion is shown in Figures 1 and 2 and demonstrates that the two free valence electron pairs are sterically active and, as expected from their increased repulsive character, occupy the two less congested axial positions of a pentagonal bipyramid. 

A cation containing two xenon atoms, Xe2Fu+, is also known, and it has a structure that can be shown as F5Xe+"-F -" XeF5+. Polyatomic anions containing xenon are produced because XeFs is also a Lewis acid. An example of this type of reaction can be shown as... 

The crystal structure of [XeF5] AgF4] consists of alternate stacks of double layers of square-pyramidal XeFlj" cations and approximately square-planar AgF anions (site symmetry D2h, Ag-F = 190.2 pm). The XeFj- ion has C4v symmetry with Xe-F(ax) = 185.2, Xe-F(eq) = 182.6 pm, and F(ax)-Xe-F(eq) = 77.7°. Each cation lying on a 4-axis interacts with one bridging F ligand of each of four anions at 263.7 pm, as illustrated below ... 

The crystal structure of XeF5 AgF4" confirms the earlier conclusion, based On Raman spectroscopy, that the material is an essentially ionic assembly. The interatomic distance in the anion is not significantly different from that given by Hoppe and Homann for AAgp4 (A = Na, K). Also, the observed Ag "-F interatomic distance of 1.902 (11) A is not significantly different... 

The XeFs anion has 12 normal vibrations, which are classified into A ](R)+A2(IR)+2 j(IR)+2 2(R)+ 2 (inactive) under D5/, symmetry. Thus, only three vibrations (A 2 and 2Il ) are IR-active and only three vibrations (A +2 2) are Raman-active. In agreement with this prediction, the observed Raman spectmm of (CH3)4N[XeF5] exhibits three bands at 502 (symmetric stretch, A j), 423 (asymmetric stretch, E 2), and 377 cm (in-plane bending, As discussed in Sec. 1.11, the trigonal-bipyramidal and tetragonal-pyramidal structures can be ruled out since many more IR- and Raman-active bands are expected for these structures. 

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