Xenon tetrafluoride XeF

Since Bartlett s discovery, many other noble gas compounds have been made. All involve very electronegative elements. Most are compounds of Xe, and the best characterized compounds are xenon fluorides. Oxygen compounds are also well known. Reaction of Xe with F2, an extremely strong oxidizing agent, in different stoichiometric ratios produces xenon difluoride, Xep2 xenon tetrafluoride, XeF and xenon hexafluoride, XeFg, all colorless crystals (Table 24-3). 

The starting point for the synthesis of xenon compounds is the preparation of xenon difluoride, XeF2, and xenon tetrafluoride, XeF4, by heating a mixture of the elements to 400°C at 6 atm. At higher pressures, fluorination proceeds as far as xenon hexafluoride, XeFfi. All three fluorides are crystalline solids (Fig. 15.27). In the gas phase, all are molecular compounds. Solid xenon hexafluoride, however, is ionic, with a complex structure consisting of XeF< + cations bridged by F anions. 

Xenon also forms many fluoroanions and their salts, mostly prepared from xenon tetrafluoride and hexafluoride. Such compounds include Na+XeFs and Cs" XeF7- formed by reactions of xenon fluorides with sodium fluoride or cesium fluoride. The dicesium xenon octafluoride, Cs2XeFs, is a stable yellow solid that decomposes above 400°C. 

Xenon tetrafluoride forms stable colorless crystals. Two other xenon fluorides, XeF2 and XeFs, were synthesized by the group at Argonne, and a highly explosive xenon oxide (XeOy) was also found. The xenon fluorides react with water to form hydrogen fluoride and oxycompounds. For example ... 

An initial 1 1 F Xe mixture will leave some imreacted xenon because of loss of fluorine to the glass, but pure XeF is produced in this way. F2.-Xe ratios greater than 1 or additional increments of fluorine to the. xenon difluoride produce xenon tetrafluoride. (Care must be taken in handling xenon tetrafluoride because xenon trioxide can be produced from it. )... 

Fig. 5a—c. Molecular Geometries of Xenon Tetrafluoride-Metal Pentafluoride Adducts, a A perspective view of the XeF SbF structural unit b The [XeF3+][Sb2F1 J structural unit c The pteFJ-[BiFJ structural unit... 

Xenon tetrafluoride and hexafluoride are very powerful oxidizing agents also. They are particularly dangerous because they react with water, giving explosive xenon trioxide. [See Cautions under the syntheses of XeF and XeO solution. The best way to dispose of xenon tetrafluoride or hexafluoride is to rinse the vessel [e.g., a protective trap) first with carbon tetrachloride and then with plenty of water, otherwise explosions may occur.)... 

This would mean that there are 10 electrons in the valence shell of the Ng atom in xenon difluoride or krypton difluoride and 12 or 14 electrons for xenon tetrafluoride or hexafluoride, and even more for the octafluoroxenate ion, [XeFs] . Since one s and three p orbitals can accommodate only eight electrons, this would require the participation of d orbitals. In fact, the currently favored model uses only s and p atomic orbitals [16]. For example, XeF2 can be constmcted with a three-center-two-electron (3c-2e) bond, like NF5 (Chapter 4, Fig. 4.5) without using d orbitals (Fig. 5.1). Perhaps one should not worry much about which orbitals are involved, because as has been pointed out, bonding is not an observable quantity only bonding distancies and electron density are amenable to observation although... 

The Compound XeF XeOFj. Xenon oxide tetrafluoride ) was vacuum distilled on to xenon difluoride(0.67 mmole) contained in a KelF weighing bottle, of lO ml capacity, provided with a KelF valve. Excess XeOF was removed under vacuum with the bottle and its contents at 0°C. The white solid residue, m.p. 29 °C corresponded to an XeOF uptake (0.67 mmole) commensurate with the composition XeFj XeOF. ... 

Infrared spectra of 1 1 adducts of xenon difluoride with molybdenum and tungsten hexafluorides and 2 1 adducts with tin, zirconium and hafnium tetrafluorides have been assigned on the basis of their containing [XeF]+ cations [XeF]+[MF6]2-(M = Ti, Zr, Hf) 84-85>. This is supported in the case of 2 XeF2 SnF4 by Mossbauer work 86). Data on nXeF2 TiF4 (n = 1.5, 1 and 0.5) are less well defined 87). 

Xenon difluoride dissolves in water, yielding a solution which contains undissociated XeF molecules which have a half-life of about 7 hours at 0 . It eventually hydrolyzes to yield the expected products, xenon, hydrogen fluoride, and oxygen. The hydrolysis reactions of the tetrafluoride and hexafluoride are somewhat more complicated. The addition of the stoichiometric amount of water to the hexafluoride results in the formation of xenon oxide tetrafluoride. Hydrolysis of either fluoride with an excess of water or acid yields in solution a stable xenon(VI) species, which has been shown to be hydrated xenon tiioxide. Removal of the excess water leaves xenon trioxide as a solid residue. Inasmuch as this solid is an extremely sensitive explosive, such solutions must be handled with care. 

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