Xenon difluoride, XeF

Xenon difluoride (XeF ) is the electrophilic fluorinating reagent whose reactions with vinyltin compounds have been studied most exhaustively. " A series of vinyl fluorides prepared according to this procedure is listed in Table 1. 

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. 

In the 1960s, scientists first produced compounds of xenon and some other noble gases at the Argonne National Laboratory located near Chicago. Xenon and krypton are the only noble gases that readily form compounds with oxygen and fluorine. For instance, when xenon combines with fluorine, it can form a series of compounds, such as xenon difluoride PCeF ), xenon tetra-fluoride (XeF ), and xenon hexafluoride pCeF ). These and other compounds of xenon are formed within metal containers at high temperatures and pressures. They are not stable. 

In an effort both to clarify the nature of Xe(PtFg)2 and related compounds and to confirm the existence of the [XeF] cation, we investigated the fluoride-ion donor properties of xenon difluoride, particularly with respect to pentafluoride acceptors. A satisfactory study of these systems was made possible by the discovery that bromine pentafluoride is a good solvent for XeF2 and... 

The effectiveness of xenon difluoride as an oxidative fluorinator in the presence of fluoride ion acceptors is probably due to the formation of [XeF]+ or [XejFj]. The electron affinity of the cation, E[XeF+fg)] is estimated from the following enthalpy cycle ... 

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. ... 

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. )... 

The formulation of complexes as salt-like species containing well-defined cations or anions should be approached with a certain amount of caution. This is particularly well illustrated with xenon difluoride adducts which span the gamut of complexes from salt-like species such as [XeF]+[Sb2Fn] to covalent adducts like XeF2 XeOF4, In the latter the components preserve their molecular identities and dimensions and the adduct is clearly a covalent adduct, but even in the former the relative short Xe F distance between the [XeF]+ cation and the [Sb2Fu] anion (2.34 A) implies considerable covalent character. 

If the [Xe2F3]+ cation is. considered in terms of an assemblage of two [XeF]+ and one F ions, each [XeF]+ species receives electron density from the associated F and the assembly becomes a multicentre bonded system with a lower overall electron affinity than that of [XeF]+ 78). The measured fluoride ion donor ability of xenon difluoride, (AH0 = 9.45 eV)14), is greater than that of XeF4 but less than that of XeF6 (see Section 2a, p. 65). The increase in Xe—F bond energy in cation formation [195.9 kJ mol-1 in [XeF]+ c.f. 135.4 kJmol in XeF2, 4)] contributes to... 

XeF2 XeF]+[TaF6]- and [XeF2 XeF]+[NbF6] , which contain loosely bound xenon difluoride, are also formed38). 

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). 

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). 

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