Compounds of xenon

Xe is comparable to, that of molecular oxygen (1175kJmol for O2 - 02 + e )- He quickly proceeded to show that deep-red PtFe vapour spontaneously oxidized Xe to produce an orange-yellow solid and announced this in a brief note/ Within a few months Xep4 and Xep2 had been synthesized in other laboratories/ Noble-gas chemistry had begun. 

Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas (p. 626). They have the ideal composition, [Gg(H20)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way (p. 358). Samples containing up to 20% by weight of Ar have been obtained. 

Clathrates provide a means of storing noble gases and of handling the various radioactive isotopes of Kr and Xe which are produced in nuclear reactors. 

Probably the most familiar of all clathrates are those formed by Ar, Kr and Xe with quinol, l,4-C6H4(OH)2, and with water. The former are obtained by crystallizing quinol from aqueous or other convenient solution in the presence of the noble gas at a pressure of 10-40 atm. The quinol crystallizes in the less-common -form, the lattice of which is held together by hydrogen bonds in such a way as to produce cavities in the ratio 1 cavity 3 molecules of quinol. Molecules of gas (G) are physically trapped in these cavities, there being only weak van der Waals interactions between 

Claassen, H. Selic and J. G. Malm, J. Am. Chem. Soc. 84, 3593 (1962). See also P. Lazlo and G. J. SCHROBILOEN, Angew. Chem. Int. Edn. Engl. 28, 636 (1989) for further detailed chronology of the first synthesis of Xep4. 

SCHROBILOEN, Angew. Chem. Int. Edn. Engl. 28, 636 (1989) for further detailed chronology of the first synthesis of Xep4. 

The solution Xe NMR spectrum of the compound exhibits an eight-line multiplet with lines of equal intensity. Accotmt for this observation. 

What would you expect to see in the F NMR spectrum of XeF4, the structure of which is consistent with the VSEPR model  

All the fluorides sublime in vacuo, and aU are readily decomposed by water, Xep2 very slowly, and XeF4 and XeFg, rapidly (eqs. 18.3-18.5 and 18.14). 

Molecular shape Linear Square planar Octahedral 

XeFg from anhydrous HF produces [XeF5]2[HF2]2 HF which contains dimeric imits of [XeFg] ions bridged by [HF2] ions (18.5). Thus, in crystalline XeFg, the most common building block is the [XeFg] imit. 

In the NMR spectroscopic characterization of Xe-containing compounds, use is made of Xe, with a natural abundance of 26.4% and I = - Although direct observation of Xe is possible, the observation of satellite peaks in, for example, NMR spectra of xenon fluorides is a valuable diagnostic tool as we illustrated for [XeFs] in Case study 5, Section 3.11. For a potential clinical application of Xe, see Box 3.6. 

Worked example 18.1 NMR spectroscopy of xenon-containing compounds 

The reaction of CF2=CFBF2 with XeF2 gives the [BFJ salt of the following cation  

The positive charge can be formally localized on Xe, leaving seven valence electrons. 

Each F atom provides one electron to the valence shell of Xe. 

The [Xep5] ion (average Xe—F = 184pm) is isoelectronic and isostructural with IF5 (16.6), but in solid state salts, 

6 (a) The structure of [XeF7] , determined by X-ray diffraction for the caesium salt [A. Ellern et al. (1996) Angew. Chem. Int. Ed. Engl., vol. 35, p. 1123] (b) the capped octahedral arrangement of the F atoms in [XeF7] . Colour code Xe, yellow F, green. 

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It follows from the preceding discussion that the unbranched H bond can be regarded as a 3-centre 4-electron bond A-H B in which the 2 pairs of electrons involved are the bond pair in A-H and the lone pair on B. The degree of charge separation on bond formation will depend on the nature of the proton-donor group AH and the Lewis base B. The relation between this 3-centre bond formalism and the 3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in... 

Table 18.2 Some compounds of xenon with fluorine and oxygen...

It is from the binary fluorides that other compounds of xenon are invariably prepared, by reactions which fall mostly into four classes ... 

The most stable binary compounds of xenon are the three fluorides, XeF2, XeF4, and XeFB. Xenon difluoride can be prepared quite simply by exposing a 1 1 mol mixture of xenon and fluorine to ultraviolet light colorless crystals of XeF2 (mp = 129°C) form slowly. 

Three common molecular shapes are associated with octahedral electron group geomehy. Most often, an inner atom with a steric number of 6 has octahedral molecular shape with no lone pairs. Example uses a compound of xenon, whose chemical behavior is described in the Chemical Milestones Box, to show a second common molecular shape, square planar. 

Interaction of the yellow hexafluoride with silica to give xenon tetrafluoride oxide must be interrupted before completion (disappearance of colour) to avoid the possibility of formation and detonation of xenon trioxide [1]. An attempt to collect the hexafluoride in fused silica traps at — 20°C after separation by preparative gas chromatography failed because of reaction with the silica and subsequent explosion of the oxygen compounds of xenon so produced [2],... 

Some oxides of xenon are known, and like most other compounds of xenon they are usually obtained from the fluorides. Two reactions that yield Xe03 have already been shown in Eq. (15.237)... 

As mentioned earlier, krypton is known to form several compounds, but they are fewer and less well characterized than the compounds of xenon. The difluoride has been obtained by electric discharge... 

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. 

All other compounds of xenon are made from xenon fluorides. 

PROBLEM 6.23 Assign oxidation numbers to the elements in the following compounds of xenon ... 

No compounds of helium, neon or argon have ever been found. However, more recently a number of compounds of xenon and krypton with fluorine and oxygen have been produced, for example XeF6. 

Xenon exhibits a rich variety of stereochemistry. Some of the more important compounds of xenon are listed in Table 17.5.1, and their structures are shown in Fig. 17.5.2. The structural description of these compounds depends on whether... 

All argon nuclei have charge +18 electronic units, so that 18 electrons orbit the nucleus of the neutral atom. Its electronic configuration is is22s22p63s23p6, which fills each of these shells. It is these closed, or full, s and p shells of electrons that make Ar and all noble gases inert. Before 1962 it was thought that these elements were absolutely inert, but at that time the first stable compound of xenon, Xe, was discovered. The newly discovered compounds of krypton, Kr, and Xe are extremely interesting to chemists. 

Not only were the fluorides the first compounds of xenon to be prepared, but also they serve as starting materials for the synthesis of most other xenon compounds. Xenon difluoride can be prepared by the reaction of excess xenon with fluorine aided by heat or electromagnetic radiation ... 

Give possible Lewis structures for Xe03, an explosive compound of xenon. Determine the formal charges of each atom in the various Lewis structures. 

The OTePs group can stabilize compounds of xenon in formal oxidation states (IV) and (VI). On the basis of VSEPR, predict the stmctures of Xe(OTePs)4 and 0=Xe(0Tep5)4. 

Nikitin (1940) made a compound of xenon vvitli phenol, Xc(C H50H)2, isomorphous with H2S(CgH50H)2. 

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