RECENT ADVANCES IN THE CHEMISTRY OFNOBLE GAS ELEMENTS*N. K. JhaDepartment of Chemistry,l ndion Institute of Technology, New Delhi 29, lndiaSection 1 : New compounds . . , . . . . . .. . . 148Simple compounds, 148Complex compounds, 15 1New preparative methods, 157Structural and other physical studies, 159Formation of XeF2 and XeF4, 163Hydrolysis of XeF2, 164Other reactions of XeF2, 164Reactions of XeF4, 165Hydrolysis of XeF4, 166Reactions of XeO3, 166XeO3, 167Xe fluorides, 167Prolonging the lifetime of tungsten filaments, 167Separation of xenon and krypton, 167Section 2: Old compounds . . . . . . . . . . .. 157Section 3 : Kinetic studies and some other reactions . . . . .. 163Section 4: Applications . . . . .. .. . . . . .. 166Conclusion, .. . . . . . . . . . . . . . .. 167References . . . . .. .. .. . . . . .. .. 168Immediately after the synthesis of the first noble gas compound, XePtF6 byBartlettl in 1962, the chemistry of noble gases was explored very extensivelyand a remarkable amount of work was reported2 during the following year.Subsequent progress has not been as rapid as in the initial stages. Thoughthe ‘inert’ gases have been proved not to be chemically inert they are nobleand form compounds only with the electronegative elements oxygen andfluorine. Moreover, only xenon forms a large number of compounds;krypton is rather unreactive and the lighter members may still be regardedas inert. The chemistry of the radioactive element radon has only veryrecently begun to yield to experiment.Nevertheless, commendable progresshas been made in the structural and other physical studies of the noblegas compounds and their reactions with compounds of other elements. Manyreview papers and a few books have appeared since 1963, and a few recent onesare listed in the references.3--9The present review is concerned with the advances made in the chemistry of* A review of literature after December 1965.Jha 14noble gases since 1966. Section 1 is devoted to a discussion of new compounds,simple and complex. Section 2 includes a discussion of some of the simplecompounds reported during 1962-1 963 (designated ‘old’ here) ; only new pre-parative methods and advances made in their structural and physical studiesare considered.In Section 3 kinetic studies are reviewed and some newreactions of noble gas compounds are discussed. Section 4 deals with theapplications of these compounds.SECTION 1: NEW COMPOUNDSOf the new compounds reported the majority are complex. Some simplecompounds reported earlier without adequate evidence have now been charac-terized and are included here. The sub-section on complex compounds alsoincludes additional information relating to materials already reported.Simple compoundsMost of the progress made pertains to the xenon compounds only, but there islittle information about radon fluorides and predictions about the possibleformation and stability of argon and helium compounds have also been made.Xenon dichloride. It has been claimed that XeCl2 can be prepared by (i) inter-action of Xe and Clz at room temperature for a few days;l0 (ii) passing highfrequency discharge through a mixture of Xe, Fz and Sic14 (or cC14) at lowtemperatures;ll (iii) passing a mixture of Xe and Cl2 through a microwavedischarge;l2 and (iv) P-decay of 1291 incorporated into ICl2,13 129ICli +129XeClz.Of the four methods, the first is uncorroborated, while the fourth isgood only for providing evidence of the existence of XeC12.Xenon dichloride is said to be a white crystalline substance, which can bestored in a sealed glass tube for a long time and may be purified by sublimationat reduced pressure at room temperature.ll Its ir spectrum shows a structuredband at 319 cm-1.12 All the structures shown in the spectrum agree withcalculated values based upon a symmetrically linear XeC12.Xenon tetrachloride. This has been identified by Mossbauer spectroscopy fromthe /I-decay of 1291 incorporated into ICl;.l3 Like XeF4, the molecule is reportedto have a square planar configuration.Xenon dibromide.The reaction 1291Bri + 129XeBr2 has been observed in aMossbauer study.14 Based on the isomer shifts in the Mossbauer studies, theassumption of p a bonding in Xe di- and tetrahalides seems to be nearly valid.The charge per halogen atom calculated from the quadrupole splittings inthe Mossbauer spectra is: F, -0.72; CI, -0.52; and Br, -0.41.l3J4Xenon dzjluoride. More sophisticated experiments are being carried out toelucidate the structure of the xenon fluorides especially from the electronictransitions between various levels.One such experiment is the high resolutionHe1 and He11 photo electron spectra of XeF2.1g1 The spectra have been com-pared with the result of a more sophisticated calculation of the electron struc-ture of XeF2.192 Such spectra for XeF4 and XeFs are also being investigated.191148 R.I.C. ReviewXenon oxide dzjluoride. A colourless volatile material obtained by the reactionof Xe, 0 2 and F2 had been assumed to be XeOF2 and had not been fullycharacterized in 1963.l5 This reaction has been reinvestigated recently and ithas been shown that no stable oxide fluoride is produced and the materialobtained in the earlier work may have been the adduct XeFz.XeF4.16Xenon oxide difluoride had been deposited on to a CsI window at - 80 "Cby the interaction of XeF4 and H2O vapour sprays and was identified by an irstudy.17 Three bands at 747,520 and 490 cm-l were observed.The first band isattributed to Xe-0 and the latter two to Xe-F stretching by comparison of theXeOF2 spectrum with that of XeF2. The corresponding values for Xe-Fstretching in XeF2 are 567 and 496 cm-1.Xenon dioxide dzjluoride. Only mass spectroscopic evidence had been obtainedin 1963 for the existence of XeOzFz which was formed, along with XeOF4, inthe hydrolysis of XeF6.18 It has been found that XeOF4 (or XeF6) reactswith XeO3 to give a mixture of Xe02F2, XeFz and XeOF4.19 From thereaction mixture XeOzF2 may be purified by fractional distillation. Xenondioxide difluoride, XeOzF2, forms white crystals which melt at 30.8"C to acolourless liquid.It has a vapour pressure between that of XeO3 and XeOF4.Although XeO2F2 is more stable than XeO3, samples of XeOzF2 have beenfound to explode violently. With moist air rapid hydrolysis to Xe03 occurs.The first two ir and Raman bands at 537, 610, 851 and 882 cm-1 havebeen assigned to Xe-F and the last two to Xe-0 stretchings.lg The vibrationalspectra have been interpreted in terms of CzV molecular symmetry associatedwith a pseudo bipyramidal structure involving two axial F atoms, two 0 atomsand a lone pair of electrons in the equatorial position.20 It is interesting to notethat the structure is the same as that predicted by Gillespie21 on the basis ofvalence electron pair repulsion theory.Gillespie also predicted that Xe02F2may polymerize but this has not been found to occur.Xenon trioxide dzjluoride. Xenon trioxide difluoride, Xe03F2, has been gener-ated by the reaction of XeF6 with solid sodium perxenate at room temperatureand studied by mass spectroscopy.22 The volatility of Xe03F2 is similar tothat of Xe04 but is much greater than that of XeOzF2, indicating that Xe03F2is more symmetrical and probably non-polar. The shape of the molecule maybe trigonal bipyramidal with two axial F atoms and three equatorial 0 atoms,as predicted by Gillespie.21Xenon chloridejluoride. Recently, an attempt has been made to prepare xenonchloride fluorides by subjecting mixtures of Xe, Cl2 and F2 in ratios 1 : 1 : 1to 1 : 2 : 2 to high frequency excitation at 10 Torr or to uv irradiation atnormal pressure.The formation of xenon chloride fluorides was not observed,CIF3 and C1F5 were produced instead.23It has been proposed that XeCl2F2 might be stable at low temperatures andunder normal conditions it may decompose with splitting off of chlorine atomsto form XeF2 due to an intramolecular redox reaction, whereas a xenonchloride fluoride with hexavalent xenon must be thermodynamically unstableeven at low temperatures.24Jha 14Xenates and perxenates. A crystalline form of monocaesium xenate has beenprepared in appreciable amount by the action of aqueous XeO3 on aqueousCsOH in presence of fluoride i0ns.~5 It is stable in dry air and sparinglysoluble in ice-cold water.It shows the characteristic ir bands of the xenate ion.The earlier samples of monocaesium xenate and other mono-alkali metalxenates prepared were in the powder form.26A perxenate of americium has been reported.27 The reaction of solid XeF4with NaOH at 0-20 "C has been found to produce Na4XeO6.2H20, NaeXeO6.2HzO.yNaF ( y - 0.8) and Na4XeO6.2.5 NaF, according to the relativeproportions of the reactants. X-ray data indicate that Na4XeO6.2.5 NaF is achemical species and not a mixture of Na4XeO6 and NaF.28Helium compounds. Stable molecule ions containing He and F have beendetected29 by producing an ion beam of He,F$ in an electromagnetic separa-tor, retarding the ions to thermal velocities with the aid of a collector andidentifying the products by mass spectrometry and by their action on thecollector. The ion source was a 1 : 1 mixture of the reagents.The ions pro-duced from He and Fz were HeF+ and HeFi+; He and BF3 yielded HeF+ andHeF+ while He and RuF5 gave rise to HeFg+ ions only.Argon compounds. The feasibility of preparing argon compounds and theirpossible stability have been discussed in detail by Ferreira.30 Extrapolation ofX-F and X-0 bond energies for groups IVb, Vb, VIb and VIIb to 0 groupindicates that Ar-F bond should be as stable as Kr-F bond and that Ar-0bond should be more stable than Kr-0 bond. Ar-F and Ar-0 compoundsshould be thermodynamically as stable as Kr-F and Kr-0 compounds andtheir non-existence is to be related to kinetic considerations. Although activa-tion barriers should be about the same for Ar-Fz as for Kr-Fz reactions, thedecomposition mechanism may prove to be different.Theoretical investiga-tions have shown that ArFf may be sufficiently stable to allow the probableisolation of ArFPt+Fi.31Radon compounds. The possibility of preparing various radon compounds andtheir expected stabilities and formation enthalpies has been discussed byVasilescu.32 Considering the formation energies and differences in ionizationpotentials available for ClF3, BrF3 and XeF2, the value of AH* for RnF2 wasapproximated as -75 kcal mol-1. Similarly, -84 kcal mol-1 was approxi-mated for RnF4. By extrapolation, the existence of RnF6, RnOF4, RnO3,Na4RnO6 and Ag4RnO6 has been predicted. Although it is quite likely thatcompounds of Rn analogous to those of Xe may be even more stable than thexenon analogues, their preparation seems to be a very difficult task as radon isavailable only as a very highly radioactive isotope (222Rn, half life, 3.83 days).Apart from the health hazard the radioactive disintegration would cause atleast a 50 per cent loss of any radon compound in addition to its decomposi-tion by the radiations.Evidence from very small samples suggest that radonforms a chemical compound with fluorine, but the compound was notcharacterized.33 However, RnFz and RnF4 seem to have been prepared.34* The SI unit for H is J mol-1 (4.184 J = 1 cal).150 R.I.C. ReviewIt has been reported that radon can be oxidized in aqueous solution byH202, KMn04 or K2S20g35 but a reinvestigation of the reactions showed thatradon is not oxidized but is only mechanically trapped.36 Recently, it has beenclaimed that solutions of oxidized radon in BrF3, BrF5 and ClF3 have beenprepared.37 The halogen fluorides may be vacuum distilled from the solution atroom temperature without volatilization of radon, which indicates that theradon may be present as an ionic species in solution.Solids obtained onevaporation were not identified but they appeared to resemble the fluoridereported earlier.33 The solid liberates elemental radon on hydrolysis just asXeF2 liberates Xe on hydrolysis.Complex compoundsThe main progress in the chemistry of noble gases is characterized by thepreparation of complexes with Lewis acids and alkali metal fluorides.Some‘onium’ complexes have also been reported.Xenonium derivatives. Compounds containing xenonium cations such asPhXe+ and MeXe+ are reported to have been detected.38~39 Phenyl xenoniumcompounds, e.g. PhXeC104, are formed by the accumulation of Xe by /3-decayof 1311 or 1331 in compounds like PhI, Ph2IC104 and other diphenyliodoniumsalts labelled with 1311 or 1331. Accumulation studies, carried out under variousconditions, e.g. by using the iodine compounds in solid form, in aqueous ornon-aqueous solutions, and by inserting PhI into crystals of KC104, KBPh4,KBF4 etc., show that the yield of the ‘onium’ compounds depends on thenature of anion present. The stabilizing capacity of the anion for PhXe+decreases in the order: ClO,, BPh,, BF-, SO:-, NO, and Cl-.The ‘onium’compounds are stable in acid medium and not in alkaline or neutral medium.It is noteworthy that the product either from PhI or Ph2I+ salts containsPhXe+. The ‘onium’ nature of the products has been established by chromato-graphy and electrophoresis studies.Complexes formed by XeF6. Xenon hexafluoride resembles halogen fluorides inits ability to combine with Lewis acids and also in forming compounds withalkali fluorides. The complexes formed by XeF6 and reported during thisperiod are shown in Fig. 1. It should be noted that this is not a complete list ofall the complexes formed by XeF6.Most of these complexes have been prepared by direct contact between thereactants. Recently it has been shown that xenon hexafluoride reacts withuranium pentafluoride at room temperature to form the adduct UF5.XeF6.XeF6 also reacts with UF4 to produce an adduct approximating to the com-position UF5.1.75 XeF6 which, on prolonged pumping, produces the 1 : 1adduct. The 1 : 1 adduct is a pale yellow compound, soluble in anhydroushydrogen fluoride.lg3 The magnetic moment of the compound is 1.57 BM,which indicates the presence of pentavalent uranium.d-Spacings for x-raypowder diffraction pattern of this adduct have also been reported. Attempts toprepare XeFs-SiF4 adducts have failed.44 In some cases a solvent such asBrF5 has also been ~ s e d . ~ 6 Certain physical properties such as mp and thermalstability of some of these complexes have also been reported.These xenonJha 15(Ref. 40) M zXe F8MXe F, 2 NOF.Xe F6 (Ref.41)1:-RbCs) IN- NO,F.XeF, (Ref. 42:SnF, 4XeF,.SnF4 (Ref.43)XeF6 .ASF5 AsF - GeF. 4XeF,.GeF, -Fl (Ref. 46, 47) 2XeF6.AsF,(Ref.47) 2Xe F6.PF52XeF6.GeF, wf. 4))i l r F 5 XeF,.GeF,2Xe F6. Ir F5 2XeF6 .VF, (Ref. 45)(Ref.46) Xe F,. Ir F,Fig. I. Complexes formed by XeFs.hexafluoride complexes are unstable to heat, have good oxidizing andfluorinating power and are easily hydrolysed by water to give Xe(vr) in solution.Raman and ir spectra indicate that some may be formulated as having ionicstructures. The complex 2NOF.XeFs can be formulated as [(NO)2]2+[XeFs]2-,41and NOzF.XeF6 as N O ~ X ~ F ? . ~ ~ The 2 : 1 and 1 : 1 adducts of XeF6 and AsF5(or IrF5) may be formulated as [Xe2F11]+[MFs]- and [xeFg]+[MFg]- (M = Asor Ir), respectively.46 X-ray d-spacings have been reported in some cases butstructural analyses have not been done.44The structure of the XeFi cation has been determined from the crystalstructure analysis of a compound FllPtXe formed by the reaction of Xe, F2and ptF6.48 FllPtXe is formulated as [XeF5]+[PtFJ.The PtF, ion contains aregular octahedron of fluorine atoms whereas the fluorine atoms in XeF; arearranged in a slightly distorted square pyramid (Fig. 2a).The Xe atom is 0.34 A below the plane of the four fluorine atoms. It shouldbe noted that the structure is same as that predicted on the basis of five bondpairs and one lone pair of electrons in terms of the valence shell electron pairrepulsion theory.That the lone pair is sterically active is shown by the direc-tion of approach of the bridging F atoms of two PtF, groups (the dotted linesin Fig. 2a).Complexes formed by XeF4. Xenon tetrafluoride does not seem to form com-plexes with Lewis acids, e.g. it does not react with IrF5, AsF546 and VF5,45 norwith alkali fluorides. The results of the reaction between XeF4 and IF5 havebeen rather conflicting. One report is that XeF4 oxidizes IF5 to IF7.49 An-other reports that if specially purified XeF4 is used no reaction takes place atall.50 Yet another report claims that an adduct XeF4.IF5 is formed.51 Theadduct is stable up to 92 "C but is readily hydrolysed in air. The formula wasconfirmed by chemical analysis and by Raman and nmr spectra in CH3CN152 R.I.C.Review\\\\1.90 + a03 A I2-14 k.03 AFig. 2. (a) Structure of XeF;, (b) structure of XezF3f.solution. The complex XeF4.2SbF5 is reported but no conclusive evidence insupport of this formulation has been published. However, another complexXeF4.4SbF5 is claimed and its x-ray powder photograph has been reported.53The reactions of XeF4 with PF5, AsF5 and SbF5 in BrF3 have recently beeninvestigated by measuring the conductivities of the solutions. The breaks inconductivities correspond to solutions containing 1 mol XeF4 for 2 and 4 molAsF5; I mol XeF4 for 0.5 and 4 mol SbF5; and 1 rnol XeF4 for 1,4 and 6 rnolPF5.53 It is from these reactions that the above mentioned XeF4.4SbF5complex has been isolated.The interaction of XeF4 with XeFz has been reinvestigated54 to confirm theearlier results55 about the adduct (XeFz.XeF4) formed between the two.Complexes formed by XeFz.The XeF2 complexes reported recently are shownin Fig. 3 on p 154.These complexes are usually formed by direct interaction between thereactants. Solvents like CH3CN,62 BrF356 and BrF546358959 have also been used.XeFz.AsF5 is claimed to be produced by exposing a mixture of Xe, Fz andAsF5 to sunlight.57Some physical properties, e.g. mp and thermal stabilities of some of thesecomplexes have been reported. There seems to be some doubt regarding thenature of XeFz.AsF5. It has been described as a fluorine bridged molecularadduct by one group57 and another group reports that it is unstable andchanges rapidly to 2XeFz.AsF5, even while manipulating for ir or x-raysamples.59 XeFz.2IF5 does not have an ionic structure on the basis of nmr andRaman studies.64 On the evidence of Raman spectra the adduct XeF2.IF5 canbe supposed to be molecular in nature.50The adducts formed by XeF2 with noble metal pentafluorides are of threetypes: 2XeFz.MFg (M = Ru, Os, Ir, Pt); XeFz.MF5 (M = Ru, Os, Ir, Pt);and XeFz.2MF5 (M = Ru, Ir, Pt), the adducts of each type form iso-morphous series.Compounds of the first two types have also been formed byXeFz and AsF5. All these complexes are thermally stable at room temperature,except XeFz.OsF5 which decomposes spontaneously ca 20 "C: 3(XeF2.0sFs)-+ 2XeFz.OsFs + Xe + 2OsF6. On the basis of Raman and ir data these haveJha 15(Ref.4 6 , s ~ ) 2 XeF, .AsF,(Ref. 46,56,57,58,59) XeF,.AsF5Xe F, .2(2,2 ' b i p y r) (Ref. 62)A s f 5 1 1 2 . 2 ' bipyridine(Ref. so. 63, 176) XeF, .I F5 IF' F] pF' '=- XeF,.PF, (Ref. 56)(Ref. 6 3 , ~ ) XeFz .2 I F, XeF, .2PF5SbFS 1 ~ P l f 5 ( t l = P r , Ir, OS, Ru, E ' Rh)(Ref. 5 6 , ~ ) XeF, .S bF5(Ref. 52,60,61) XeF,.2SbF52 XeF,.MF5(Ref.46,s8,59.also 65 for Nb, Ta)(Ref. 60) XeF, . I .5S bF5(Ref. 60) XeF, .6SbF5XeF,.MF,XeF2 .2MF5(a~s0 Ref. 63 for Irand 65 for Nb, Ta & Ru)Fig. 3. Complexes formed by XeF2.been formulated as salts [Xe2F3If[MF6]-, [XeF]+[MF6]- and [XeF]+[M2F11]-,respectively.58~59A detailed x-ray analysis of XezF,f[AsF,]- shows that AsFG has nearly anoctahedral shape and XeZFi is a planar V-shaped cation.The cation Xe2Ficontains a bridging F atom and is symmetrical about that atom (Fig. 2b). Onthe basis of shorter (1.90 5 0.03 A) terminal Xe-F distances (cf. 2.0 A inXeF2) the cation is represented as F-XefF-Xe-FS.58959 XezFf salts arecharacterized by strong bands in Raman at - 575 and 591 cm-1 (stretchregion) and - 160 cm-1 (bend region).Raman spectra of the 1 : 1 complexes are similar to those of relatedestablished salts A+MF, (A = alkali metal and M = transition metal). Thesedepart slightly from the ideal salt spectra which suggest that MF, symmetry inthese cases is lowered by fluorine bonding of the anion with cation: F-Xef . . .F-MF-. Raman spectra of XeFf salts show an intense doublet in the region of600-612 cm-1.The doublet nature of the band is attributed to weak interact-ions between cations.59The crystal structure analysis of the complex XeF2.2SbFs has also beenreported.61 On the basis of shorter Xe-F bond length (1.84 3 0.04A) inXeF2. SbF5, this compound may be formulated as [XeF]+[Sb2F$, but it isalso found that a fluorine atom of Sb2Fll unit is very close to Xe (2.35 A, sumof the van der Waals radii of Xe and F = 3.5A), indicating considerableinteraction, and hence XeF2.2SbF5 may be regarded as essentially a covalentmolecule.61Conclusions regarding complex formation by XeF2, XeF4 and xeF6. XeF2,XeF4 and XeF6 seem to form most of their complexes by fluoride ion donation.It is apparent from the above list of complexes that the fluoride ion donor154 R.I.C.Reviewability is the least in XeF4. The fluoride ion donor abilities of these three fluor-ides has been nicely compared experimentally by Bartlett and Sladky.46Treatment of 1 : 1 : 1 mixture of XeF2, XeF4 and XeF6 with AsF5 in BrF5solution followed by the removal of BrF5 and excess of AsF5 yields a mixturecontaining [XeF5]+[AsF6]-, [Xe2F3]+[AsF6]- and XeF4. This shows that the F-donor ability decreases in the order XeF6 > XeF2 > XeF4. This reaction hasalso been suggested as a method for purification of XeF4 from XeF6 and XeF2contaminants .46One expects XeF2 to be the best fluoride ion donor as F- can be more easilyseparated from it because of low effective charge on Xe in XeF2. The order onthis basis should be XeFz > XeF4 > XeF6 but XeF6 is found to be a betterdonor than XeFz.This has been explained46 on the basis of the tendency ofnon-octahedral XeFs to change over to pseudo octahedral XeF5f.Complexes formed by XeOF4. Xenon oxide tetrafluoride forms a number ofcomplexes analogous to those formed by XeF6. These are shown in Fig. 4.XeOF4 forms complexes with K, Rb and Cs fluorides but not with NaF.Thermogravimetric studies indicate that a number of intermediates, e.g.3CsF.2XeOF4,6KF.XeOF4 etc., are formed before the final decomposition toalkali fluorides.66 It forms an adduct with AsF5 at -78 "C which is unstableat room temperature.66 XeOF4 also reacts with NO2F at about 100 OC.67Nearly all the complexes of XeOF4 dissociate completely in the vapour phase.The ir spectrum of solid NOF.XeOF4 shows bands due to NO+.41 Thus,XeOF4 may be acting as an F- acceptor in the complexes with NOF and alsowith alkali metal fluorides and as an F- donor with Lewis acids like VF5 andSbF5.CompZexes formed by XeO3.The first such complex reported was mono-caesium fluoroxenate, CsF.XeO3 which was formed by exposing CsF.XeFs40or CsF.XeOF466 to air. CsCI.XeO3 was prepared by mixing aqueous solutionsof CsCl and X e 0 ~ . ~ ~ A few more compounds, viz. MF.Xe03 (M = K, Rb, Cs)Fig. 4. Complexes formed by XeOF4.XeOF4.2SbF, (Ref.66) 1 SbF,RbF - 3RbF.2XeOF4 (Ref. 66)CrF : CsF.XeOF, (w. u)KF(Ref. 66) 3KF.XeOF4(Ref. 41) NOF.XeOF, 4 NOF2XeOF, .VF5 (Ref. 45)Jha 15and CsBr.XeO3 have recently been reported, prepared by mixing aqueousXeO3 and MF solutions followed by acidifying with aqueous HF, or neutral-izing aqueous XeO3 containing HF (obtained by hydrolysis of XeFs) withaqueous MOH and evaporating the solution in either case until crystals areformed.69 The stability of these compounds decreases with increase in atomicweight of the halogen: CsF.XeO3 > CsCl.XeO3 > CsBr.XeO3.It was realized in the beginning that such compounds are not equimolecularmixtures of MF and Xe03 because no lines due to MF or XeO3 showed inthe x-ray powder patterns of these compounds;40$66 they were assumed to bemolecular addition complexes of the type MF.Xe03 since no absorptionoccurred in the Xe-F band region though bands were present in Xe-0region.66These salts are much more stable than XeO3 and hence it is unlikely thatthey are simply loose molecular complexes of MF and XeO3.Recently anx-ray crystal and molecular structure analysis of one of these compounds,namely KF.Xe03, has been carried out70 which shows that the crystalstructure consists of infinite chains of XeO3 units linked by bridging F atoms,with K+ ions at non-bonding distances from 0 and F atoms. Thus, the formu-lation of this compound as molecular addition compound is incorrect, thecorrect formulation being nK+ (Xe03F-),. Other fluoroxenates should belikewise formulated.The x-ray analysis shows70 that the coordination around xenon is analogousto that found in XeFl and XeOF4, and may be considered to be a distortedoctahedron with three coordination sites occupied by oxygen atoms (nearlysame positions as in XeOs), two sites by fluorine atoms (Xe-F distances beinglarger than normal covalent Xe-F bonds but smaller than bridging Xe-Fdistances) and the sixth site occupied by the Xe lone pair of electrons.The absence of absorption in Xe-F band region is easily explained as theXe-F distances in these compounds are longer than the normal covalent Xe-Fdistance, hence the absorption due to Xe-F in these compounds would occurat lower frequencies. The stabilities of these compounds have been attributedto the special stability of octahedral coordination in Xe(v)r compounds.70A potassium salt containing Xe in (VI) and (VIII) oxidation states, namelyK4XeO6.2Xe03, which was reported earlier71 has been reinvestigated.72 It isprepared by ozonolysis of an aqueous solution of XeO3 and KOH.It is stableup to 201 "C but is sensitive to mechanical shock. It decomposes according tothe equation K4XeO6.2Xe03 = &XeOs + 2Xe + 302.Ir, x-ray diffraction and thermal study suggest that XeO3 is likely to be co-ordinated to the central perxenate moiety through an 0x0 bridge to give a salt,K4Xe3012.72Other complexes. All the compounds described so far are those in whichxenon is attached to F or (and) oxygen atoms. Some new compounds haverecently been reported in which xenon is attached to oxygen which is itselfattached to some other groups.The reactions of XeF2 with appropriate amounts of fluorosulphuric acid orperchloric acid at -78 "C or below produce compounds such as xenon(I1)fluoride fluorosulphate, FXeS03F ; Xenon@) bis(fluorosulphate), Xe(S03F)z ;156 R.I.C.Reviewand the corresponding perchlorates, FXeC104 and Xe(C10&.73 These seemto be formed by the replacement of fluorine atoms on XeFz by - OSOzF or- OC103 groups. All these compounds are colourless solids at room tempera-ture; their melting points have been reported. The fluorosulphates are kineti-cally more stable than the perchlorates but all are thermodynamically un-stable at room temperature. They decompose as shown:FXeS03F -+ X e + XeFz + S%OSF~Xe(S03F)z + Xe + SZOSFZThe perchlorates give Xe, 0 2 and C1207 with some ClOz.The x-ray structure of FXeS03F has been reported.73 The Xe is bicovalent,bonded on one side to F and on the other to one 0 of the S03F group.In thebis compounds Xe is bonded as 0-Xe-0.Similar compounds F X ~ O T ~ F S ~ ~ and Xe(OTeF&75 have been produced bythe reactions of XeFz with an equimolar amount of HOTeF5 and a five-foldexcess of HOTeF5, respectively. The tellurium compounds are much morestable than the above mentioned sulphur and chlorine derivatives. They maybe distilled under vacuum at room temperature without decomposition.FXeOTeF5 acts as a fluoride ion donor in the same manner as XeFz toproduce [F5TeOXe]+[AsFs]-.74 It is expected that the corresponding fluoro-sulphate and perchlorate will behave the same way.All these compounds of Xe react with water to produce the correspondingacid, xenon and oxygen.[Xe(OTeFs)z reacts slowly with HzO but reactsvigorously in strongly alkaline medium.741The reaction of XeFz with trifluoroacetic acid at -24 "C gives xenon@)fluoride trifluoroacetate, FXeOC(O)CF3 and xenon@) bis(trifluoroacetate),Xe[OC(O)CF3] 2 both of which are pale yellow solids and detonate on thermalor mechanical shock. Their ir spectra in acetonitrile solution have beenreported.76A new complex of approximate composition XeMnF5, a wine red solid, hasbeen reported to have been obtained during an investigation on the influenceof manganese trifluoride on xenon-fluorine reaction.181 The reaction betweenxenon and fluorine was carried out at 120°C in the presence of MnF3. Thevolatiles had been removed by prolonged pumping off at 50°C and theresidue left in the vessel had the composition XeMnF5.SECTION 2: OLD COMPOUNDSIn this section the 'old' compounds, i.e.the simple compounds reported priorto 1966 are discussed, and those compounds are included about which newpreparative methods and new structural and other physical data have beenobtained.New preparative methodsXeFz. A simple and elegant method of preparation of XeFz has been describedin which a mixture of xenon and fluorine (or FzO) in Pyrex glass is exposed tosunlight or diffused light at room temperature.77978 A slight modification ofthis method has been described in detail.79 Crystals of the difluoride can beseen within two hours in bright sunlight and in diffuse daylight within two days.Jha 15A static thermal method similar to that for the preparation of XeF4 andXeF6 has also been evolved to produce XeF2 under controlled conditions.8lThe advantage in this method is that the same vessel (Monel or nickel) can beused for the preparation of all the xenon fluorides; quartz or glass beingnecessary for preparing XeF2 by other methods.KrF2. A simple preparative method for KrFz has been reported by whichKr-F2 or Kr-OF2 mixtures are exposed to sunlight,@ but similar independentexperiments have failed to produce any krypton fluoride.83It is worth mentioning here that attempts to repeat the preparation ofkrypton tetrafluoride have only resulted in the isolation of K ~ F z .~ ~Manufacture of noble gas Jluorides. Various methods have been patented forthe manufacture of noble gas fluorides. The methods for production of xenonfluorides are by (i) contact of Xe with chlorofluorocarbons like CFzClz underhigh electric discharge86 and by treating Xe with (ii) NzF287 or (iii) 02F2.34A method similar to (i) has been patented for the manufacture of kryptonfluoride86 and one similar to (iii) for radon fluorides.34Gas mixtures of Xe or Kr with F2 have been found to react upon protonbombardment to yield the various fluorides of Xe or Kr.88The conditions for the synthesis of xenon fluorides have been studied againand it is reported that the main product from an Xe-F mixture in the moleratio 1 : 10 with the total pressure - 33 atm in a closed system is XeF2 at120 "C, XeF4 at 150 "C and XeF6 at 200 OC.8931 These conditions are rathermild compared to the others which require higher temperatures.Xe0F4.In earlier preparations it was assumed that the most satisfactory wayto produce XeOF4 was to treat XeF6 with H2O in the vapour p h a ~ e , ~ O ~ ~ ~ but ithas since been realized that the reaction is successful even when the reactantsare largely not in the vapour phase.92Xe-0 compounds. Radioactive xenates, XeO3 and Xe04 have been reportedto be formed by decay of l3lI and 1331 in some iodates, periodatesl3~93-95 andiodoxybenzene, CsHsIO2.96 The reactions are of the type 131105 -+ 131Xe03.Aqueous Xe03. A detailed method of preparation of pure aqueous Xesolution has been described by Appelman.97 In this, xenon hexafluoridehydrolysed and fluoride ion is precipitated by MgO.Mg2+ is removed wZr3(PO4)4 and the resulting H3P04, along with the last traces of fluoride,hydrous ZrO2.Perxenate. A method for the manufacture of perxenates has been patented.98Alkali metal perxenates are made by the alkaline hydrolysis of XeF6 or by thedisproportionation of alkaline solution of XeO3 and oxidation of such solu-tion by ozone. CU(III), Ag(rI), La(@, Zn(rr), Pb(Ir), UOZ(II) and Th(1v)perxenates are formed by adding the solutions of these ions to perxenatesolution. A laboratory method of preparation of sodium perxenate by the158 R.I.C. Reviewreaction of ozonized oxygen with aqueous XeO3 solution has been describedin detail.99Structural and other physical studiesxeF6. Among all the xenon compounds XeF6 has been the most controversialwith regard to structure. The other 15 known hexafluorides are all octahedraland so the same octahedral structure was expected for XeF6.At the very out-set it was realized that the valence shell electron pair repulsion model predicteda distorted octahedron whereas empirical MO calculations suggested a regularoctahedral structure.2 At that time electron diffraction studies indicated a dis-torted octahedral structure, whereas ir and Raman studies did not give anunambiguous result.It has now been established beyond any doubt that xenon hexafluoridepossesses a slightly deformed octahedral structure in the vapour phase. Thecause of this deformation is not well known yet. The various structural studiesare summarized below.Theoretical studies by Claxton and BensonlOO indicate that the structure ofXeF6 is determined by a net electron attraction term, causing distortion.Contrary to earlier conjectures it has been shown that MO calculations mayalso predict a distorted structure for XeF6.1019102Comparison of the measured equilibrium constant for the reactionXeF4 + F2 --f XeF6 with thermodynamic functions calculated for variousmodels of XeF6 suggests that the molecule has less than octahedral sym-metry.103Electron diffraction studies have again favoured a distorted octahedron forXeF685J04 but the distortion is much smaller than expected from Gillespie’smodel.21 In view of the small static distortion of the order of magnitude ofvibrational amplitude in XeF6 required to fit the electron diffraction data, analternative model of dynamic distortion symptomatic of Jahn-Teller inter-action was proposed and found to fit the diffraction data better.105 Realizingthat no single geometry is capable of accounting for the electron diffractionpatterns, Bartlett and BurbanklOG have offered an explanation for the reportedelectron diffraction data which involves intramolecular rearrangement betweenseveral molecular geometries ; there being little difference in energy betweenCzv and C3v configurations a transformation between the two along a path ofC8 configuration is assumed (Fig.5). By two successive transformations eitherC3v + Czv -+ CSv or Czv -+ CsV -+ Czv a molecular rearrangement may beaffected. A very good agreement between the synthetic radial distributioncurves obtained by this model and those reported has been claimed.A similarmodel has been used to explain the electron diffraction data of the relatedIF7.106 Another detailed analysis of the electron diffraction results183J84points to a similar model consisting of rapidly inverting non-Oh structures;however, a satisfactory model can be described as having a CsV structure, adistorted octahedron in which the xenon lone pair occupies the centre of one ofthe faces of the octahedron. The electron diffraction data gives a mean Xe-Edistance of 1.890 & 0.005 A.The Raman spectra of solid, liquid and gaseous XeF6 have been reported.107There are three bands in the Raman spectrum of monomeric XeF6 vapour asJha 1591CSlone pairc3 v@.fluorine atomsFig.5. Transformations of configurations in XeFG.expected for Oh symmetry, but one band is very broad compared with thespectra of other hexafluorides. Thus either the ground state vapour moleculespossess a symmetry lower than Oh or they have some very unusual electronicproperties that markedly influence the region of the spectrum usually con-sidered to be the vibrational-rotational. In the liquid and solid compoundmore bands are observed due to the lowering of symmetry by aggregation incondensed phases (see p 161). During an ir, far ir and microwave study108 morebands than expected for Oh symmetry were observed in the bond stretchingregion. Definite absorptions were not observed in the bond bending regionand no microwave absorptions occurred in the range 3.7 to 8.6 cm-1. Glassloghas analysed the reported Raman and ir data and has concluded that theground state vapour molecules of XeFs possess Oh symmetry but have unusualelectronic properties which influence the band widths.Magnetic susceptibility measurements show that XeF6 exhibits a tempera-ture-independent diamagnetism from 77 K to 325 K.llO*lll The deflection of amolecular beam of XeF6 in an inhomogeneous magnetic field indicates thatgaseous XeF6 at room temperature does not contain paramagnetic componentsgreater than - 1 per cent in abundance.l12 These results are not consistentwith Goodman's hypothesis113 that a low lying 3Tu state of XeF6 is appreciablypopulated at room temperature.The value of electric dipole moment of XeFG is found to be less than 0.03D114by deflection of a molecular beam of XeF6 in an inhomogeneous electric field.Such a small value of the dipole moment eliminates Gillespie's electron pairrepulsion model.The electrical conductivity of liquid XeF6 is (1.45 & 0.05) x 10-6 0-1at 50 "C and the dielectric constant is 4.10 & 0.05 at 55 "C.115 The low value ofthe dielectric constant accounts at least in part for the lack of ionization ofsalts, e.g.of CsF40 in liquid XeF6. A reported transition of XeF6 from acolourless crystalline solid to a pale yellow solid at 42 "C116 has been ascribedto premelting in impure sampIes.117A preliminary study of the heat capacity and other thermodynamic func-160 R.I . C. Reviewtions of solid XeFs has indicated that XeF6 is polymorphic; it exists in threemodifications.118 A similar detailed study has confirmed the above conclu-sion.llg In this study a more accurate value of the melting point of XeF6 hasbeen obtained, 322.63 5 0.10 K, and the enthalpy of fusion has been found tobe 5743 J mol-1. Two modifications, namely, monoclinic and cubic, have beenidentified crystallographically120 and they are assumed to correspond totwo of the three phases indicated by heat capacity data. A detailed x-raystructure analysis has been reported but crystallographic data was onlyobtained for the cubic form.120 Recently, during an x-ray structure analysis ofthe cubic form, it was also observed that the cubic form was stable in thetemperature range 301 K and 103 K.121J22 This observation is in conflict withthe heat capacity data according to which three modifications should havebeen observed in that temperature range.The single crystals of cubic XeF6 have been further investigated and it hasbeen found that cubic XeFs is stable from the melting point to 93 5 5 K;122thus, this phase is capable of existence at temperatures characteristic of all thephases inferred from the thermal measurements. In fact, the single crystals ofboth the monoclinic and cubic modifications were found to grow side by sidein the same capillary at 296 K.The stability of the cubic form of XeF6 hasbeen explained on the basis that a solid-solid transition would not be possibleat an observable rate between the two phases (cubic and monoclinic) becauseof the lack of a simple relationship between the two structures.122 The mono-clinic form apparently contains tetrahedra of xenon atoms linked by bridgingfluorine atoms120 whereas the cubic form contains ions of X e P and F-associated in tetrameric and hexameric rings of point group symmetries 4 and32.121It has been concluded122 that the cubic XeFs is a phase additional to thoseinferred from the heat capacity rnea~urements1~8J~ and the structuralmodifications do not involve the cubic phase; the transition observed duringthe thermal study most probably ensues from the monoclinic modification.Further work is evidently required to completely elucidate the nature ofpolymorphism in XeF6.However, in view of the stability of the cubic form ofXeF6 and its ability to coexist with other modifications, care must be taken toconsider the crystallographic phase present in addition to chemical purity,when making physical measurements on solid XeF6.XeFz, XeF4 (and XeF6). The linear structure of XeF2 and planar of XeF4 arewell established,2 hence very few new structural investigations have beenmade. Mossbauer study seems to have validated the assumption o f p bondingin XeFz and XeF4.13914 Recently, the v3 region of the spectrum of XeF2 hasbeen studied under high resolution. The Xe-F bond length obtained from thisstudy is 1.977 0.001 5 &l23 which compares well with the previous value. Anextensive study of the Xe-Fz system1o3 has yielded equilibrium constants forvarious fluorination reactions of Xe from which a few thermodynamic func-tions of these fluorides have been calculated.The values of the heats of forma-tion [AHf(g)] of XeF2, XeF4 and XeF6 have been found to be -25.9,- 51.5 and - 70.4 kcal mol-1, which lead to the Xe-F bond energy values of31.3,3 1.2 and 30.2 kcal mol-l in XeF2, XeF4 and XeF6, respectively. It may beJha 16noted here that the Xe-F bond energy in various fluorides of xenon remainsnearly constant whereas I-F bond energy seems to decrease with increase incoordination number of I in different iodine fluorides.85For XeF2 two more values of heat of formation have been reported. AHf(g)obtained by the measurement of the heat of reaction of XeF2 with ammonia isreported to be - 28.5 1 kcal mol-1.124 AHf(s) obtained by measuring theheat of combustion of XeF2 is found to be - 41.5 0.6 kcal rnol-1,125 whichyields the value of AHf (g) as - 28.4 kcal mol-l taking the heat of sublimationof XeF2 to be 13.1 kcal mol-1.126 These values are on the high side as com-pared to the data obtained by Weinstock et ~ 1 .~ 0 3Vapour pressure-temperature relationships have been studied for XeF2 andXeF4 by Schreiner et a1.,126 giving heats of sublimation of 13.1 and 14.4 kcalmol-l respectively (cf. earlier values of 12.3 and 15.3).127 Accurate triplepoints, obtained by the thermal arrest method, are 129 "C for XeF2 and 117 "Cfor XeF4. Earlier reported values were 140 "C and 114 "C, respectively.128 Avalue of 130 "C for XeF2 has also been obtained ~eparately.1~9XeOF4. The melting point of XeOF4 has been redetermined and found to be- 46.2 "C,66 the earlier reported values were - 41 "C90 and - 28 OC.91 Theelectrical conductivity at 24 "C is 1.03 x 10-5 Q--1 cm-1 which indicatessome self-ionization in the liquid and also accounts for the enhanced con-ductivity on addition of alkali metal fluorides.The dielectric constant at 24 "Cis 24.6 which lies between those of IF5 and B ~ F s . ~ ~ The dipole moment isreported to be 0.65 5 0.09D.lS5Normal coordinate analysis of XeOF4 and XeF4 has been carried out tocalculate various vibrational functions.130 A recent microwave study185confirms the C4v symmetry of this molecule and gives precise bond distances:Xe-F = 1.900 & 0.005 Xe-0 = 1.703 5 0.01 5 A, the 0-Xe-F angle =91.8 " 5 0.5 *.The Raman spectrum of XeOF4 in liquid HF is similar to thatof the pure substance.186KrF2. A Mossbauer study,131 an electron diffraction study85 and a rotationalfine structure analysis of 590 cm-l ir band of KrF2132 support the earlier con-clusion that the molecule is linear. A Kr-F bond length of 1.875 & 0.002 or1.867 & 0.002 A accords with the rotational structure satisfactorily; the valueobtained from electron diffraction measurements is 1.889A vapour pressure temperature study has given a value of 9.9 kcal mol-1 forthe heat of sublimation.133 The heat of formation of KrF2(g) has been deter-mined to be + 14.4 & 0.8 kcal mol-1. From this the Kr-F bond energy isestimated to be 11.7 kcal mol-1.134 KrF2 has a special importance: so far thisis the only fluorine compound known which is formed endothermically fromthe elements.Hence it follows that it is a stronger oxidizing agent thanelementary fluorine.0.01 A.Xe-0 compoundsXeO3. XeOs has a very low vapour pressure at room temperature but XeO3molecules have been observed in the vapour phase by mass spectrometry.187I62 R. I. C. ReviewElectrophoretic and chromatographic studies indicate that the hypotheticalH6Xe06 (aqueousThe enthalpy of formation of XeOs(aq) at 298.15 K has been estimated tobe 99.94 $ 0.24 kcal mol-l from the calorimetric measurements of enthalpiesof reactions of Xe03(aq) with HI(aq) and Iz(c) with HI(aq).137exists in solution either as XeO4+ or XeOgt.136Xe04.The molecular structure of xenon tetroxide has been investigated in thegas phase by electron diffra~tion.l7~ The data is compatible with the tetra-hedral structure proposed from analysis of the ir spectrum. The Xe-0distance is deduced to be 1.736 A as compared with 1.6 8, (approx.) from irresults.180 A detailed analysis of the vibrational spectrum has been reportedand vibration amplitudes have been calculated.lB8Xenate and perxenate. Structural data on xenate and perxenate ions in solidcompounds were available earlier but the aqueous solutions of these ions haveonly recently been investigated by Raman spectroscopy.l38 The investigationsindicate that the HXeO, ion is the predominant species present in aqueoussodium xenate solution and Xe0;- ions in the solutions of caesium perxenate.In perxenate solutions certain details of the spectra imply the presence of otherionic forms.A Mossbauer study has indicated that the measured isomer shift13is consistent with directed sp3d2 hybrid bonds for Xe04,-.SECTION 3 : KINETIC STUDIES AND SOME OTHER REACTIONSFormation of XeFz and XeF4Kinetic studies on the formation of XeFz and XeF4 from the gaseous elementsreveal that the reactions are heterogeneous and occur, for the most part, onthe fluorinated walls of the Monel reaction vessels or on the surface of addedmetal fluorides like COF3, NiFz and CaF2, which act as catalysts.139J40 Adetailed study of the catalytic formation of XeF2 has been carried out byBaker et al.141 They report that the formation of XeFz is catalysed by Pd andNi and other metals which form ionic fluorides, e.g.Co, Cu and Al, and not bymetals which form covalent fluorides, e.g. Ti, Zr, Mo, Ta, W, Re, Ir, Fe, Cr, V,Rh and Pt. The catalytically active metals become coated with an ionicfluoride layer. The Pd and Ni systems were investigated in detail; for bothmetals the reaction (Xe + Fz --f XeF2) was found to be zero order for xenonand fluorine with partial pressures > 50 Torr.That the rate of formation of XeFz is zero order in fluorine pressure has beenexplained on the basis that fluorine is known to be strongly adsorbed. That therate is zero order in xenon pressure is explicable only if the xenon is alsostrongly adsorbed at the surface, by chemical combination with fluorine.Baker et al.142 have demonstrated by radioactive techniques that xenon isadsorbed onto the metal surface in the presence of fluorine.The xenon isprobably chemically bound to fluorine in the adsorbed state and this mayconstitute the intermediate in the catalytic formation of XeF2. Attempts arebeing made to elucidate the mechanism of the photolytic preparation of XeFz.Flash photolysis of XeF2 has been carried out alone and in the presence of othergases to investigate the mechanism involved.143Jha 16A very interesting observation has been made during an investigation of thecatalytic role of magnesium fluoride and nickel fluoride of high surface area inthe reaction of xenon and fluorine (1 : 10 molar ratio) at low temperature,namely 120°C.In the presence of MgF2, xenon difluoride was produced asexpected89 but in the presence of nickel fluoride the only product was xenonhexafluoride.ls1 Xenon hexafluoride was produced even when the mole ratioof xenon to fluorine was only 1 : 5.Hydrolysis of XeF2The reaction representing the hydrolysis of xenon difluoride (XeF2 +H2O -+ Xe + 302 + 2HF) is a first order reaction. The rate constant in0.01 MHC104 solution is reported to be 2.58 x low2 min-1 at 25 OC; AH* andAS* are 19.6 kcal mol-land -8.1 cal OC-1, respectively. Mechanisms have beenproposed invoking XeO and Xe02.144 The rate constant in pure water isreported to bc (1.51 min-l at 25 OC.145 Another estimate ofthe activation energy for the hydrolysis is 18.4 & 2.1 kcal mol-1 which gives afrequency factor in the Arrhenius equation of 1.2 x 10l2 min-1.The frequencyfactor calculated from the number of active collisions of XeF2 and H2Omolecules is 0.9 x 1012 In this case the mechanism is supposed toinvolve the XeF. radical.Studies of the effects of pH and of different cations and anions on the rate ofhydrolysis show that the rate is minimal in the pH range 4-9; cations capableof forming stable fluorocomplexes, e.g. Th4+, A13+, Be2+ and La3+ and anionsNO,, HCO, and ClO, have an accelerating effect on hydrolysis.147 Cationsprobably form active (F-Xe-F-M)n+ complexes or else take up F- from XeFzto form MFn+ complexes, thus releasing an active XeF+ species.Anions alsopossibly form active intermediate fluoro complexes. Hydrogen fluoride alsoaccelerates the hydrolysis rate, perhaps by picking up fluoride ion to formHF;.l48It has been concluded that at least 97 per cent of XeF2 sample dis-solved in water is initially present in solution as molecular XeF2. Side reactionsproducing XeF(OH), HF, XeF+ and F- etc., are believed to occur only toan extent of about 3 per cent.1490.04) xOther reactions of XeF2Some of the reactions of XeF2 where definite products have been obtained areshown in Fig. 6. XeF2 acts as oxidative fluorinator in most of the cases.Xenon difluoride dissolves in acetonitrile with solvation but without furtherreaction at 25"C153 but at the boiling point of the acetonitrile fluorinationtakes place.49 Xenon difluoride also dissolves in dimethyl sulphoxide andpyridine giving some gaseous products; it does not dissolve in liquid NH3, butNz and F2 are slowly evolved with the formation of NH4F.49XeF2 dissolves in BrF5, HF, IF5, S02, CH3CN and WF6 without oxidationor reduction.But in the presence of a trace of a Lewis acid such as BF3, HF orSO2 it acts as an oxidative fluorinator,l51 e.g. 1 2 and XeF2 do not react inCH3CN but in presence of acid, iodine is oxidized; liquid SO2 and XeF2 do notreact but in the presence of a trace of BF3 the sulphur dioxide is converted tosulphuryl fluoride. The role of the Lewis acids may be explained by supposing164 R.I.C. ReviewSZOs F, (Ref. 151)ArF,(Ref. 49) AS F 5 -CCI,FCCIF,(Ref. 49) CCI F3 - - BrF, (Ref.150)(Ref. 152) KIO, IF 5 (Ref. 150)Fig. 6. Reactions of XeF2.that they facilitate XeF2 ionization, XeF2 + A --f XeFf + AF- or2XeF2 + A 4 Xe2F+ + AF-. These cations then act as oxidative fluorinators.Solutions of XeF2 in CH3N02, dioxane, cc14, THF, DMF, DMSO, pyridineetc. have been studied by ir spectroscopy.l54 It has been concluded that theXe-F bond is weakened with increasing donor strength of these solvents.The solutions of XeF2 in liquid NOF,3HF have been investigated bysolubility, conductivity and 9F nmr measurements.l55 XeF2 is highly soluble,rather unusually, in this solvent, e.g. 83.8 per cent at 80 "C and 73.2 per cent at16.8 "C. XeF2 remains as molecular species in this solvent.An interesting experiment on gas chromatographic analysis of XeF2 invapour phase using a KelF-10 oil Fluoropac 80 column has been described.156It is suggested that the retention properties of XeF2 should offer possibilitiesof preparative separation of XeF2, HF and F2.Reaction of molten XeF2 with TiF4 has been investigated by conductometricmeasurements which shows that XeF2 forms a 2 : 1 adduct with TiF4 whichhas been formulated as [FXe][TiF6][XeF].157Xenon difluoride reacts with anhydrous nitric acid at 20°C giving red-brown products which decompose rapidly and have not been characterized.Ithas been suggested that unstable FXeN03 and Xe(N03)~ are produced in thisreaction.76 Aqueous solutions of XeF2 oxidize iodate to periodatel52 andbromate to perbromate.l89XeF2 reacts with excess benzene in cc14 in the presence of small amounts ofHF to give fluorobenzene in 68 per cent yield.190Reactions of XeF4XeF4 reacts with NH3, MezSO, C5H5N, MeCN, CC~.LF-CCIF~ and AsF3 togive the same products as XeF2 but it is more oxidizing in nature as it appar-ently reacts with IF5 to produce IF7.49 The reaction of XeF4 with IF5 is doubt-ful as Bartlett et al.have observed5O that especially purified XeF4 does notreact with IF5. The solutions of XeF4 in liquid NOF,3HF have been investi-Jha 16gated by solubility, conductivity and 19F nmr techniques.155 The solubility is5.95 per cent at 18.4 "C and 21.22 per cent at 80.9 "C. These values are muchlower than those for XeF2.Hydrolysis of Xe F4During the hydrolysis reactions of XeF4 a transient yellow colour has beenobserved by many workers.The yellow colour was generally assumed to bedue to XeO and reactions involving XeO and Xe04 were proposed for thehydrolysis of XeF4.71 Recently, it has been proved that the transient yellowcolour may be due to XeOF217 and in the light of this finding the reactionschemes for the hydrolysis of XeF4 may have to be modified.Reactions of Xe03A thermal decomposition study showed that the decomposition of XeO3begins at 40 "C and is complete at 140 "C. The decomposition is smooth andtakes place without explosion,l58 contrary to earlier reports.The oxidation of PU(III) to PU(IV) by XeO3 in aqueous solution has beenstudied kinetically. The mechanism of reaction appears to involve either twosuccessive one-electron changes or one two-electron change to form a Pu(v)species, other than PuOg, which then reacts with PU(III) to form Pu(1v).l59 Asimilar study of the oxidation of Np(v) to N ~ ( v I ) indicates that a photo-chemical process is involved in the reaction.lG0 The decomposition of xenontrioxide in aqueous solution is increased by the addition of XeF2; the kineticdata have been explained by assuming the intermediate formation of H202.l61Xenon trioxide dissolved in tertiary butyl alcohol behaves like an acid and maybe titrated with standard K or Rb t-butoxide dissolved in t-butyl alcohol. Theacid may be represented as ButOXe03H.162Aqueous Xe03 in acidic or neutral medium oxidizes primary and secondaryaliphatic and aromatic alcohols to C02 and H2O.The reaction with tertiaryalcohols is slow; some tertiary alcohols apparently do not react at all.163J64Carboxylic acids, including hydroxy and dicarboxylic acids, are also oxidizedto C02 and H~O.165 Benzyl alcohol and benzaldehyde are oxidized to benzoicacid by aqueous Xe03.166Rough estimates for oxidation potentials have been obtained experi-mentally.71~167 In acid solution the Xe-Xe(v1) potential should be about 1.8 Vand that of Xe(vI)-Xe(vIIr) about 2.3 V. Recently, more accurate assessmentsof the electrode potentials of the Xe-XeO3 couple in acidic solution and the Xe-HXeO, couple in basic solution have been made; these values are 2.10 0.10and 1.24 & 0.01 V, respectively.l37SECTION 4 : APPLICATIONSAfter the synthesis of noble gas compounds potential applications were sug-gested.168 Since then, from time to time, this topic has been alluded to byvarious authors.6~44~169J70 A few reactions which are of potential practicalimportance have been carried out recently and are summarized below.166 R .I. C. Re viewXeO3The potential uses of xenon trioxide are based on its strong oxidizing powercoupled with the fact that the reduced product is xenon gas which does notcontaminate the oxidation products.Aqueous XeO3 oxidizes carboxylic acids and primary and secondaryaliphatic and aromatic alcohols to COz and HzO.l63-l65 Micro- and semi-micro-amounts of the acids and alcohols have been estimated by adding aknown excess of Xe03 and determining the excess of XeO3 iodometrically.Acids like acetic and succinic which are otherwise difficult to oxidize canalso be estimated.Other dicarboxylic, hydroxy and amino-acids have beenestimated in amounts as low as 100 pg165 whereas methanol, ethanol and pro-panol have been determined in amounts as low as 30 ~ 8 . ~ 6 3A direct spectrophotornetric titration between XeO3 and H202 has beenused to determine small amounts (- 50 pg) of HzO2. Even lower amounts ofH202 (- 0.9 pg) can be estimated by using a catalytic method which utilizesHzO2 to initiate the reaction between t-butyl alcohol and Xe03.171Xe fluoridesXenon difluoride acts as an oxidative fluorinator in the presence of a trace ofacid.151 In aqueous solution the difluoride can be used as an oxidant, e.g.oxidation of K103 to KIO4.152 XeF has been proposed for the detection anddetermination of I- and Cr3+.172 XeF2 oxidizes I- to 10, and Cr3+ to CrzO;-.Xenon fluorides have proved to be useful initiators in polymerization.87Advantage has also been taken of the fluorinating properties of the xenonfluorides.They may be used as fluorinating agents for inorganic and organiccompounds under certain conditions.Prolonging the lifetime of tungstep filamentsThe lifetime of tungsten filaments in incandescent lamps is prolonged by mixingxenon fluorides with the gas to fill the lamps.l73Separation of xenon and kryptonIt had been suggested earlier1G8 that Xe and Kr can be separated from eachother by fluorination reaction. The conversion of xenon to its fluorides isrelatively simple whereas krypton does not react under those conditions.Aproblem which may utilize such an application is the separation of radioactivexenon and krypton which are formed during the fission in nuclear reactors.Such a separation has been reported recently.182CONCLUSIONAs the advances have been made in the knowledge of the chemistry of noblegases the inherent danger in working with these compounds has also beenrealized, in view of the explosive nature of XeO3 and other compounds.A good account of safety precautions while working with noble gas com-pounds has been given by H o l l o ~ a y . ~ y ~ 7 ~ Recently, there has been anotherreport regarding the explosion hazard during work with fluorine containingJha 16xenon compounds.l75 Shock sensitivities of mixtures containing XeFz and XeF4were determined.XeFz and XeF4 are reported to be resistant to detonationbut partially hydrolysed XeF4 samples were found to be explosive. Explosionhazards are there when these fluorides come in contact with various materialslike paper, wool, sawdust, lubricants etc.There have been only a few recent general reports regarding the nature ofbonding in the noble gas compounds apart from the review literature quoted inthe reference section.177 A theoretical calculation based on configuration inter-action for XeF2 is reported to show that the valence bond structures whichincorporate the 5d,2 orbital at xenon contribute approximately 69 per cent ofthe total wave function and the resonance of F-Xe+F- and F-Xe+-F based onthe xenon 5pz orbital contributes only about 16 per cent of the total wavefunction.178 This brings out the importance of the valence bond structureswhich involve the 5d,2 orbital at Xe in covalent bonding.ACKNOWLEDGEMENTThe author expresses his gratitude to Professor R.D. 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