7 Halogens and noble gases E. G. Hope Department of Chemistry, University of Leicester, UK LE1 7RH 1 Introduction This chapter reviews the 1998 literature for the elemental halogens and the noble gases and compounds containing these elements in their positive oxidation states only. Publications which involve halide or oxohalide anions as counter ions are not described. 2 Halogens Following the report1 of the observation, by mass spectrometry, of molecular chlorine concentrations at a North American coastal site which cannot be explained using established atmospheric chlorine chemistry, it has been shown, in a controlled laboratory experiment, that chlorine is the product of photolysis of ozone in the presence of aqueous sea-salt particles.2 This has significant implications for marine troposphere chemistry since early-morning photolysis of molecular chlorine may yield su¶ciently high concentrations of chlorine atoms to render the oxidation of common gaseous molecules 100 times faster than the analogous oxidation with hydroxide radicals.Further improvements in procedures to control the extreme reactivity of elemental fluorine have been outlined.The beneficial e§ects of the addition of protic solvents (e.g. trifluoromethanesulfonic acid,3 ethanol4), Lewis acids5 or metal salts (e.g. hydrated copper nitrate6) have been illustrated in the direct fluorination of a range of organic substrates. In comparison with that for tert-butoxide, the chemistry of the tertbutoxylium moiety (Bu5O`) is virtually non-existent. The reaction of fluorine with tert-butyl alcohol, in situ, a§ords tert-butyl hypofluorite which represents a unique source of this novel moiety and which has been used in further synthetic chemistry.7 Fluorination at very low temperatures in solution leads to F 3 SCF 2 CF 2 SF 3 8 and the novel FSCSN 3 species9 which has been characterised by vibrational spectroscopy and the structure investigated by theoretical calculations.At room temperature, NiF 2 is oxidised by sunlight or UV light-irradiated fluorine in anhydrous HF in the presence of alkali metal hydrofluorides to give the corresponding NiF 6 2~ salts.10 In a very interesting series of developments in the quest for highest T# superconductors,11,12 annealing lanthanocuprate or mercurocuprate superconductors in F 2 –N 2 mixtures increases the T# to the maxima (yet reported) in these systems; increases of up to 60K Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 79have been achieved. X-Ray data suggest that, for the fluorination of La 2 CuO 4 thin films, a complex, multistage process occurs, yielding two, distinct, La 2 CuO 4 -type phases. Following the fluorination of the fullerenes and the subsequent application of the products in synthetic chemistry, progress in the bulk synthesis and purification of single-wall carbon nanotubes (SWNTs) has allowed the chemical modification of these species to occur.Between 150 and 325 °C the tube structure is maintained on fluorination with the fluorine atoms covalently attached to the side walls of the SWNTs. These can be cleanly removed by reaction with hydrazine to regenerate the starting material. 13 Chlorine has been used as the oxidant in an improved preparation of SF 5 Cl14 and the oxidation of a series of phosphines with bromine yields [R 3 PBr]`Br~, except for R\C 6 F 5 when the trigonal bipyramidal R 3 PBr 2 is formed.15 Interest in charge-transfer complexes of halogens and donor molecules continues apace.An ab initio study of C 6 H 6 –X 2 (X\F, Cl, Br, I) as model complexes for host–guest interactions has revealed the favoured T-shaped geometries within which the binding energy increases in the order F 2\Cl 2\Br 2\I 2 , and represent shallow minima.16 The reactions of 2,3-dihydroimidazol-2-ylidenes with 1,2-dichloroethane give a series of stable carbene adducts of chlorine (I) which possess unusually reactive Cl–Cl bonds, i.e.in contrast to the established Friedel–Crafts conditions, benzene can be chlorinated with these complexes under extremely mild conditions.17 N N R R Me Me .Cl2 I R = Me, Et, Pri Charge-transfer complexes incorporating iodine include the structurally-characterised [Pd(Et 2 timdt) 2 ] · I 2 ·CHCl 3 ,18 2([15]aneS 5 )·7I 2 , [18]aneS 6 ·I 2 , [18]aneS 6 ·4I 2 , [24]aneS 8 ·I 2 , [24]aneS 8 ·6I 2 ,19 M2,4,6-(MeO) 3 C 6 H 2N3 P·I 2 ,20N,N@-bis(mercaptoethyl)- 1,5-diazacyclooctanenickel(II)–diiodine21and the elusive 1: 1 Ph 3 PS·I 2 adduct,22 all of which contain linear S · · · I–I interactions.The species [18]aneS 6 ·4I 2 and [24]aneS 8 ·6I 2 are the first macrocyclic thioether–iodine charge transfer complexes to show both exo- and endo-diiodine coordination which is thought to arise from the large ring size.19 Theoretical23 and in-depth analyses of the experimental work24 on the pre-reactive intermediates of the halogens with Lewis bases have been published.The experimental and theoretical similarities between the series of B· · ·X 2 and B · · ·HX intermediates were identified o§ering a quantitative scale of gas-phase electrophilicities of the halogens and information on the electron distribution in elemental fluorine. 3 Interhalogen and polyhalide anions The e§ect of relativity on the properties of the interhalogens (ClF, BrF, BrCl, IF, ICl and IBr) has been established from relativistic and non-relativistic calculations of bond lengths, harmonic frequencies and dissociation energies.25 Hybrid Hartree–Fock/den- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 80Fig. 1 Crystal structure of NO`NO 3 ~·IF 5 (reproduced by permission from Z. Anorg. Allg. Chem., 1998, 624, 667). sity functional theory (DFT) calculations of molecular structures, electron a¶nities and dissociation energies of BrFn and BrFn ~ (n\1–7) reveal good agreement with experimental structural data (where available) and the first data (theoretical or experimental) on the electron a¶nities of bromine fluorides.26 Of interest is the prediction of unusually long Br–F!9 bond lengths for the hypothetical BrF 3 ~ and BrF 5 ~ suggesting that the bonding in these anions is di§erent from that in their neutral counterparts.Some thermochemical data on some iodine fluorides and their mono-substituted derivatives have been discussed.27 After an extensive study of a range of solvent systems, iodine pentafluoride has been found to be the only solvent which can be used in the synthesis of nitrosonium nitrate; this unique behaviour of IF 5 has been ascribed to its large dielectric constant and its chemical inertness toN 2 O 4 .The solvateNO`NO 3 ~·IF 5 has been structurally characterised (Fig. 1).28 The interhalogen molecules in the lattice, NO`NO 3 ~ double chains separated by layers of IF 5 , act as a diluent and are virtually unchanged if compared to the structure of the free molecule. However, there are a plethora of short, interchain, contacts from which the role of the IF 5 molecules can be interpreted as a Lewis acid to NO 3 ~ and as a Lewis base to NO`.Iodine penta- and hepta-fluorides have been used as mild fluorinating agents for transition metal oxo-anions29 and the dinitramide anion MN(NO 2 ) 2 ~N has been shown to be superior to the nitrate anion as a reagent for the controlled, stepwise, replacement of fluorine by oxygen for BrF 5 , ClF 5 and IF 7 .30 On reaction with ClF 5 at [13 °C, an equimolar mixture of KClOF 4 and KClF 4 is obtained where the formation of the oxyfluoride anion is remarkable since similar exchange processes give exclusively FClO 2 .With BrF 5 at[45 °C, KBrOF 4 is formed, whilst with IF 7 , deoxygenation of the desired IOF 6 ~ product yields KIF 6 which, in the presence of excess IF 7 , gives the novel KIF 6 ·2IF 7 adduct.Three di§erent reaction pathways are observed when BrF 3 reacts with X–C 6 H 4 SiF 3 .31 When X\H, p-Me or p-MeO, the aryl groups are exclusively oxidised. When X\m- or p-F, oxidation also occurs, but ca. 10% disubstitution at bromine yields [(FC 6 H 4 ) 2 Br][SiF 5 ]; the bromonium salts have been isolated after metathesis with BF 4 ~ or PF 6 ~. When X\m- or p-CF 3 , monosubstitution a§ords CF 3 C 6 H 4 BrF 2 as the first examples of hydrogen-containing phenylbromine difluorides.Iodine monochloride finds diverse applications in organic chemistry as a radical additive to double bonds,32 for iodination33 and chlorination, e.g. in the synthesis of C 70 Cl 10 34 and Cl 4 ARC 59 N35 (Ar\4- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 81Fig. 2 Anionic matrix in [Pd 2 Cl 2 ([18]aneN 2 S 4 )] 1.5 I 5 (I 3 ) 2 (dicationic chains run through vertical channels). Starred atoms identify one I 8 2~ unit (reproduced by permission from Angew. Chem., Int. Ed., 1998, 37, 293). Fig. 3 Primitive cubic lattice of iodide ions bridged by I 2 molecules in (DMFc) 4 (I 26 ) (reproduced by permission from from Z. Anorg. Allg. Chem., 1998, 624, 679).C 6 H 4 X; X\Me, OMe or OPr*) which contains a pyrrole moiety in the fullerene cage. The structurally characterised charge transfer complexes between Ph 3 PS and ICl/IBr consist of discrete molecular units with linear S · · · I–Cl/Br interactions.36 The weak intermolecular complexes between benzene and small molecules has been a fascinating problem for over 50 years.The first theoretical and the first UVPES study of the C 6 H 6 · · · ICl complex indicates, as expected, donation from a p-orbital of benzene to the r*-orbital of ICl in a bond-centred oblique structure.37 Numerous gas-phase pre-reactive intermediates between ClF and Lewis bases have been identified by microwave spectroscopy,38,39 including the related C 6 H 6 · · · ClF adduct.40 Here, the spectra were established to be of the symmetric-top type for which the data were interpreted in terms of a complex in which, at equilibrium, the ClF axis is inclined at ca. 14° to the C 6 axis of benzene, with the two axes intersecting at the centre of mass of the ClF molecule. The ClF axis points towards the centre of the C–C bond, intersecting the plane of the ring ca. 0.24Å inside the ring, and can rotate around the C 6 axis of the benzene molecule. For the furan · · · ClF and thiophene · · · ClF complexes, a similar interaction of the interhalogen with the p-bonding electrons occurs in preference to Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 82that with the heteroatom, non-bonding, lone pairs.39 A theoretical study of the, previously reported, SO 2 · · · ClF pre-reactive intermediate suggests an intermolecular interaction energy of 1.93 kcal mol~1.41 The extensive studies of polyiodide anions and polyiodide anionic networks continue with 30 examples structurally characterised in 1998.As the anionic networks become larger (and it is di¶cult to predict whether discrete anions or extended networks will be produced) the term ‘polyiodide architecture’ has been coined.The formation of these iodine/iodide networks often occurs at complexed metal cations as templates whereby the resulting polyanionic network is dependent, primarily, upon the charge and shape of the cation. This area has been reviewed by one of the leading groups.42 Linear I 3 ~ anions have been identified with the following cations: [Pd 2 Cl 2 (C 12 H 26 N 2 S 4 )]2` in which the cations and anions link into sinusoidal chains,43 [Ni(MeCN) 2 (C 11 H 26 N 4 )]2`,44 [Pd(C 11 H 26 N 4 )]2` where the terminal iodine atoms coordinate to the metal centre and have long interactions with each other to give cations plus pairs of anions in zigzag chains,44 [Co(C 6 H 12 S 3 ) 2 ]2`,45 dimethylferrocinium and decamethylferrocinium,46 [Pr/Me 2 PhN]` and [Pr*Me 2 PhN]`,47 [Ni(NH 3 ) 6 ]2`,48 [(NH 3 ) 3 M(l-OH) 3 M(NH 3 ) 3 ]2` (M\Cr, Co),49 and [Ag([18]- aneS 6 )]`.50 The I 3 ~ and I 4 2~ anions are both present in [Cr(NH 3 ) 6 ](I 3 )(I 4 ).51 Alarger structural diversity is observed for the pentaiodide anions isolated with the following cations: Decamethylferrocenium where the anion is unusually isolated,46 dimethylferrocenium where the anions form chains with alternating planar and helical regions,46 [Pr*Me 2 PhN]` where the anion is a iodide–diiodine zigzag chain with extra I 2 molecules attached to the iodide ion in the trans-position,47 [Ni(NH 3 ) 6 ]2` where the V-shaped anions line up to give a novel pentaiodide chain,48 [Ag([9]aneS 3 ) 2 ]` where charge transfer S · · · I interactions contribute to the extended structure featuring polymeric successions of cations and anions,50 and [BiI 2MOP(NMe 2 ) 3N4 ]` where the central iodine atom is located at an S 4 symmetry site in the crystal.51 This a§ords a very unusual bent anion which is statistically disorded over two sites whence the anion e§ectively forms a three-dimensional network giving an ‘open’ cage structure with the cations occupying the cavities.In [Pd 2 Cl 2 ([18]aneN 2 S 4 )] 1.5 I 5 (I 3 ) 2 , infinite chains of the binuclear cations are embedded in a matrix of the anions (Fig. 2).52 The anionic networks identified for (DMFc) 4 (I 26 )46 and [Ag([18]aneS 6 )](I 7 ),50 despite the signifi- cantly di§erent empirical compositions, are quite similar. For the former, the network may be derived from a primitive cubic lattice of iodide ions with I 2 bridges on all edges by systematically removing 1/12 of the I 2 molecules (Fig.3) whilst for the latter, the iodide ions occupy the lattice points of a primitive rhombohedral lattice with I 2 bridges along all edges.Cations embedded in a three-dimensional polyiodide network of cages of I 5 ~ ions and diiodine molecules are present in [RhCl 2 (C 12 H 24 S 4 )](I 5 )·I 2 .53 In [Pr*Me 2 PhN](I 8 ),47 the anion is best described as I 16 2~ which is present as 14-membered rings, catenated by diiodine molecules and linked into layers with 10-membered and two types of 14-membered rings.Regular anionic shapes are present in the solid-state structures of [(Crypt-2,2,2)H 2 ](I 8 ), [Ni(phen) 3 ](I 8 )·2CHCl 3 and bis(N-methylurotropinium) octaiodide, but that in bis(N-ethylurotropinium) octaiodide represents a new configuration which is somewhere between (I 3 ~·I 2 ·I 3 ~) and broken (I 3 ~, I 5 ~).54 Nonaiodides are very rare, but two, as [Pr*Me 2 PhN]`47 and [K([15]aneO 5 ) 2 ]`52 salts, have been structurally characterised this year. The anion in the former consists of 14-membered rings tied by two iodine bridges into 10-membered Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 83rings, whilst that in the latter is best described as an [I 3 ~·(I 2 ) 3 ] charge-transfer complex.The dodecaiodide dianion in [Ag 2 ([15]aneS 5 ) 2 ](I 12 ) is, again, best represented as a charge-transfer complex [(I~) 2 ·(I 2 ) 5 ] bound to the cation by Ag–I bonds with weak I · · · S interactions combining to build up an extended three-dimensional, spiral, superstructure.50 In marked contrast, the same anion in bis[potassium(dibenzo-18- crown-6)] dodecaiodide is nearly a discrete entity (I 2 ·I 3 ~·I 2 ·I 3 ~·I 2 ).55 A mixture of anions are identified in [Me 3 PNHPMe 3 ] 4 (I 28 ); linear I 3 ~, Z-shaped I 8 2~ and zigzag chains of I 14 4~.56 Unremarkable anionic geometries are reported in [Pd(C 6 H 12 S 3 ) 2 ](IBr 2 ), [Pd(C 10 H 20 S 4 )](IBr 2 )57 and [Cs(18-crown-6)](ICl 2 ).58 Following the preparation of a new IF 2 ~ salt, [NMe 4 ](IF 2 ), from the reaction of tetramethylammonium iodide and XeF 2 in a 1: 1 ratio at[31 °C,59 ab initio calculations on the series of related molecules and ions (XeF 2 , KrF 2 , IF 2 ~, BrF 2 ~ and ClF 2 ~) show that the previous assignments for the vibrational spectra of the IF 2 ~ anion are in error; highly unusually, the antisymmetric stretching modes for both IF 2 ~ and BrF 2 ~ have lower frequencies than their symmetric ones.This is accounted for by mass e§ects and the enhancement of three-centre-four-electron bonding by the antisymmetric stretch.Increasing the XeF 2 :NMe 4 I ratio to 2: 1 gives a route to another new interhalide salt, [NMe 4 ]- (IF 4 );60 the structural characterisation of this anion as the 1,1,3,3,5,5-hexamethylpiperidinium salt was reported in 1997. Addition of NMe 4 F to [NMe 4 ](IF 4 ) in acetonitrile results in the formation of the insoluble [NMe 4 ] 2 (IF 5 ).60 This anion, only the second example of a pentagonal planar AX 5 species, has been characterised by vibrational spectroscopy, ab initio calculations, and by comparison to the isoelectronic XeF 5 ~ anion.Interestingly, this characterisation shows, unequivocally, that the first AX 5 species was prepared 25 years ago, whence Cs 3 IF 6 is actually a 1: 1 mixture of Cs 2 IF 5 and CsF. 4 Halogen oxides and organoiodine oxygen compounds Theoretical studies on chlorine oxides by DFT61,62 correlate well with experimental data (geometries, dissociation energies, vibrational frequencies). Importantly, the C 27 geometry for ClO 4 · is calculated to be the minimum, whereas the hitherto, assumed, C 37 structure is shown to be an energetically higher-lying second-order saddlepoint.Heats of formation of BrO, BrO 2 , BrO 3 , BrONO 2 , BrONO, BrOOH and Cl 2 O 5 have been estimated.63,64 The failure to detect IO in the lower stratosphere (sensitivity^ 0.2 ppt) suggests that the role of iodine in stratospheric ozone depletion is negligible.65 A low-temperature crystal structure determination of Cl 2 O reveals an essentially molecular structure [d(O–Cl)\1.7092(4)Å] with weak secondary interactions [d(O · · · Cl)\2.7986(4)Å] a§ording a distorted tetrahedral coordination around the oxygen atom.66 Vibrational spectroscopic data for BrNO 2 and cis- and trans-BrONO, acquired in a low temperature matrix,67 are in good agreement with earlier theoretical calculations.Vibrational spectroscopic and X-ray structural investigations of the isostructural [Mg(IO 3 ) 2 ·4H 2 O], b-[Ni(IO 3 ) 2 ·4H 2 O] and [Co(IO 3 ) 2 ·4H 2 O] show octahedrally-coordinated metal centres with trans-monodentate iodate ligands and some internal hydrogen bonding.68 The room temperature, ferroelectric phase of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 84Fig. 4 Polymeric network in HIS 2 O 8 (reproduced by permission from Angew. Chem., Int.Ed., 1998, 37, 1426). [C 5 H 5 NH][IO 4 ] (prepared by the reaction of pyridine with H 5 IO 6 ) contains isolated, non-disordered, tetrahedral periodate anions linked by disordered pyridinium cations. 69 Although polymeric networks of ternary oxides of non-metals are usually regarded as mixed anhydrides of oxoacids and their constitution described in terms of known oxoanions, the structure of HIS 2 O 8 (Fig. 4), for which the iodine atom geometry is most comparable to that in IF 2 `, is based upon an IO 2 structural unit for which neither the free acid nor the free anion are known.70 Whilst the reactions of aluminium or gallium salts with H 5 IO 6 yield [M(H 2 O) 6 ][IO 2 (OH) 4 ] 3 , slow evaporation of an aqueous solution of In(NO 3 ) 3 ·nH 2 OandH 5 IO 6 (1: 3 molar ratio) produces H 11 I 2 InO 14 .A single crystal X-ray study indicates that this represents the first example of a chelating I 2 O 10 unit coordinating via vertices (rather than edge sharing) to give ‘I 2 InO 12 ’ chains.71 OMe OMe OR I OH OTs– R¢OH R¢¢CONH2 V R' = Me, Et, Pri, 2-Adamantyl VI R¢¢ = Et, Pri, But, Ph III R = Me, Et + II OMe I(OAc)2 IV O I HN O R¢¢ O I OR¢ NH2 +OTf– NH2 +OTf– NH I OTf O Significant new studies on organoiodine oxygen/nitrogen compounds, which find widespread application in organic synthesis, have been published.Of note are, a review Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 85of their use in carbohydrate chemistry,72 a cleaner, faster and higher yield, route to diacetoxyiodoarenes using CrO 3 ,73 and the synthesis and structural characterisation of some, relatively rare, chiral hypervalent iodine compounds (II, III); asymmetric oxygenation reactions with up to 53% ee were outlined.74 Compound III shows a strong interaction between the iodine and the methoxy group, allowing its formulation as a salt, whilst II shows no intramolecular interactions.The new benziodazole (IV) reacts with alcohols or amides to give novel 1-alkoxy-3-iminiumbenziodoxoles (V) or 1-amido-3-iminiumbenziodoxoles (VI) for which the isopropyl derivatives have been structurally characterised.Ab initio calculations indicate that the driving force for the novel internal rearrangement is the relative thermodynamic stability of the N- vs. O-protonated species.75 5 Cationic iodine and other organoiodine compounds In the last 20 years, interest in hypervalent iodine compounds has surged.The preparations, properties and applications (in electrophilic alkynylating reactions, Michael-type conjugate additions and Diels–Alder and 1,3-dipolar cycloadditions) of the newest class, the alkynyliodonium salts, has been elegantly reviewed.76 A series of alkynylphenyliodonium salts ([R–C–– – C–I–Ph]`OTf~; R\Me 3 Si, Ph, Bu5, Bu/) have been prepared in high yields from the reactions of the appropriate trimethylsilyl alkyne with diacetoxyiodobenzene in the presence of triflic anhydride.77 The reaction of XCN (X\Br, I) with phosphine gives a mixture of [R 3 P(CN)]X and [R 3 PX]CN20 and a theoretical study confirms the linear structure for the [ICNI]` cation.78 A structural characterisation of CF 3 IF 2 , prepared from the reaction of CF 3 I and CF 3 OCl, con- firms the trigonal bipyramidal structure suggested by earlier NMR studies.79 6 Noble gases Surface structural investigations of silica aerogels,80 AgA zeolite81 and cryptophane-A in organic solution82 using 129Xe NMR have been reported.Polarisation transfer from hyperpolarised 129Xe, the high solubility of xenon in blood and the long T 1 relaxation time of dissolved 129Xe have been exploited in ground-breaking wholebody, thorax, lung and brain MRI studies which open the way for developments using xenon in a wide variety of diagnostic procedures.83,84 The laser polarisation approach, in the gas phase85 and in liquid Xe,86 has also been extended to surface nuclei with low gyromagnetic ratios, e.g.in a 13C NMR study of C 60 and C 70 .85 As postulated in Volume 94 of the Annual Report of the RSC, laser-polarisation in supercritical xenon has now been demostrated which, in view of the particular solvent properties of supercritical fluids, o§ers the extension of laser-polarisation to a wide range of applications. 87 The [Au–Xe]` complex has been examined theoretically as a benchmark for a gold(I) cation a¶nity scale,88 and a theoretical study of XeX (X\F, Cl, Br, I) cations, radicals and anions has indicated weak van der Waals interactions in the radicals and anions, and strong interactions for the cations.89 On annealing difluorovinylidene in a xenon-doped low temperature matrix, additional peaks in the IR spectrum are ob- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 86served which have been ascribed to a Xe · · ·C––CF 2 charge-transfer complex for which theoretical calculations suggest a Xe · · ·C distance of 2.379Å and a Xe–C–C angle of 102°.90 No comparable krypton charge-transfer complex has been observed on annealing difluorovinylidene in a krypton-doped matrix. 7 Noble gas compounds A detailed review of recent advances in noble gas chemistry has been published.91 Following the reports on noble gas–hydride compounds (H–Ng–Y) produced during the photolysis of HY in noble gas matrices, the first examples of xenon–sulfur (in H–Xe–SH)92 and krypton–carbon (in H–Kr–CN)93 bonds have been identified after the photolysis of H 2 S in xenon and HCN in krypton respectively.For HXeSH, only the Xe–H stretch has been observed experimentally which shows a characteristic shift on deuterium labelling and whose position is in good agreement with theoretical calculations which suggest a bent structure with the H–S–Xe angle close to 90°. Extensive theoretical studies on krypton fluorides has revealed, for the first time, that KrF is a bound molecule (in agreement with experiment), that triplet state KrF 2 is bent (71°), and that the Kr · · ·F 2 van der Waals complex has the expected T-shape.94 R R R R R R H F FXeO Me3SiO O O O O O XeF2 FXe+ (1) Xenon difluoride continues to find application as a mild fluorinating agent in organic systems, e.g.in the generation of 18F labelled methionine derivatives for positron emission tomography studies.95 In an interesting paper96 on the modes of reaction of XeF 2 with organic substrates, it is shown that the choice of reaction vessel and solvent are crucial.It is presumed that the acidic surface of the glass promotes the generation of electrophilic XeF` whereas in FEP un-ionized XeF 2 acts as a oneelectron oxidant [eqn. (1)]. The intriguing outcome, occasionally explosive, of the addition of water to a mixture of XeF 2 andUO 2 or U 3 O 8 or ZrO 2 (which do not react at room temperature in the absence of water) has been described.97 Xenon difluoride has been used in the synthesis of a rare example of an organoselenium(VI) species, [SeF 2 (C 12 H 8 ) 2 ],98 and a well ordered Y 2 BaCu 7 O 14 F superconductor (T# \62 K).99 The related Xe(OEF 5 ) 2 (E\Se, Te), have been used in the oxidation of M 2 (CO) 10 (M\Mn, Re).100 A structural characterisation of XeF 2 ·2CrF 4 reveals independent, octahedrally coordinated, chromium atoms where two of the fluorine ligands are provided by di§erent, linear, XeF 2 molecules.101 A new assignment of the vibrational spectra of the [XeF 5 ]~ anion has been made,60 and in the crystal structure of [XeF 5 ]`[CrF 5 ]~, prepared by the reaction of XeF 6 with either CrF 4 or CrF 5 , the cations show three short contacts to terminal fluorine atoms on the cis-bridged anionic chain.101 Cyclic voltametric measurements on pentafluorophenylonium cations (C 6 F 5 X)` Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 87Fig. 5 ORTEP drawing of the two discrete molecules in the unit cell of [C 6 F 5 Xe] [AsF 6 ] (reproduced by permission from Inorg.Chem., 1998, 37, 4884). [X\Xe, N 2 , C 6 F 5 Br, C 6 F 5 I, (C 6 F 5 ) 3 P] reveal that the xenon cation has the lowest reduction potential whence a one-electron reduction a§ords, after loss of xenon, C 6 F 5 ·.102 Metathesis of [C 6 F 5 Xe][(C 6 F 5 ) 2 BF 2 ] with either AsF 5 (g) or AsF 5 ·MeCN a§ords [C 6 F 5 Xe][AsF 6 ] which has been structurally characterised (Fig. 5).103 The Xe–F"3*$'% distance for this complex is less than that in [2,6-C 6 H 3 F 2 Xe][BF 4 ] which, from theoretical calculations, appears to be related to a lower r-charge on xenon in the BF 4 ~ salt. Further work on the reaction of XeF 2 in anhydrous HF with tetra- fluorobenzenes C 6 F 4 HR (R\Xe`Y~; Y~\AsF 6 ~, BF4 ~) has a§orded the first F F F F Xe+Y– H F F F F Xe+Y– H F F F F Xe+Y– F F F F F Xe+Y– F F F F F F F F F Xe+YH F F F F F F F F Xe+YF F F F F (2) XeF2 XeF2 aHF aHF + + + VII VIII IX X Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 79–91 88hydrogen-containing (polyfluorocycloalken-1-yl)xenon(II) salts [eqn. (2)].104 The spectroscopic data indicate close similarity between these hydrogen-containing cations (VII, IX) and their perfluorinated analogues (VIII, X) which precludes the presence of a ‘through space’ or chelate-like stabilization of the xenon–carbon bond as previously supposed.References 1 C.W. Spicer, E. G. Chapman, B. J. FinlaysonPitts, R. A. Plastridge, J. M. Hubbe, J. D. Fast and C.M. Berkowitz, Nature (London), 1998, 394, 353. 2 K.W. Oum, M.J. Lakin, D. O. DeHaan, T. Brauers and B. J. FinlaysonPitts, Science, 1998, 279, 74. 3 P.L. Coe, A. M. Stuart and D. J. 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