me Typical Elements PARTIV Groups VI-VIII By R. H. Cragg 1 GroupVI During 1975 a considerable amount of work has been published concerning the chemistry and properties of compounds of the Group VI elements. Two major areas which merit special mention are (i) the continuing interest in 'crown' ethers and (ii) the synthesis and properties of organic polymers such as (SN), which have metallic properties. A major characteristic of crown ethers is their ability to stabilize anions. For example dibenzo-18-crown-6 and 18-crown-6 ethers have been reported to facili- tate a simple and direct route to anionic derivatives of Group VI metal hexa- carbonyls.' The crown ether-[W(CO),OH]- compound is obtained in 57% yield from a mixture of hexacarbonyltungsten crown ether and potassium hydroxide in methylene dichloride which has been irradiated for two hours using a mercury lamp.The analogous fluoride compound is obtained in 27% yield by substituting potassium fluoride for the hydroxide. However if tetraethylammonium fluoride is used only 7% of the anion is obtained. Other complex anions reported are [M(CO),X]-(M = Cr X =For OH). One of the major problems in the development of the properties of crown ethers has been the lack of a convenient method of synthesis. However recently optically pure configurationally chiral 18-crown-6 and 9-crown-3 cryp- tands have been synthesized from L-tartaric acid and D-mannitoL2 In order to obtain an assessment of the complexing power of the 18-crown-6 derivatives the stability constants defined as equilibrium constants (K in 1mol-l) \for the equilibrium M'X-+ cryptand $ cryptate'x-have been measured for metal and primary alkylammonium cations.The results given in Table 1 indicate that the 18-crown-6 cryptands form strong cationic constants defined as equilibrium constants (K in 1 mol-l) for the equilibrium Table 1 Stability constants for cryptate complexes Cryptate K Cryptate K LL-a ButNH3+ 2 x lo4 DD-c Na+ 3.9 x lo3 LL-b ButNH3+ <l.OX lo4 DD-c K+ 3.0X lo4 DD-c Bu'NH3+ <30 DD-cRb+ 4.6X104 DD-c PhCH2NH3+ 1.5%.lo6 One important property of these crown ethers is their ability to differentiate in complex equilibria between (f)(RS)-a-phenylethylammonium hexafluorophos- hate.^ This is observed when the substituent groups on the configurationally chiral J.L. Cihonskii and R. A. Levenson Inorg. Chern.. 1975,14,1717. 2 W. D. Curtis D. A. Laidler J. F. Stoddart and G. H. Jones J.C.S. Chern. Cornrn. 1975,833. 3 W. D.Curtis D. A. Laidler J. F. Stoddart and G. H. Jones J.C.S. Chern. Comm 1975,835. A.J. Carty,R.H. Cragg and J. D. Smith LL-a R =CH20CH2Ph LL-b R =CH20CPh3 DD-c R = LL-d R = CH20Ac Me DD-d R =CH~OAC DD-e R = DD-fR= MeO Rgure 18-crown-6 cycle are bulky. For example host (S)-LL-b HPF is ca. 1.00kJ mol-' more stable than (R)-LL-b HPF6. In contrast (R)-DD-c HPF6 is ca. 1.25 kJ mol-' more stable than the (S)-DD-c HPF complex. The important property of crown ethers in chiral recognition has been further extended. The cyclic ether (l),containing two 2,2'-substituted- 1,l'-binaphthyl units as chiral barrier has been synthesized and observed to complex somewhat selectively the enantiomers of the hexafluorophosphate salt of racemic methyl phenylgly~inate.~ (1) R = H or Me Analogous macrocyclic ethers (2) are observed to complex differently with the enantiomers of the hexafluorophosphate salts of racemic methyl phenylglycinate or methyl ~alinate.~ Compounds (3) or (4)are obtained by condensation of a diaza-18-crown-6 ether with an acid chloride followed by reduction with diborane of the resulting cyclic diamide,6 and have been found to catalyse nucleophilic substitution reactions as well G.W. Gokel J. M. Timko and D. J. Cram J.C.S. Chern. Comm. 1975,394. G. W. Gokel J. M. Timko and D.J. Cram J.C.S. Chern. Cornm. 1975,444. M. Cinquini F. Montanari and P. Tundo J.C.S. Chem. Comm. 1975 393. The Typical Elements as alkylation at carbon cyclopropanation and borohydride reduction. Table 2 gives some idea of the catalytic effect in the reaction of an alkyl bromide with potassium iodide. (2) e.g.,X = Y = CH,OCH X = CH,0CH2 Y = CH,CH,CH R (3) R = n-C,,H, (4) R = n-C,,H, (5) R = H i-(6) n-C,,H,,PBu",Br - (7) crown ether Table 2 Relative catalytic effects of the compounds (3)-(7) in the reaction n-C,H,,Br + KI +n-C,H,,I +KBr Yield of Catalyst T/"C Reaction time/h n-CsHl71 (YO) (3) 60 0.2 100 (4) 60 0.5 92 (5) 60 14 90 (6) 60 1 93 (7) 80 3 100 The molecular recognition of the spherical alkali- or alkaline-earth cations by an organic ligand should ideally be achieved by a system containing a spherical intramolecular cavity into which the cation may be included.Recently the macro- cyclic system (8) has been synthesized. When solid CsBr is added to a CDCl solution of (8) the salt dissolves slowly and the initial n.m.r. spectrum is slowly replaced by a new spectrum of the 1 1 complex. Complex formation is also observed by n.m.r. with KBr CsBr or BaBr in D20 as well as with NH,I in CDCl,. As all the bridges in the ligand are equivalent in the exchanging complex in CDC13 the complex must have a centre of symmetry. This strongly suggests that the cation is trapped inside the cavity of the ligand. The cryptate has a cavity radius of about 1.8 A and its high connectivity introduces considerable rigidity in the molecule.7 E. Graf and J. M. Lehn J. Amer. Chem. Soc. 1975 97 5022. A.J. arty R.H. Cragg,and J. D.Smith Preliminary measurements show that the stability constants for the K' Rb' and Cs' complexes are about 3.4,4.2 and 3.4 respectively (log k in water at 25 "C) and the Cs' complex is probably the most stable to date. The cation exchange rates (determined from 'H n.m.r. coalescence temperatures) are amongst the lowest observed with free energies of activation of 64.8 (at 28 "C) 69.8 (51 "C) and 67.3 (41 "C) kJ mol-' for the K' Rb' and Cs' complexes respectively. The structures of a number of crown ethers have been reported. X-Ray diffraction studies of two of the five possible isomers of dicyclohexyl-18-crown-6 show that the oxygen atoms lie approximately in a plane with the cavity elliptical in shape and the shorter distance across the ellipse slightly more than 4 A.*In both isomers the cavity surrounded by the six oxygen atoms is elliptical in shape with the two axial oxygen atoms pointing out of the cavity.The structures of three macrocyclic thioethers 1,4,7-trithia-( 12-crown-4) 1,4-dithia-( 15-crown-5) and 1,lO-dithia-( 18-crown-6) with ring sizes varying from 12 to 18 atoms have been determined by X-ray diffraction and the donor atoms have been found to be nearly coplanar with all the sulphur atoms directed out of the cavitie~.~ The oxygen atoms are directed into the cavities with the exception of one oxygen atom in the crown-5 compound.The average C-C distances in the ring are 1.49 1.51 and 1.50 A shorter than the expected 1.54 A. Polyether antibiotics are important monocarboxylic acid isophores owing to their ability to solubilize cations and facilitate their passage through membranes. The absolute configuration and constitution of the polyether antibiotic R021-6 150 has been established by X-ray crystallographic analysis of the silver salt." The co- ordination about the silver ions is irregular with eight Ag-0 contacts which are less than 3.01 A. Carbon-based polymers such as polyacetylene are known to exhibit electrical insulating properties. In contrast it has recently been recognized that polymers containing sulphur and nitrogen or sulphur and selenium often have properties similar to those of a metal.The observation of superconducting phenomena in the fluctuation region in the 'organic metal' tetrathiafulvalene-7,7,8,8-tetra-cyanoquinodimethane (TI'F-TCNQ) has aroused considerable interest in the use of T-donors with TCNQ in the hope of obtaining new 'organic metals'. Recently the 8 N. K. Dalley J. S. Smith S. B. Larson J. J. Christensen and R. M. Izatt J.C.S.Chem. Comm. 1975,43. 9 N.K.Dalley J. S. Smith S. B. Smith S. B. Larson K. L. Matheson J. J. Christensen and R. M. Izatt J.C.S. Chem. Comm. 1975 84. lo J. F. Blount R. H. Evans C. Liu T. Hermann and J. W. Westley J.C.S. Chem. Cbmm. 1975 853. The Typical Elements 141 cis-(9) and trans-forms (10) of diselenadithiafulvalene (DSeDTF) have been reported and on mixing with TCNQ in acetonitrile a black 1:lcharge-transfer salt is instantly precipitated." Single-crystal electrical conductivity measurements show a metallic-like temperature dependence with a room-temperature conductivity of ca.550 i2-' cm-'.Charge-transfer salts containing the organic donor TTF or a derivative have the highest electrical conduction of organic solids presently known and it has been observed that the selenium analogue has led to an improvement in the metallic-like properties of its charge-transfer salt with TCNQ. Other workers have found the d.c. electrical conductivity to be 700* 300 K1cm-' at room temperature; the conductivity has a negative temperature coefficient upon cooling.12 The TCNQ salts of TTF and its selenium analogues form an isostructural series of highly conducting organic salts l3 having remarkably similar electrical conductivities with peaks at 59 40,and 64K respectively.(9) X' = X3= S x2= x4= Se (10) X' = X4 = S X2= X3 = Se The X-ray diffraction pattern and unit cell constants for the TCNQ salts of TIT (ll) DSeDTF (9) and (lo) and TSeF (12) show these three materials to be isostructural and the 'H n.m.r. spectra show the presence of equal amounts of the cis- and trans-isomers in neutral DSeDTF. (1 1) (12) Another organic system has been observed to have similar proper tie^;'^ 1,2-dithiolylium 5-thioxo-l,2-dithiole-3-thiolates have been observed to be charge- transfer salts with semiconducting electrical properties.The specific d.c. resistance pzOwas ca. 10f-108 i2 cm-' for 3,5-diphenyl-1,2-dithialylium 4-phenyl-5-thioxo-1,2-dithiole-3-thiolate (13). This is a little surprising since the corresponding salts derived from 3-phenyl-l,2-dithiolylium salts and 3,4-diphenyl-1,2-dithiolylium salts were found to be insulators. I s-s s-s s-s S-Ph S S* H Ph H Ph (13) l1 E. M. Engler and V. V. Patel J.C.S. Chem. Comm. 1975,671. l2 M. V. Lakshmikantharn M. P. Cava and A. F. Garito J.C.S. Chem. Comm. 1975,383. l3 S. Etemad T. Penney E. M. Engler B. A. Scott and P. E. Seider Phys. Rev. Lmers 1975 34,741. l4 N. Loayza and C. T. Pedersen J.C.S. Chem. Comm. 1975,496. 142 A.J. Carty R.H. Cragg and J. D.Smith An all-valence-electron CND0/2 MO calculation predicts that the boat form of cyclohexasulphur is the stable conformer and that its potential energy is ca.16.7 kJ mol-' less than that of the chair form which is found in the rhombohedra1 crystals. The interconversion has a barrier of 96.1 kJ m01-l.'~ The crystal structures of Ba2S and Bas, determined from three-dimensional single-crystal X-ray diffraction data show that the former contains a sulphide ion and an S22-polysulphide ion with S-S $stance 2.32 A.16In the latter a poly- sulphide anion with S,2-has S-S 2.074 A and an SSS angle of 114.8'. The Ba2S phase is apparently formed only at elevated temperature. The volume available per S atom by subtracting the volume of Ba2' from the unit cell volumes of Bas Ba2S, and Bas gives the values 54.84 42.78 and 29.37A3 respectively.Thus at high pressures the formation of polysulphide anions is favoured becaise more of the available space is utilized by the S atoms. Perhaps the most significant contribution to Group VI chemistry during 1975 has been the synthesis of analytically pure single crystals of the metallic conductor polymeric sulphur nitride (SN), polythiazyl in a convenient form for solid-state inve~tigafion~.'~"~ The significance of this work may be seen as an extension of the study of 'organic metals' and inorganic conductors the electrical properties of which are quasi-one-dimensional to potentially conducting polymeric materials. Polymeric (SN) was first reported in 1910 but the potential of this polymer as a metallic conductor has only recently been recognized.This is because the intrinsic electronic properties of anisotropic solids are extremely sensitive to impurities and defects. Ultra-pure polythiazyl is obtained by the following method. The vapour of S4N4is passed over heated silver wool S2N2collecting on the surface of a cold-finger containing liquid nitrogen. Polythiazyl is then obtained by slowly growing crystals by 8Ag + S4N4 --+ 4Ag2S + 2N2 A%*S S4N4(g) + 2S2N2(g) sublimationof S2N2at 0 "Cover a period of two days followed by room-temperature solid-state polymerization over a period of three days and then completing the polymerization by heating at 75 "Cfor two hours. During the formation of (SN) the colourless tabular monoclinic crystals of S2N2 turn dark blue and become paramagnetic (g = 2.005) and then change to gold diamagnetic crystals which are pseudomorphous with and have the same space group (P2Jc) as the S2N2crystals from which they are obtained.The purity of (SN) can be confirmed by the fact that the polymer is diamagnetic has not the characteristic iodine-like odour of S2N2and there is no vapour pressure of S2N2above the polymer at room temperature. Scanning electron micrographic studies indicate that the .crystalline polymer is composed of an ordered array of parallel (SN) fibres. At room temperature the d.c. conductivity is ca. 2.5 X lo3R-' cm-' in a direction parallel to the fibre and this value compares favourably with those obtained for metals such as mercury. The value of the conductivity is temperature-dependent a characteristic property of a 15 2.S. Herman and K. Weiss Znorg. Chem. 1975 14 767. 16 S. Yamaoka J. T. Lemley J. M. Jenks and H. Steinfink Znorg. Chem. 1975. 14 129. 17 C.M. Mikulski P. J. Russo M. S. Saran A. G.MacDiarmid A. F. Garito and A. J. Heeger J. Amer. Chem. SOC.,1975,97,6358. 18 A. G.MacDiarmid C.M. Mikulski P. J. Russo,M. S. Saran A. F. Garito and A. J. Heeger J.C.S. Chem. Comm. 1975,476. The Typical Elements metal and on decreasing the temperature to 10K the conductivity increases ca. 225-fold. Indications that (SN) remains metallic at low temperature have been obtained from heat-capacity studies and at 0.26 K (SN) is superconducting. Careful experimental technique is needed to obtain pure (SN) since the polymerization of S,N appears to take place at the surface of the crystals and consequently incom- pletely polymerized crystals can be obtained which appear to be identical with (SN) but have the same X-ray intensity data and cell dimensions of pure S2N2.Although the polymer can be sublimed in uucuoat 140-150 "C when heated above 208 "C or in an evacuated vessel at 40-50 "C (SN) decomposes into sulphur nitrogen and other as yet unidentified compounds.No change is observed in the X-ray diffraction pattern when (SN) is exposed to the atmosphere for two weeks or after exposure for six days to an atmosphere of one mole of dry or moist oxygen. However slow decomposition takes place when the polymer is added to degassed distilled water. Polythiazyl consists of an almost planar chain of alternating sulphur and nitrogen atoms (14) with intrachain distances of Sa-N 1.593(4) S,-Nb 1.628(7) sa-sb 2.789(2) N,-N, 2.576(7) and S,-N 2.864(5) A.The bond angles SNS and NSN have values of 119.9(4) and 106.2(2)" respectively. ?=-T I S-Nc ,Nb S-N sb (14) An X-ray single-crystal study of S2N2,at -130"C shows that the molecule is square planar the S-N bond lengths having approximately the same value [1.65 1( 1) and 1.657(1) A] as in (SN) the values of the SNS and NSN angles being 90.42(6) and 89.58(6)" respectively. 2 Group VII The main areas of importance in the chemistry and properties of the halogens have been the synthesis and structural assignments of polyhalogen species. An X-ray crystallographic structure determination of (theobromine),H,I shows that the compound is a polyiodide salt containing cationic and anionic layers the cation consisting of hydrogen-bonded protonated theobromine species and the anion being 1164-.19This polyiodide ion is the largest polyanion to be reported.The shortest distance between adjacent 1164-anions is 3.54 A which is of the same order as that observed in the tri-iodide chains in (benzamide) HI3. The large distance between the anions is indicative of there being only a weak interaction and suggests that the 11$-species can be regarded as a discrete polyiodide anion. The C1,-anion has been identified by Raman and i.r. studies from the products of alkali-metal atom matrix reactions with molecular chlorine.2o The yellowish M' C1,-species produced resonance Raman spectra and the dissociation energy of the C12- anion ranges from 1.19 *0.06 eV for LP3'C12- to 1.38 f0.06 eV for Cs' 35Cl,-.The vibrational assignments to the (vl)and intraionic (v2)symmetric modes of M' C1,-based on a triangular geometry are given in Table 3. F. H. Herbstein and M. Kapon J.C.S. #em. Comm. 1975,677. W. F. Howard and L. Andrews Znorg. Chem. 1975,14 767. A.J. Carty,R. H. Cragg and J. D. Smith Table 3 Vibrational assignments for alkali-metal dichlorides Species u1 (waoenurnber/cm-') u2 (wuuenurnber/cm-') 6~i~12 246 552 7~i~12 246 518 NaC12 225 (270)* KC12 264 (200)* RbC12 260 (160)* csc12 259 (140)* * Estimated. Salt-molecule reactions in a matrix have been found to be very effective for the synthesis of polyatomic ionic molecules for spectroscopic study.Reaction of the chloride of sodium potassium rubidium or caesium with hydrogen chloride in an argon matrix results in the formation of the HCl anion in the species M' HCl,-.'l A comparison of the u3 frequencies and the observed shifts for the deuterium com- pounds has led to the conclusion that this species is in fact the isolated HC1,- anion and not the HC1 radical as previously assigned. A similar reaction of a Group I metal salt with chlorine results in the formation of the M' C1,- species identified by the u3 of the C13- anion. Argon-matrix reactions of alkali-metal atoms with molecular fluorine have been studied using laser Raman and i.r. spectroscopy.22 The Raman signals appropriate for the ul intraionic (F-F)- mode in the M' Fa-species show an alkali-metal shift because of the interaction with the v2 intraionic M+-F2- mode.The vibrational assignments for the symmetric modes of the M'F,-species based on a triangular geometry are given in Table 4. Table 4 Vibrational assignments for alkali-metal dijluorides Species v1(wuuenurnber/cm-') v2(wavenurnber/cm-') 6LiF2 452 708 7LiF2 452 NaF 475 454 KF 464 342 RbF2 462 (266)* CSF~ 459 (248)* * Possibly due to (MF),. The HF2-anion has aroused considerable attention owing to the possibility of a double minimum potential for the hydrogen atom. Structural studies by X-ray analysis of p-toluidinium bifluoride show the anion to be linear and symmetric in contrast to the results from neutron diffraction studies which found the (F..-H-F)- ion to be linear but with different H-F bond lengths.However calculations based on the effect of an external point charge on the bifluoride geometry have been made,23 and show that both fluorine atoms move towards the positive charge hence shortening one H-F bond and lengthening the other. A point charge therefore will affect the geometry of the HF2- anion and hence the asymmetric geometry of the HF2-anion in p-toluidinium bifluoride can be partly explained by its asymmetric crystal field. 21 B. S. Ault and L. Andrews J. Amer. Chem. SOC.,1975,97 3824. 22 W. F. Howard and L. Andrews Inorg. Chem. 1975 14 409. 23 N. S. Ostlund and L. W. Ballenger J. Amer. Chem.SOC.;1975,97 1237. The Typical Elements 145 The products of the argon-matrix reaction of iodine and an alkali metal have been investigated by Raman The six-membered vibrational progression beginning near 115cm-' decreases in intensity and increases in bandwidth in a regular manner with inckeasing vibrational quantum number for lithium sodium potassium rubidium and caesium. The values obtained for the dissociation energies for lithium sodium and potassium are 88 84 and 75 kJ mol-' respectively. The chlorine hexafluoride radical has been obtained by y-radiolysis of sulphur hexafluoride containing 5 mole '/o of chlorine pentafluoride at -196 0C.25 The e.s.r. spectrum has been assigned and ClF has been found to have a large 35Cl coupling of 77.1 mT which is more than twice the value for CIF,.It is concluded that the unpaired electron in ClF must populate the chlorine 3s-orbital. The i.r. spectrum and force field of the hexafluorobromine cation obtained by the action of excess BrF and a 2 :1 molar ratio of KrF,-AsF, has been recorded.26 The stretching force constant for the [BrF,]' cation has a value of 4.9 mdyn A-' which is the highest value observed for any BrF bond. As these bonds are much stronger than in other bromine fluorides the reactivity of [BrF,]' salts must be due to its high oxidizing power. The chlorine n.q.r. frequencies have been reported for Ph,AsCl, Et4NC13 Me,NBrCl, and Me,NICl,." In the trichloride ion the negative charge is divided evenly between the two terminal atoms with the central chlorine atom having a slight positive charge The negative charge on the chlorine atom increases as the central atom varies from chlorine through bromine to iodine.The charge distributions observed are consistent with Rundle delocalized three-centre four-electron bonds involving s-and p-orbitals and are indicative of little or no d-orbital contribution. The i.r. and Raman studies on the 1 1 complex between iodine heptafluoride and antimony pentafluoride are consistent with the complex having the ionic structure [IF6]+[SbF6]-.28 In contrast to most halogenofluorine-metal fluoride complexes which usually react violently with water [IF,]'[SbF,]- undergoes a smooth exo- thermal hydrolysis [IF6]+[SbF6]-+ 4Hz0 + HIO4 + HSbF + 6HF Although [IF,]'[SbF6]- reacts with carbon monoxide 7CO + 2[IF6]+[SbF6] + 7COFz + I + 2SbF5 there is only a slight reaction with methane or sulphur dioxide and no reaction with carbon dioxide.Both nitric oxide and nitrogen dioxide react to form stable non- volatile complexes 2NO 4-[IF,]+[SbF6]-+ NO+[SbF6]-+ NO+[IF6]-1 FNO + IF 24 W. F. Howard and L. Andrews J. Amer. Gem. SOC.,1975,97,2956. 25 K. Nishikida F. Williams G. Maniantov and N. Smyrst J. Amer. Chem. SOC.1975,97 3526. 26 K. 0.Christie and R. D. Wilson Inorg. Chem. 1975,14,694. 27 E. F. Riedel and R. D. Willet J. Amer. Chem. Soc. 1975,97 701. ** F. A. Hohorst L. Stein and E. Gebert Inorg. Chem. 1975 14 2233. A.J. Carty,R.H. Cragg,and J. D.Smith However the most significant property of [IF6]+[SbF6]- is its reaction at ambient temperature with radon forming'an unidentified non-volatile Rn compound.This has important implications for the analysis of radon in air and also for gas purifica- tion. However although the oxidation potential of [IF6]+is high enough to oxidize Rn no reaction with Xe was observed. 3 GroupVIII Xenon difluoride forms adducts with some metal pentafluorides and these adducts have been assigned ionic structures. However recent spectral evidence suggests that in many of these compounds there is considerable covalent bonding involving fluoride bridges between the cation and the anion. For example from the reaction of xenon difluoride with the pentafluorides of antimony tantalum and niobium three types of adduct have been identified having the XeF :MF mole ratios 2 :1,l 1 and 1 :2.On the basis of Raman and X-ray crystallographic studies the adducts have been formulated as [Xe,F,]' [MF6]- [XeF]+[MF6]- and [XeF]'[M,F,,]-. However recent Raman spectroscopic studies strongly suggest some covalent bonding in that the spectra of the XeF,,MF adducts can be more satisfactorily assigned on the basis of c4"symmetry for the hexafluoro-anion than oh symmetry. The results are consistent with a significant lowering of the symmetry of the octahedral anion by fluoride bridging to the [XeF]' cation. As the v(XeF) is totally symmetric the splitting of v(XeF) in many adducts has been attributed to factor-group splitting. However the mean value of the stretching frequency associated with v(XeF+) for the XeF2,2MF and XeF,,MF series of adducts decreases in the order SbF >TaF > NbF,.These results are consistent with the suggestion that the XeF bond length in [XeF]' is progressively increasing. A comparison of the peaks which have been assigned to v(Xe..-F) shows a progressive increase in value of the mean frequency which is indicative of an increasing strength of the bridging bond.29 Previously the spectrum of XeF,,SbF5 was assigned on the assumption of oh symmetry for the anion. However definitive assignment for the anion modes has proved difficult since in the case of the Sb and Ta adducts more than six anion modes were observed. If the adducts which were previously formulated as the ionic species [XeF]'[MF,]-are reformulated as having a fluoride-bridged structure of the type XeF+,FMF,- in which the anion can be regarded as distorted from oh to C4" symmetry then the observed spectral bands can be assigned more satisfactorily.The bands in all the spectra in the region 450-490cm-' are not easily assignable to vibrations of octahedral [MFJ anions and are better assigned to additional v (Me* * F). In conclusion the i.r. and Raman spectra of the adducts 2XeF2,MF (M =Sb or Ta) XeF,,MF, and XeF2,2MF (M = Sb Ta or Nb) have been measured and although the spectra have been interpreted in terms of an ionic formulation involving [XeF]' and [Xe,F,]' the results indicate an increasing covalent character in the series XeF,,SbF <XeF,,TaF <XeF,,NbF <XeF2,2TaF <XeF2,2NbF, and 2XeF2,SbF <2XeF2,TaFS.The Typical Elements Further evidence for the covalent nature comes from a Raman and 19F n.m.r. study of adducts of xenon difluoride with WOF4.30 Stoicheiometric amounts of XeF and 'WOF react in HF at room temperature and in the melts at 30-75 "C to give stable crystalline solids at room temperature. Two possible structures are the ionic form (15)and the partially covalent form (16). The low-temperature 19Fn.m.r. spectra of FOF \$/ + [XeF] [WOF,] -F/i\ (15) iF solutions in BrF and S0,ClF support the covalent stru~ture,~" which is further supported by bands in the Raman spectrum which can be assigned to a fluoride- bridged structure. In XeF,,WOF the struc!ural unit has approximately C' sym-metry. The terminal XeF bond length (1.89 A) is shorter than that of XeF (2.00 A) while the Xe...F bridge bond length (2.04 A) is longer than the XeF bonds in XeF,.The W-.-F--Xe bridge angle is 147". The Raman spectrum of XeF,,2WOF4 is also consistent with a bridged structure (17). However the 19Fn.m.r. spectrum of a S0,ClF solution of XeF,,2WOF4 is complex and in addition to lines associated with the fluoride-bridged structures lines were also observed consistent with an oxo-bridged structure. Xe \ F (17) Krypton difluoride is a very powerful oxidative fluorinating agent and has been used to synthesize gold(v) species:31 7KrF + 2Au 'i 2[KrF]+[AuF6]-+ 5Kr Raman studies at -80 "C under a layer of HF are consistent with a formulation in which the [KrF]+ cation is fluoride-bridged to a [AuF6]- anion with lines assignable to a C, symmetry for [AUF,]-.[KrF]'[AuF,]-on pyrolysis gives AuF, a powerful oxidative fluorinating agent which react with an excess of XeF, [KrF]+[AuF,]-60~~ocb AuF + Kr + F XeF LHF .[Xe,F3]+[AUF6] -30 J. H. Holloway G. J. Schrobilgen and P. Taylor J.C.S. Chem. Comm. 1975 40. 31 J. H. Holloway and G. J. Schrobilgen J.C.S. Chem. Comm. 1975,623. A.J. Carty,R. H. Cragg,and J. D.Smith Raman and 19Fn.m.r. studies on the products of the reaction of [KrF]' salts with excess of XeOF show them to be 02'and [XeOF,XeF,]' salts and not XeOF as previously reported. Relativistic quantum mechanics applied to radon (or element 118) fluoride structures indicates that ionic crystalline forms are probably more stable for the fluorides of these elements in contrast to the molecular forms of xenon An ionic crystalline form of RnF would be expected to be non-volatile as found and show the observed migration of Rn as a cation upon electrolysis.32 K. S. Pitzer J.C.S. Chem. Comm. 1975 760.