THE CHEMISTRY OF COMPOUNDS CONTAINING SULPHUR- FLUORINE BONDS By H. L. ROBERTS (RESEARCH DEPARTMENT IMPERIAL CHEMICAL INDUSTRIES LIMITED SULPHUR has the electronic configuration (3~)~(3px)~(3py)(3pz) in the ground state and as would be expected is commonly bivalent in its compounds with other elements. However its 3d-orbitals are available and there is considerable evidence1 that these play an important role in its chemistry. This is particularly evident for sulphur-fluorine compounds where sulphur is normally either quadri- or sexi-valent. For sulphur to increase its valency in this manner one or two electrons must be promoted to the 3d-shell to permit the formation of more bonds whose energy must be sufficient to make up for the energy of promotion. The formation of strong bonds depends on the overlap of the orbitals of the two atoms form- ing the bond.A study of overlap integrals shows that for sulphur the 3d-orbitals in the free atom are too weakly bound and diffuse to contribute significantly to the bond energy. Craig and Magnusson2 have made a theoretical study of this problem and described a model in which the ligand atoms in a sexivalent sulphur compound are replaced by six point positive changes in octahedral disposition. In such a model the d-orbitals are contracted and for a ligand such as fluorine effective overlap of sp3d2 hybridised orbitals becomes possible. The contraction is most effective for small electronegative ligands and is therefore in accordance with the fact that sulphur has its highest covalency when bonded to oxygen and fluorine. In addition to this ligand effect on the d-orbitals the low dis- sociation energy of the fluorine molecule (F2 38; Cl, 58; H2 104 kcal.mole-l) means that for a given bond strength a fluorine compound will be more exothermic than a chloride or hydride. It is therefore not surprising that sulphur forms a stable hexafluoride but no hexachloride and that only a limited replacement of fluorine in sulphur hexafluoride by other atoms or groups is possible. This contraction of d-orbitals which permits the formation of sulphur hexafluoride also means that the 2p- electrons of oxygen can overlap effectively with the 3d-orbitals of sulphur so that in sulphuryl fluoride SO,F the oxygen atoms are linked to sulphur by a double bond and the sulphur atom is thus sexivalent in this compound also. In this case where the oxygen atoms play an important role there are chlorine and bromine analogues of similar structure.In sulphur tetrafluoride where the co-ordination number of the sulphur atom is four it is still necessary to promote one electron to a d-orbital and even here there is no chlorine analogue as sulphur tetrachloride exists ALKALI DIVISION NORTHWICH CHESHIRE.) Cilento Chern. Rev. 1960 60 147. Craig and Magnusson J. 1956 4895. 30 ROBERTS COMPOUNDS CONTAINING S-F BONDS 31 only in condensed phases where it is present as SCl,+Cl-. Thus it is only in certain oxyfluorides and oxychlorides that there is any similarity between the fluorides and chlorides of sulphur. While it is usual for there to be a marked difference between the fluorides and chlorides of a given element there is probably no other element where this is quite so striking as with sulphur.One interesting feature of sulphur-fluorine chemistry is that it provides a series of compounds in which both the maximum covalency and the co-ordination number of six are simultaneously reached. In this respect it is similar to carbon in the first row of the Periodic Table where covalency and co-ordination number have a maximum of four. This leads to several interesting parallels between the derivatives of sulphur hexafluoride in which one or two fluorine atoms are replaced by other atoms or radicals and those of carbon tetrafluoride. Thus both have a stability to hydrolysis which is kinetic rather than thermodynamic but the range of possible substituents in the case of sulphur hexafluoride is much more restricted than with carbon tetrafluoride.The known sulphur-fluorine compounds are classified in this Review under five headings (1) Sulphur monofluoride S,F2 sulphur difluoride SF, and their derivatives of the type R-SF where R is a univalent element or radical. (2) Sulphur tetrafluoride SF and its derivatives RmSF,. (3) Sulphur hexafluoride SF6 and its derivatives R-SF and SF,R,. (4) Fluoro-oxy-acids and oxyfluorides of sulphur. ( 5 ) Sulphur-nitrogen- fluorine compounds. This classification is one of several possible and is chosen for clarity of presentation rather than for its chemical significance. Sulphur Monofluoride and Sulphur Difluoride and Their Derivatives The preparation of sulphur monofluoride has been reported by several worker^^*^*^ by the action of silver(1) fluoride or mercurous fluoride on sulphur.The compound is said to attack glass; although it was stated to be impure a boiling point of -99" and freezing point of -120.5" are given3 while other workers4 give -38" as the boiling point of S,F2. Sulphur monofluoride is said4 to decompose when heated giving sulphur and sulphur difluoride but this product is also poorly characterised. More recently both S,F and SF2 have been reported as present in the products of electrical breakdown of sulphur hexafl~oride,~~~ but in this case the identification given depends on unpublished infrared spectra. It must be concluded that neither of these compounds has been adequately charac- terised. Centnerszwer and Strenck Ber. 1923 56 2249; 1925 58 914. Trautz and Ehrmann J. prakt. Chem. 1935 142 79. Matutano and Otero Anales real Soc.espafi. Fis. Quim. 1955 51 B 223. Schumb Trump and Preist Ind. Eng. Chern. 1949 41 1348. Edelson Bieling and Kohrnann Ind. Eng. Chein. 1953 45 2094. 32 QUARTERLY REVIEWS The only derivative of sulphur difluoride which has been claimed is trichloromethanesulphenyl fluoride CCl,.SF which was obtained* by the action of mercuric fluoride on the corresponding chloride. However no attempt was made to establish the structure of the product which could have been CFCl2.SC1. The existence of fluorides of bivalent sulphur is therefore not established. Sulphur Tetrafluoride and its Derivatives In 1950 Sidgwickg described reports of sulphur tetrafluoride as " almost certainly wrong". Brown and Robinson,lo who review the earlier literature on the lower fluorides of sulphur were however able to isolate and characterise sulphur tetrafluoride as a result of reaction of fluorine with a thin film of sulphur at -75".Sulphur tetrafluoride is also obtained by the action of chlorine trifluoridell or iodine pentafluoride12 on sulphur. Sulphur dichloride sulphur monochloride or sulphur dibromide may be used in place of sulphur in any of these reactions. This has the advantage that the reagent being a liquid can be more readily cooled to remove the heat of reaction and minimise the tendency to form sulphur hexafluoride but the product is then contaminated with chlorine which can be troublesome to remove.12 All these reactions involve the use of elementary fluorine or a halogen fluoride and thus need special apparatus which is not available in labora- tories that do not specialise in fluorine chemistry.A ' more convenient preparative reaction12 is the fluorination of sulphur dichloride in the presence of acetonitrile at 70-80° by sodium fluoride 3SCI + 4NaF -+ SF + S,CI + 4NaCI This remarkable reaction which proceeds readily to give a high yield of sulphur tetrafluoride may occur through the unstable SCl which arises13 from the equilibrium 3Scl2 + SCl + S2C12 the function of the catalyst being to stabilise the SCl,. In the absence of acetonitrile or other catalyst the reaction proceeds only at elevated temperatures.12 Sulphur tetrafluoride is a colourless gas (b.p. -40") which condenses to a colourless liquid whose Trouton constant (27-1) indicates that it is associated. It solidifies at - 121 ". Its critical temperature is 91 O.The structure of the sulphur tetrafluoride molecule by analogy with other halides of Group V VI and VII elements may be expected to be derived from a trigonal-bipyramidal arrangement of valency electrons in 3s3p33d-orbitals around the sulphur atom1 For sulphur tetrafluoride this Sidgwick "The Chemical Elements and their Compounds," Oxford 1950 p. 943. * Kober J. Arner. Chern. Suc. 1959 81 4810. lo Brown and Robinson J. 1955 3147. l1 B.P. Appln. 16,404/1959. l2 Tullock Fawcett Smith and Coffman J. Amer. Cliem. SOC. 1960 82 539. l3 Rosser and Whitt J. Appl. Chem. 1960 10 229. l4 Gillespie and Nyholm Quart. Rev. 1957 11 339. 33 ROBERTS COMPOUNDS CONTAINING S-F BONDS leaves two possible structures in which the unshared pair of electrons occupy either an equatorial or a polar position.A study of the infrared and Raman spectral5 strongly suggested that the lone pair occupied an equatorial position a result which was confirmed by nuclear magnetic resonance s p e c t r o ~ c o p y . ~ ~ ~ ~ ~ The structure is therefore intermediate between those of phosphorus pentafluoride and chlorine trifluoride the former being a trigonal bipyramid with a fluorine atom at each corner and the latter having two lone pairs in equatorial positions. It is of interest that in chlorine trifluoride the two polar fluorine atoms are bent towards the single equatorial fluorine on account of the repulsion of the lone pairs; a similar effect might arise in sulphur tetrafluoride although here there are only one lone pair and two equatorial fluorine atoms. By a study of the variation of the fine structure of the nuclear magnetic resonance spectrum with temperature a value of 4.5 -j= 0-8 kcal.mole-1 is obtained for the activation energy of fluorine exchange. The heats of hydrolysis18 and hydrogenati~nl~ of sulphur tetrafluoride have been measured and lead to values of 176 and 172 kcal. mole-l respectively for the heat of formation of gaseous sulphur tetrafluoride. The sulphur-fluorine bond energy in sulphur tetrafluoride can be cal- culated from these values and known thermochemical data20 to be 78-79 kcal. mole-l. Sulphur tetrafluoride is very readily hydrolysed by aqueous media to thionyl fluoride which although it can be hydrolysed further to sulphur dioxide is less reactive than the tetrafluoride This ready hydrolysis makes it difficult to handle sulphur tetrafluoride in glass apparatus unless special precautions to avoid moisture are taken.SF + H,O -+ SOF + 2HF l6 Dodd Woodward and Roberts Trans. Faraday SOC. 1956,52 1052. l6 Cotton George and Waugh J. Chem. Phys. 1958 28 994. l7 Muetterties and Phillips J . Amer. Chem. Soc. 1959 81 1084. l9 Muetterties 17th Cong. Pure Appl. Chem. Munich 1959. Nicholls Ph.D. thesis Durham University 1958. Cottrell “Strengths of Chemical Bonds,” Butterworths Scientific Publications 2nd edn. London 1958. 2 34 QUARTERLY REVIEWS In conjunction with the near-identity of the boiling points of sulphur tetrafluoride and oxychloride it may account for much of the confusion in the early literature concerning the lower fluorides of sulphur. The reaction of sulphur tetrafluoride with water has a parallel in its reaction with other compounds containing a hydroxyl group and there is some correlation between the acidity of this group and the yield of product.Carboxylic and sulphonic acids are converted in high yield at room temperature into acid fluorides While alcohols do give the corresponding fluoride the appropriate ether is always a major by-product. In addition to reacting with hydroxyl groups sulphur tetrafluoride reacts with organic compounds containing a carbonyl group,21 such as aldehydes ketones carboxylic acids and their derivatives the oxygen atom being replaced by two fluorine atoms R.C02H + SF4 -+ R+COF + SOF 50" 35% 110" 60% CHSCHO + SF4 -+ CHS.CHF2 + SOF2 CH,*C0.CH3 + SF4 -+ CH,*CFz*CH + SOF 1 20" C,H,.COF + SF4 -+ C,H,*CF + SOF2 41 % The reaction is very specific; for example unsaturated acids may be used 120" CH=CCO,H + 2SF4 -+ CHGCCF + 2SOF + HF Certain ketones and all esters react in this manner only with difficulty but even in these cases reaction will proceed in the presence of a catalyst such as hydrogen fluoride boron trifluoride or arsenic pentafluoride and other fluorides which are Lewis acids.A possible mechanism21 for this reaction is shown in the annexed scheme. The initial step may be the co-ordination of the fluoride XF, which may be SF in the uncatalysed I I s+ s- s+ L- F-SF3 SCO + X G - -C-0-XF -C-O-XFn 4 -C-O-sF f X$ 'x i F I i I I -5-F + SOF )-t - 5 - 0 7 F F F -XF,,- + XF \ / reactions with the carbonyl group and the reaction can then proceed as depicted. That sulphur tetrafluoride is a weak Lewis acid is shown by its interaction with tertiary amines to form a complex in which the nitrogen is the donor.22 Sulphur tetrafluoride has also been shown to be a useful 21 Hasek Smith and Engelhardt J.Amer. Chem. Soc. 1960 82 543. 22 Muetterties U.S.P. 2,897,055. ROBERTS COMPOUNDS CONTAINING S-F BONDS 35 reagent for the preparation of other inorganic It reacts with most inorganic oxides and sulphides to give the corresponding fluorides or oxyfluorides I,O + 5SF4 -+ ZIF + 5S0F2 With oxides thionyl fluoride is produced but with sulphides sulphur is the usual by-product SF + SnS -> SnF + 3s This reaction is of little value for the readily obtainable alkali-metal or alkaline-earth metal fluorides but the production of iridium pentafluoride selenium and tin tetrafluoride and molybdenum and tungsten hexa- fluoride from the corresponding oxides or sulphides provides the most con- venient route to these compounds.With certain oxides such as nitrogen dioxide and chromium trioxide which are strong oxidising agents sulphur tetrafluoride is oxidised to thionyl tetrafluoride. Sharpz4 has extended the scale of this reaction to the preparation of complex fluorides by carrying out the reaction of sulphur tetrafluoride with oxides sulphides and carbonyls in the presence of alkali-metal fluorides. Thus with molybdenum hexacarbonyl or molybdenum dioxide the following reaction takes place P205 + 3SF4 -f ZPOF + 3soF s F4 NaF + Mo(CO) -4 NaMoF 205 O SF4 NaF + MOO -+ NaMoF 350' Mo(CO) + SF4 -+ MoF,,SF 205 O In each case the final product contains MoV although the starting com- pounds contained Moo and MoIV.Other oxides studied were boron trioxide tantalum and niobium pentoxide cerium and titanium dioxide and silica ; in each case a complex fluoride with the metal in the same valency state as in the oxide was obtained. This method of preparation can probably be extended to the preparation of complex fluorides of Group VIIl metals. Sulphur tetrafluoride forms an interesting series of solid complexes with certain inorganic fluorides which can accept fluoride ions. These complexes BF,,SF, AsF,,SF, and SbF,,SF, written in ascending order of stability were first discovered by Bartlett and Robinson25 who postulated that they were simple Lewis acid-Lewis base complexes SF,+BF,. This formulation was criticised on theoretical grounds by Cotton and George.26 They pointed out that sulphur tetrafluoride would be a poorer electron- donor (because the extra fluorine atom would decrease the lability of the 23 Oppegard Smith Muetterties and Engelhardt J .Arner. Chem. Soc. 1960 82 24 Sharp J. personal contribution. 25 Bartlett and Robinson Proc. Chem. Soc. 1957 230. 2G Cotton and George J . Inorg. Nuclear Chem. 1958 7 397. 3835. 36 QUARTERLY REVIEWS lone pair) than phosphorus trifluoride which has been shown not to react with boron trifluoride. Further while boron trifluoride can accept fluoride ion giving BF,- and can function as general Lewis acid arsenic and antimony pentafluoride are known to be very strong fluoride-ion acceptors but are not known to be good general Lewis acids. These authors therefore prefer the formulation BF,-SF,+. More direct evidence for this structure is given by Seel and D e t ~ e r ~ ' who have been able to obtain an infrared spectrum of a thin film of BF,,SF on a rock-salt plate.This shows a strong band at 1050 cm.-I which is typical of BF4- and of greater significance because the BF,- could arise by reaction of the fluorides with sodium chloride a doublet at 908 and 940 cm.-l which by comparision with phosphorus trifluoride is assigned to the SF3+ ion. In the gas phase the complexes are completely dissociated and from a study of the vapour pressure-temperature relationship a value of 25-5 kcal. mole-l is deduced2' for the heat of dissociation of BF3,SF,. Sulphur tetrafluoride reacts with organic chlorides in presence of boron trichloride to give fluorides and a mixture of sulphur dichloride and chlorine; no compounds of the type SF3CI SF2C12 or SC1,F were detected and in view of the low activation energy required to produce isotope exchange in sulphur tetrafluoride it is likely that such compounds would disproportionate very readily.With fluorine sulphur tetrachloride readily gives sulphur hexafluoride; with chlorine trifluoride reaction is slow at room temperature but at 200" gives an equimolar mixture of sulphur hexafluoride and sulphur chloride pentafluoride ; Somewhat higher temperatures are needed before reaction takes place with chlorine monofluoride but at 380" reaction is rapid and gives sulphur chloride pentafluoride.28 With bromine trifluoride26 and iodine penta- fluoride12 there is no reaction below 200". Sulphur tetrafluoride does not react with oxygen alone but in the presence of nitrogen ZSF + CIF + SFJI + SF it is oxidised to thionyl tetrafluoride 2SF + 0 + 2S0F4 Derivatives of Sulphur Tetrafluoride.Several derivatives of sulphur tetrafluoride of the type RsSF are known. Trifluoromethylsulphur TABLE 1. Sulphur tetrafluoride and its derivatives. Compound M.p. B.p. -121" - 40.4 O -110 - 96.5 - 51- 60'15 mm. 7 35 26 27 Seel and Detmer 2. anorg. Chem. 1959,301 113. 28 Roberts B.P. Appln. 2543/1959. 49 Smith and Engelhardt J. Amer. Chern. SOC. 1960,82 3838. ROBERTS COMPOUNDS CONTAINING S-F BONDS 37 trifluoride CF,.SF, was prepared by Tyezkowski and Bigelow30 by reaction of carbon disulphide vapour with fluorine highly diluted with nitrogen; and SF,CF2-SF has been isolated from the electrochemical fluorination of carbon di~ulphide.~~“ These are similar to sulphur tetra- fluoride in ease of hydrolysis.In contrast to this 2,4-dinitrophenylsulphur trifl~oride,~~ obtained by fluorination of the corresponding disulphide in solution in anhydrous hydrogen fluoride is stable in air and can be recrystallised from acetone or ethanol. A general method for the preparation of aromatic trifluoro- sulphur derivatives has been published32 in which aromatic disulphides dissolved in the inert 1,1,2-trichloro-l,2,2-trifluoroethane are treated with silver difluoride. Phenylsulphur trifluoride is a colourless liquid which slowly attacks glass and like sulphur tetrafluoride can be used in place of sulphur tetrafluoride to replace carbonyl groups by CF,. As it is a high-boiling liquid the reaction can be carried out in ordinary laboratory equipment.Authenticated derivatives of this series are listed in Table 1. Sulphur Hexafluoride and its Derivatives Sulphur hexafluoride is a colourless gas which was first isolated and characterised in 1900 by Moissan and L e b e a ~ . ~ ~ It was prepared by burning sulphur in an atmosphere of fluorine and removing lower fluorides by washing the products with aqueous alkali and this is still the most convenient method of making it;34 it is also produced in vigorous fluorination of organic compounds which contain sulphur. Alternative methods which have been suggested for the preparation of sulphur hexa- fluoride are reaction of metal fluorides on sulphur in the presence of an excess of chlorine35 at 500° 6NaF + S + 3CI -f 6NaCI + SF and pyrolysis and electrochemical fluorination of sulphur tetrafluoride;29 but neither of these methods gives high yields.Sulphur hexafluoride is a colourless odourless non-toxic gas which at atmospheric pressure condenses to a white solid at -63.8”. Its physical properties have been studied in great detail on account of its value as a symmetrical molecule in checking the predictions of various theories of the liquid Chemically sulphur hexafluoride is almost totally inert being un- affected by aqueous or fused alkali ammonia or oxygen; and even alkali 30 Tyezkowski and Bigelow J. Amer. Chem. SOC. 1953 75 3523. 30u ClifFord El-Shamy Emelkus and Haszeldine J. 1953 2372. *l Chamberlain and Kharasch J. Arner. Chem. SOC. 1955 77 1041. 3a Sheppard J. Amer. Chem. SOC. 1960 82,4751. 33 Moissan and Lebeau Compt. rend. 1900,130 865 984.34 Dodd and Robinson “Experimental Inorganic Chemistry,” Elsevier Amsterdam 35 Muetterties B.P. 805,860. 36 Cady “Advances in Inorganic Chemistry and Radiochemistry” (ed. Emeltus and 1955. Sharpe) Academic Press New York 1960. 38 QUARTERLY REVIEWS metals react appreciably only at elevated temperat~res.~~ This lack of reactivity and its high dielectric strength have led to the use of sulphur hexafluoride as an insulating atmosphere for high-voltage electrical equipment,38 although when breakdown does occur a very corrosive atmosphere containing lower fluorides of sulphur and fluorine is pro- duced.' The extreme chemical stability of sulphur hexafluoride must be kinetic rather than thermodynamic as the S-F bond energy differs little from that in sulphur tetrafluoride. The heat of formation of sulphur hexafluoride has recently been redetermined by Hayaman Gross and L e ~ i ~ ~ who give a value of 288.9 kcal.mole-l which is 27 kcal. mole-l higher than the previously accepted value.4o The S-F bond energy20 in sulphur hexafluoride is therefore 76 kcal. mole-l. The hydrolysis of sulphur tetrafluoride which is rapid and that of sulphur hexafluoride which does not proceed at a detectable rate under ordinary experimental conditions have free energies that may be calculated and compared viz. SF,(g) + 2H,O(g) -f SO,(g) + 4HF(g) SF,(g) + 3H20(g) -+ SO,(g) + 6HF(g) Several explanations have been advanced for the high activation energy of hydrolys of sulphur hexafluoride. Direct attack of water or hydroxyl ion on the sulphur atom in the covalently saturated sulphur hexafluoride could only take place by extensive electronic rearrangement and may be ex- pected to be difficult whereas attack could take place easily on sulphur tetrafluoride.This theory receives support from the ready fluorine-isotope exchange in sulphur fetrduoride,l6J7 compared with the lack of such exchange in sulphur he~afluoride.~~ The other factor which must also be important is the failure of a water molecule or a hydroxyl ion to co- ordinate on to a combined fluorine atom because when one fluorine in sulphur hexafluoride is replaced by chlorine to form sulphur chloride pentduoride (see below) attack by hydroxyl ions is rapid. The structure of sulphur hexafluoride is octahedral as would be expected for sp3d2-hybridisation.14 This has been established by electron diffrac- tion,42p43 the S-F bond length being 1.56 & 0.02A.The infrared,44 R a ~ a n ~ ~ and nuclear magnetic resonance17 spectra can be fully assigned on the basis of an octahedral structure. They have been used to afford thermodynamic functions for sulphur hexafluoride over a range of temperature^.^^ Derivatives of SuIphur HexaJEuoride. The similarity between the chemistry of sulphur hexafluoride and carbon tetrafluoride suggests that 37 Cowan Riding and Warhurst J. 1953 4168. Bucchner van de Graaf Spertuto Burril McIntosh and Urquhart Phys. Rev. Hayaman Gross and Levi Internat. Symp. Fluorine Chemistry Birmingham 1959. A G O = -58 kcal. mole-' AGO = -48 kcal. mole-1 1946 69,692. 40 Nat. Bur. Stand. Tables Circular No. 500. 41 Rogers and Katz J . Amer. Chem. SOC. 1952,74 1375. 42 Braune and Knoke 2.phys. Chem. 1933 B 21 297. 43 Brockway and Pauling Proc. Nut. Acud. Sci. 1933 19 68. 44 Lagemann and Jones J . Chem. Phys. 1951 19 534. 45 Gaunt Trans. Faraduy SOC. 1953 49 1122. ROBERTS COMPOUNDS CONTAINING S-F BONDS 39 there might exist a homologous series of the general formula SnF4n+2 similar to that of the fluoroalkanes CnF2n+2 and that an SF group may show properties similar to those of the CF group. It is to be expected however that the range of possible substituents on an SF5 group will be somewhat limited as the condition necessary for sulphur to form six bonds is that the ligand atoms shall be both small and electronegative.2 Also there will always be a tendency for SF,X to lose XF and give sulphur tetrafluoride a possibility not present with CF,X.At present only S,F1 is known of the series SnF4n+2 while the range of SF compounds is confined to SF,C1 SF,.OF SF5.0-SF, SF,.O.O.SF, and R-SF where R is an alkyl substituted alkyl aryl or fluoroalkyl radical. The number of R,SF4 compounds is even more restricted only perfluoroalkyl derivatives being known with any certainty. Disulphur DecaJEuoride. Disulphur decafluoride S 2F10 was first isolated and characterised by Denbigh and Wh~tIaw-Gray~~ as a by-product from the formation of sulphur hexafluoride by the action of fluorine on sulphur. These authors give a melting point of -92" and a boiling point of 29" but later workers4' showed that the melting point was very sensitive to small traces of impurities and give a melting point of -55" a vapour pressure curve indicating a boiling point of 29" and a Trouton constant of 21.3.It is more conveniently prepared by the photochemical reduction of sulphur chloride pentafluoride with hydrogen :48 2SF,CI + H2 +. S,F, + 2HCl Disulphur decafluoride is a colourless liquid which like sulphur hexa- fluoride is stable to aqueous alkali. It is however rapidly absorbed by molten potassium hydroxide and reacts vigorously with molten sodium. In marked contrast to sulphur hexafluoride disulphur decafluoride is extremely and its use as a war-gas has been suggested. Thermal decomposition of disulphur decafl~oride~~ to sulphur hexafluoride and sulphur tetrafluoride is quite rapid at 200" and is thought to proceed by a preliminary fission of the S-S bond to two SF,. radicals. The structure of disulphur decafluoride is the expected one of two SF groups linked by an S-S bond.Electron-diffraction meas~rements~l show that the S-F bond length 1.56 &- 0.02 A is similar to that in sulphur hexafluoride but that the S-S bond 2.21 & 0.03 A is considerably longer than the 2.08 A observed for disulphides containing bivalent sulphur.52 This structure is supported by the infrared and Raman spectra53 which can be assigned completely on this model. Force-constant calculation^^^ show 46 Denbigh and Whytlaw-Gray J. 1934 1346. 47 Hollies and McIntosh Canad. J. Chem. 1951 29 494. 48 Roberts B.P. Appln. 30,908/1960. 49 Lester and Greenberg Arch. Ind. Hyg. Occup. Med. 1950,2 348. 6o Trost and McIntosh Canad. J. Chem. 1951 29 508. 51 Harvey and Bauer J . Amer. Chem. SOC. 1953 75 2840. 62 Sutton "Interatomic Distances," Chem.SOC. Special Publ. No. 11 London 1958. 63 Dodd Woodward and Roberts Trans. Faraday SOC. 1957 53 1545. 64 Woodward and Roberts Trans. Faraday SOC. 1957 53 1557. 40 QUARTERLY REVIEWS that the S-F stretching force constant is the same as that for sulphur hexafluoride while the S-S stretching force constant has an unusually low value. Although the SF group is very electronegative it is not small and it is likely that the substitution of a second SF in place of a fluorine atom is not possible. Sulphur Chloride PentaJuoride SF,Cl. The replacement of a fluorine atom in sulphur hexafluoride by another halogen has been ac- complished only in one case sulphur chloride pentafluoride SF,Cl. This compound was first isolated from the products ~ b t a i n e d ~ ~ ~ by fluorination of liquid sulphur dichloride at -lo" but the yield was very low.Some sulphur chloride pentafluoride is also obtained5' when disulphur decafluoride reacts with chlorine at 200" and when chlorine trifluoride reacts with sulphur.ll The best preparative method however is the reaction of chlorine monofluoride with sulphur tetrafluoride at 380" which gives an almost quantitative yield:28 SF + CIF -+ SF,CI Sulphur chloride pentafluoride is colourless and has b.p. -21" and m.p. -64". The latent heat of vaporisation is 4560 cal. mole-1 and the Trouton constant (18.2) is somewhat lower than the usual value (21) for an unassociated liquid. The critical temperature is 1 15 0.58 The structure of sulphur chloride pentafluoride is that expected for a monosubstituted octahedron. This has been established by a study of the microwave which leads to a bond length of 1-58 & 0.01 A for S-F and 2.030 & 0.002 A for S-C1.These values are larger than those quoted for sulphur hexafluoride and disulphur di~hloride,,~ but the difference is less than the probable experimental error. The infrared and Raman spectra are also consistent with this structure.60 The heat formation of sulphur chloride pentafluoridesl has been deduced from its heat of hydrolysis to be 245 kcal. mole-l. Toegther with the S-F bond energy deduced from sulphur hexafluoride this leads to a value of only 45 kcal. mole-1 for the S-C1 bond energy which is considerably lower than the value of 61 kcal. mole-1 found for the much less stable disulphur dichloride. It is probable therefore that the S-F bond energy is less in sulphur chloride pentafluoride than in the hexafluoride.This is consistent with the theoretical work of Craig and Magnusson2 who suggest that the effect of replacing fluorine by a larger atom would be to weaken all the bonds in the molecule. Thermally sulphur chloride pentafluoride is somewhat more stable than disulphur decafluoride. It is recovered unchanged after being heated 65 B.P. Appln. 31,320/1958. 66 Roberts and Ray J. 1960 665. 68 Leach unpublished work. 6B Kewley Murty and Sugden Trans. Faraday Soc. 1960,56 1732. 6o Cross Roberts Coggin and Woodward Trans. Faraday SOC. 1960 56 945. 61 Leach and Roberts J. 1960,4693. George and Cotton Proc. Chem. Soc. 1959 317. ROBERTS COMPOUNDS CONTAINING S-F BONDS 41 to 350” in an inert container,5s but at 400” it is decomposed to sulphur hexafluoride sulphur tetrafluoride and chlorine Hydrolytically it is less stable being rapidly hydrolysed by aqueous sodium hydroxide although it is not attacked by neutral or acidic media SF,CI + 8NaOH -+ Na,SO + 5NaF + NaCl + 4H,O It liberates iodine rapidly from aqueous potassium iodide and bromine from potassium bromide but appears not to react with alkali-metal chlorides.Calcium chloride however is slowly attacked and its use as a drying agent for sulphur chloride pentafluoride should be avoided. The ease of hydrolysis of sulphur chloride pentafluoride is in marked contrast to the stability of disulphur decafluoride and other compounds containing the pentafluorosulphur group and suggests that attack is by the hydroxyl ion on the chlorine atom. As such a reaction does not easily take place with chlorotrifluoromethane this indicates that the chlorine in sulphur chloride pentafluoride is positive and a possible mechanism for the hydrolysis is 2SF,CI -f SF + SF + CI 6- s+ SF,CI + OH- -+ SF,- + ClOH SF,- 3 SF4 + F- SO,2- + HOCl -+ SO,2- + H+ + CI- the overall result being that given in the equation above.The positive nature of the chlorine atom receives support from the observation that sulphur chloride pentafluoride is a powerful oxidising agent. In addition to the halogen displacements mentioned it oxidises alcohols and aldehydes to carboxylic acids and aromatic amines to azo- compounds. Organometallic reagents such as phenyl-lithium or Grignard reagents react vigorously and reduce sulphur chloride pentafluoride to sulphur tetrafluoride.With benzene under Friedel-Crafts conditions it gives chlorobenzene again in agreement with the positive nature of the chlorine atom:s2 SF + 60H- -+ SO:- + 4F- + 3H,O SF,CI + C&& -f C,H,CI + [SF,H] 5. SF + HF The presence of positive halogen in sulphur chloride pentafluoride suggests that the properties of this molecule may be similar to those of trifluoromethyl iodide. Trifluoromethyl iodide can add across the double bond of olefins to give trifluoromethyl compounds the reaction taking place readily under the influence of ultraviolet light :63 CFSI + CZH4 + CF3.CH,*CH,.I 62 Case Ray and Roberts unpublished work. 63 Haszeldine “Fluorocarbon Chemistry,” Roy. Inst. Chem. Monograph No. 1 1957. 42 QUARTERLY REVIEWS This reaction always proceeds through primary attack of the CF,.radical on the olefin and usually takes place at a CH group where this is available or with fluoro-olefins at a CF group:g4 With sulphur chloride pentafluoride a similar series of reactions can take place;65 with olefins and acetylenes the reactions are similar to those of trifluoromethyl iodide. Thus with propene either under the influence of ultraviolet light at one atmosphere or at 90" in an autocalve 2-chloro- propylsulphur pentafluoride is produced Additions of this type have been carried out with ethylene propene butadiene and cyclohexene but with less reactive olefins lsuch as 1,2- dichloroethylene no reaction takes place and isobutene and styrene poly- merise in the presence of sulphur chloride pentafluoride. With fluoro- olefins which are less reactive thermal reactions must be catalysed by a free-radical initiator such as benzoyl peroxide but when this is done they proceed in a manner analogous to those of trifluoromethyl iodide by a mechanism involving attack of SF,.radicals on the olefin. In the case of addition to tetrafluoroethylene initiated by benzoyl peroxide CF,I + CH,CH:CH 4 CH,CHI*CH,-CF CH,.CH:CH + SFSCI + CH,.CHCI.CH,.SF5 SFSCI + C6H5. + SF,. f C6H5CI SF5 + CF2:CFz + sF,.CF2*CF2- SF,CF2CF2* + C2F4 -f SF5-[CFz]4. (propagation) SF,CF2.CF2- + sF,cI -f SF,*CF2-CF2CI + SF,. (chain transfer) products of the form SF,.[ CF2CF,] ;C1 are obtained. trifluoromethyl radical With unsymmetrical olefins the point of attack is the same as that of a SF,CI + CHF:CF2 -+ SF6-CHFCF2CI SF,CI + CFz:CFCI + SF5.CF2*CFC12 In photochemical reactions the primary step is the photolysis of sulphur chloride pentafluoride hv SF,CI -+ SF,.+ CIS Both these radicals then react with the olefin e.g. (a) CIS + CH2:CH2 + CICH2.CH2* (b) SF + CH2:CH2 4 SF,CHz*CH2- Both these radicals can now abstract a chlorine atom from sulphur chloride pentafluoride to liberate another SF,. radical. Reaction (b) therefore is self-sustaining but reaction (a) goes over to (b) after one step and thus the 64 Haszeldine and Steele J. 1957 2800. 65 Case Ray and Roberts J. in the press. ROBERTS COMPOUNDS CONTAINING S-F BONDS 43 SF,.CH,CH2C1 is present to the virtual exclusion of ClCH,CH,Cl. With unsaturated fluorocarbons of the two reactions (a) CIS + CF2:CF -+ CICF2-CF2. (b) SF,. + CF,:CF2 -+ SFs.CF2*CF2. (a) is very much the more rapid and the compound ClCF,*CF,Cl is found in addition to the SF,.[CF,-CF2] ,Cl produced in the thermal reactions. The mixture of products is difficult to separate and the method is of little preparative value. The reaction of sulphur chloride pentafluoride with acetylenes has been studied in two cases only. In these addition of only one molecule takes place CH:CH + SF,CI 3 SF,.CH:CHCI CH,-C:CH + SF,CI -+ SF,CH:CCI*CH In contrast to sulphur chloride pentafluoride itself these organic sulphur pentafluorides are stable to aqueous alkali. However the compounds in which hydrogen and chlorine are present on adjacent carbon atoms react with alcoholic potassium hydroxide to give alkenylsulphur penta- fluorides KOH-EtOH SF6*CH2*CH&I - 3 SF,*CH:CH A similar reaction with 2-chloropropenylsulphur pentafluoride leads to decomposition but if powdered potassium hydroxide in dry light petroleum is used prop- 1-ynylsulphur pentafluoride is produced SF,*CH:CCI*CH -+ SF5C i CCH The compound SF,.CHF.CF,Cl like SF,CHF, is decomposed by aqueous or alcoholic potassium hydroxide but with solid potassium hydroxide gives trifluoroviriylsulphur pentafluoride SF,CF :CF,.This instability to aqueous media of the group SF,-CFH is rather curious as SF,CH,R SF,CF2R and CHF are all inert to aqueous alkali. Compounds produced in these reactions are included in Table 2. Arylsulphur pentafluorides cannot be produced by this type of reaction but they have been obtained by reaction of silver difluoride with aryl- sulphur trifluorides :32 C6H,*SF3 + 2AgF2 -+ C6H5.SFs + 2AgF The SF group has a strongly deactivating influence on the benzene ring and nitration gives a m-nitro-derivative as would be expected for a strongly electronegative substituent.Perjluoroalkyl Derivatives of Sulphur Hexafiuoride. A number of perfluoroalkylsulphur pentafluorides and di(perfluoroalky1)sulphur tetra- fluorides have been obtained by oxidative fluorination of aliphatic thiols and bis-sulphides. The method most commonly used is electrochemical 44 QUARTERLY REVIEWS TABLE 2. Pentafluorosulphur compounds obtained by addition of SF5CI to unsaturated compounds. Compound M.p. SF5CH2CH2Cl SF5.(CH2*CH&Cl SF5.CH2.CHCl.CH3 SF5CH2CHCICH :CH2 2-Chlorocyclohexylsulphur pentafluoride SF5.CH :CCl.CH SF5CH2CHCI2 SF5*CH2CHClCH2-CHC12 SF5CH CH2 SF5CH CHCH SF,.CH,.CH :CH2 1 C yclo hexen ylsulp hur SF,-CrCMe SF,CH CHCl SF5CHClCH2Cl SF,.CF,.CF&I - 113" pentafluoride SF5-(CF2*CF.J2Cl -90 SF5.(CF2*CF.J3CI -5 SF,CF2CFCl SF5CF CF2 SF,CHFCF2Cl SF5CFCI-CF2CI B.p.92" 171-172 109 78-80/26 mm. 188-1 90 92 108 72'19 mm. 41 8&82 161 66 111 47 99 142 80-83 19 79-5-42 nto d20 1.3590 1.64 1.3900 1.3686 1 -4320 1.4783 1.3760 1,3840 1 -4269 1.4282 1 -3052 1.3079 fluorination in which the aliphatic compound is dissolved in anhydrous hydrogen fluoride and electrolysed under conditions such that hydrogen but no fluorine is evolved although other methods have been used.66-s* The scope of this type of preparation is indicated by the compounds listed in Table 3. These fully fluorinated compounds are chemically very inert being quite similar to sulphur hexafluoride and aliphatic fluorocarbons. At high temperatures trifluoromethylsulphur pentafluoride has been shown to act as a fluorinating agent.Thus at 500" it reacts with phosphorus nitride P,N5 to give a mixture of trimeric and tetrameric phosphonitrilic fluoride (PNFc3 and (PNFc34,69 while with hexafluoropropene it affords perfluoro- neopentane and other fluorocarbons.6e The structure of these perfluoroalkylsulphur pentafluorides and bisper- fluoroalkylsulphur tetrafluorides is that expected for derivatives of sulphur hexafluoride. The 19F nuclear magnetic resonance spectra indicate that 66 Lovelace Rausch and Postelnek "Aliphatic Fluorine Compounds," Amer. Chem. SOC. Monograph 138 Reinhold New York 1958 Chapter 13. 67 Dresdner and Young J . Amer. Chem. SOC. 1959 81 574. 68 Dresdner Reed Taylor and Young J. Org. Chem. 1960 25 1464.Mao Dresdner and Young J. Amer. Chem. SOC. 1959 81 1020. ROBERTS COMPOUNDS CONTAINING S-F BONDS TABLE 3. Fluorocarbon derivatives of sulphur hexafluoride. Compound M.p. B.p. n t5 CF3*SF5 -20" to -21" C2F5-SF5 11.3 n-C,F,SF 42 n-C4F,.SF5 70-5 n-C,F,,. SF 118.2 ClCF,.SF 16.5 -31 " SF5CF2COF (C2F5)2.NCF2CF2-SF5 CF,*SF4-CF3 C2F,- S F4 - C2F5 CYC~O- CGFI ,.SF5 C,F,*SF,*CF C3F7-SF4C3F7 C4Fs.SF4-C4Fs \ 17.2 1 24 110 20.5 47.1 70 116 154 80.3 88 112 82"/61 mm. 144.5 60 60 65 90-91 126"/37 mm. 71-72 65"/1 mm. 1 -2594 1.2710 1 ~2829 1.2921 1.3041 1 -2674 1 *2753 1.2856 1 -2943 1.3041 1.3010 1.3258 1-2884 1 -2598 45 d25 1.801 1.8541 1.8910 1.86 1.9530 1.803 1 -875 1 *865 1 -903 1.9031 2.041 1-772 the fluorine atoms in the SF group form a square pyramid while the C-S-C bond angle in compounds containing the group -SF,- may be either 90" or 180" as expected for octahedral co-ordination of the sulphur atom.70 Oxygen-containing Derivatives of Sulphur HexaJuoride.The two compounds bispentafluorosulphur oxide and peroxide SF5.0.SF5 and SF,.O.O.SF, are by photochemical oxidation of sulphur chloride pentafluoride. This reaction is formally analogous to that of trifluoromethyl iodide with sulphur to give first CF,.S.SCF and on further irradiation CF3.S*CF,.72 The probable mechanism is hu 2SF,CI + 0 --+ SF,*O*O.SF hu SF,*O*O*SF6 -+ SF,.0*SF6 + *02 this is supported by the fact that continuous removal of the products from 'O Muller Lauterbur and Svatos J. Amer. Chem. Soc. 1957 79 1043. 72 Haszeldine and Kidd J. 1953 3219. Roberts J. 1960,665. 46 QUARTERLY REVIEWS the reaction enhances the yield of the peroxide with respect to the oxide while irradiation of the peroxide leads to formation of the oxide.Bispentafluorosulphur oxide is a colourless liquid of b.p. 31". It is stable to aqueous alkali and appears to be quite inert. The peroxide b.p. 49" is also stable to alkali but reacts with quite a variety of organic compounds sometimes violently. Bispentafluorosulphur oxide like disilyl ether has the possibility of interaction of the lone-pair electrons on the central oxygen atom with vacant d-orbitals on the outer atom. With disilyl ether this leads to a large increase in the Si-0-Si angle from that observed in diethyl ether,73 which causes it to obey the spectroscopic selection rules for a linear molecule. A study of the infrared and Raman spectra of bispentafluoro- sulphur oxide while leading to no definite conclusions concerning the exact symmetry of the molecule does eliminate any possibility that the molecule has a linear S-0-S skeleton.74 Bispentafluorosulphur peroxide has been studied by electron diffra~tion~~ and has a structure similar to that of hydrogen peroxide the two SF groups replacing the hydrogen atoms. The S-F bond length 1-56 &- 0.02 A is normal and the S-0 bond length is 1.66 & 0.05 A. In addition to the two compounds SF,.O.SF and SF,-O-O.SF, there exists also pentafluorosulphur hypofluorite SF,.OF in which the SF group is bonded to oxygen. This compound is obtained by fluorination of thionyl fluoride over a silver difluoride catalyst :76 SOF + 2F2 -+ SF,.OF As would be expected for a compound containing a fluorine-oxygen bond it is very reactive.It attacks mercury and dissolves in aqueous alkali with the evolution of oxygen SF,*OF + 6OH- -+ $02 + SF- + S0,F- + 3H20 In this it is similar to trifluoromethyl hypofluorite. Its structure has been confirmed by electron diffra~tion;~' the SF g.roup is octahedral with the S-F bond length 1.53 & 0.04 A the same within experimental error as for other SF compounds which have been studied. The S-0 bond length 1.66 It is evident that where they exist SF compounds are similar to the CF compounds. There are however important differences. The SF group is reduced by organometallic compounds the CF group is not and the extensive chemistry based on metal and metalloid compounds of CF is therefore not likely to be reproduced by SF compounds.There is also a limit to the extent to which other atoms can replace fluorine in 0-05 A is similar to that in SF,.O.O.SF,. 73 McKean Taylor and Woodward Proc. Chem. Soc. 1959 321. '* Goggin Thesis Oxford 1960. 75 Harvey and Bauer J. Amer. Chem. SOC. 1954,76 859. '6 Dudley Cady and Eggers J. Amer. Chem. Soc. 1956 78 1553. 77 Crawford Dudley and Hedberg J. Amer. Chem. SOC. 1959 81 5287. ROBERTS COMPOUNDS CONTAINING S-F BONDS 47 sulphur hexafluoride which is not present for carbon tetrafluoride. Thus all members of the series CF, CF,Cl CF,C12 CFCl and CC14 are stable while from sulphur hexafluoride only sulphur chloride pentafluoride has been prepared and even the dichloride tetrafluoride is likely to be unstable. Neither pentafluorosulphur bromide nor the iodide has been reported.With hydrogen even the replacement of a single fluorine in the hexafluoride to give SHF, is unlikely. Despite these limitations however there is here a wider field of chemistry to be developed. Fluoro-oxyacids and Oxyfluorides of Sulphur 1 he known fluoro-oxyacids and oxyfluorides of sulphur are shown along with their parent oxyacids in Table 4. Fluorosulphurous Acid. Fluorosulphurous acid may be regarded as the monoacid fluoride of sulphurous acid and like sulphurous acid itself does not exist as a definite compound at room temperature though it gives rise to a series of salts with alkali-metal and ammonium cations. These salts are obtained by the reaction of sulphur dioxide on the ap- propriate fluoride; a reaction which proceeds more readily for larger cations such as K+ Rb+ and Cs+ than for the smaller Na+ and does not proceed at all with Lif.The fluorosul phites so formed have a measurable dissociation pressure even at room temperature KS0,F + SO + KF but X-ray investigation shows that they are distinct chemical compounds structurally similar to the isoelectronic chlorates and are not molecular complexes. The stability of the fluorosulphites and their solubility in sulphur dioxide also increase with increasing size of the cation. When heated at 170-180" in the presence of excess of sulphur dioxide the fluorosulphites disproportionate to fluorosulphates and sulphur. 2KSOzF + SO -f 2KS0,F + S Thus as where sulphur tetrafluoride yields sulphur hexafluoride and sulphur on pyrolysis this reaction indicates that sexivalent have higher stability than quadrivalent sulphur compounds.The fluorosulphites are very useful as reagents for replacement of chlorine by fluorine. The reactions take place under mild conditions and usually proceed cleanly and in high yield. Sulphuryl chloride can be converted successively into the chloride fluoride and the phosphonitrilic chlorides can be converted into fluorides with the same degree of poly- merisation (PNCI,) + 6KS0,F + (PNF,) + 6KCI + 6S0 Other compounds prepared from the corresponding chlorides by this reaction are benzoyl fluoride sulphuryl fluoride phosphorus trifluoride, 48 QUARTERLY REVIEWS TABLE 4. Fhoro-oxyacids and oxycfluorides of sulphur. Oxyacid OH / o=s 'OH Sulphurous acid 0 OH \s/ 0' \OH Sulphuric acid 0 0-OH Xs/ 0' \OH Permonosulphuric acid 0 0 II II H 0-S-0-S-0 H I1 I1 0 0 Disulphuric acid 0 0 II II HO-S-0-0-S-OH II II 0 0 Perdisulphuric acid Peroxydisulphuric acid 0 0 0 /I II II HO-S-0-S-0-S-OH 11 II II 0 0 0 Trisulphuric acid { Fluoro-oxyacid F / o=s 'OH Fluorosulphurous acid O F B.p.162-6" XS/ 0' 'OH Fluorosulphuric acid 0 0 I1 II II I/ 0 0 HO-S-0-S-F Fluorodisulphuric acid Oxyfluoride F / M.P. -1 o= s \ B.P. -43.7" F Thionyl fluoride 0 F M.P. -120" \s/ / \F B.P. -55.4" 0 Sulphuryl fluoride 0 0 . F \-/ M.P. -158.5" 0 /-\F B.p. -31.3" 0 Fluorine fluorosulphate (sulphuryl fluoride hypo- fluorite) 0 0 /I !I M.P. -48" F-S-0-S-F I/ I1 B.p. 51" 0 0 Disulphuryl difluoride 0 0 II 11 M.P. -55.4" F-S-0-0-S-F II 11 B.p. 67.1' 0 0 Peroxydisulphuryl difluor- ide 0 0 0 II It II F-S-0-S-0-S-F II II II B-P. 120" 0 0 0 Trisulphuryl fluoride F M.p.-99.6" o=s B.P. -49.0" F I'F Thionyl tetrafluoride ROBERTS COMPOUNDS CONTAINING S-F BONDS 49 and arsenic trifluoride ; complex-forming fluorides such as boron trifluoride are isolated as the salt of the fluoro-acid BCI + 4KS0,F -+ KBF + 3KCI + 4S0 This reaction cannot however be used to prepare sulphur difluoride or disulphur difluoride as no reaction takes place between fluorosulphites and the sulphur ~ h l o r i d e s . ~ ~ ~ ~ ~ Thionyl Fluoride. Thionyl fluoride which is the diacid difluoride of sulphurous acid is a colourless gas with a pungent and irritating smell. It is obtained by a variety of reactions usually involving the replacement of chlorine by fluorine.36 The most convenient small-scale preparation is the reaction of thionyl chloride with antimony trifluoride which can be con- veniently carried out in ordinary Pyrex vessels but on a larger scale the reaction of anhydrous hydrogen fluoride with thionyl chloride in steel vessels is better.The product can be freed from traces of sulphur dioxide by distillation.80 It is also the first product of hydrolysis of sulphur tetrafluoride. Structurally thionyl fluoride is similar to thionyl chloride both mole- cules having a trigonal pyramidal structure based on the tetrahedral arrangement of four a-bonding pairs of electrons one of which is a lone pair.14 The microwave spectrum of thionyl fluorides1 indicates the following structural parameters rso = 1.412 i- 0.001 A YSF = 1-585 i- 0.001 A LFSF = 92” 49’ & 5’ LOSF = 106” 49‘ 5‘. The bond length rso is therefore shorter than the value of 1.43 found for sulphur dioxide and sulphuryl chloride;52 YSF however is similar to that in sulphur hexa- fluoride and other sulphur-fluorine compounds.The infrared and the Raman spectras2 of the thionyl halides have been studied and are consistent with the above structure. It is of interest that the bond-stretching force constant kso is 11.0 x dynes cm.-l for thionyl fluoride and 9.69 x dynes cm.-l for thionyl chloride. These variations in bond length and force constant are consistent with the expected increase in the pr-dr- double-bond character of the S-0 bond due to the increasing electro- negativity of the halogen. Thionyl chloride fluoride SOClF is also known but apart from its preparation and characterisation has not been further studied.s3 Chemically thionyl fluoride is somewhat less reactive than the cor- responding chloride.It is only slowly hydrolysed by water to give sul- phurous and hydrofluoric acid and with ammonia gives the amide SO(NH&2.36 Unlike sulphur tetrafluoride it does not react with carbonyl Seel and Riehl 2. anorg. Chem. 1955 282 293. Seel and Langer 2. anorg. Chem. 1958,295 316. Brauer (ed.) “Praparative Anorganische Chemie,” Ferdinand Enke Verlag Ferguson J. Amer. Chem. SOC. 1954 76 850. Stuttgart 1960. 82 Cotton and Horrocks Spectrochim. Acta 1960 16 358. 83 Jonas 2. anorg. Chem. 1951 265 273. 50 QUARTERLY REVIEWS groups but with carboxylic acids under forcing conditions it gives acid fluorides R-C02H + S 0 F 2 -+ R-COF + HF + SO Fluorosulphuric Acid. Fluorosulphuric acid is the monoacid fluoride of sulphuric acid. It can be prepared in a variety of ways which are all essentially the reaction of hydrogen fluoride with sulphur trioxide or the fluorination of chlorosulphuric acid.Industrially it is prepared by com- bining sulphur trioxide with anhydrous hydrogen fluoride :36984 SO + HF -f HSO,F The anhydrous acid is a colourless liquid which fumes in moist air and dissolves in water with evolution of much heat. Rather remarkably it can be distilled at atmospheric pressure in Pyrex apparatus (b.p. 162-6”) a property which makes it a convenient laboratory fluorinating agent. The acid is isoelectronic with perchloric acid and as would be expected is a strong acid in aqueous solution. The free acid in water is to some extent hydrolysed an equilibrium being established HSO,F + H20 + H,SO + HF If however the solution is made alkaline it is possible to drive off the water and isolate the alkali-metal fluorosulphates.This method cannot be used for the alkaline-earth or other metals whose sulphates or fluorides are insoluble but these fluorosulphates as well as those of the alkali metals can be prepared by the reaction of sulphur trioxide on the metal fluoride at 200”.85 CaF + ZSO -+ Ca(SO,F) This reaction is similar to the formation of fluorosulphites from fluorides and sulphur dioxide but in this case is not confined to the alkali-metal and ammonium salts. When strongly heated the alkali-metal and calcium fluorosulphates decompose by reversing their formation reaction and so give fluorides and sulphur trioxide but strontium and barium fluorosulphate give sulphates and sulphuryl fluoride Ba(SO,F) -+ BaSO + S02F2 This difference in behaviour on pyrolysis may be due to some minor variation in the structure of the solid The fluorosulphates of the elements of Groups other than IA and IIA can also be prepared both by the action of fluorosulphuric acid on chloridess6 and by the reaction of sulphur trioxide on fluoride~.~~,~’ ** Lange in “Fluorine Chemistry” (ed.J. H. Simons) Academic Press New York 86 Hayeck Puschmann and Czaloun Monarsh. 1954 85 359. 13’ Clark and Emeleus J. 1957 4778. 1950 Vol. I pp 167-182. Muetterties and Coffman J. Amer. Chem. SOC. 1958 80 5914. ROBERTS COMPOUNDS CONTAINING S-F BONDS 51 These compounds are frequently double salts such as AlCl(SO,F), TiCl(SO,F), ZrF2(S03F)2,86 WF2(S03F), NbF,(SO,F), and TaF (SO,F),. They are salt-like in nature and are hydrolysed readily by water.Fluorosulphates of non-metallic elements are also known. Nitrosyl fluorosulphate which is a white hygroscopic solid can be obtained by reaction of dinitrogen trioxide with fluorosulphuric acid :36988 N,O + 2HS0,F -f 2NO+SO,F- + H20 Nitronium fluorosulphate is obtained in a similar manner from dinitrogen pentoxide and fluorosulphuric acid :36989 N20 + HS0,F -f NO,+SO,F- + H,O Phosphorus does not appear to form a fluorosulphate as phosphorus trifluoride and sulphur trioxide do not react,85 but phosphorus penta- chloride and fluorosulphuric acid give phosphorus oxyfluoride.86 In contrast with this arsenic trifluoride does react with sulphur trioxide giving a product of b.p. 142" whose empirical formula corresponds to 2AsF3,3SO,. A study of the AsF,-SO system by nuclear magnetic reson- ance ~pectro~copy~~ indicates that the interaction is complex and that several compounds are formed.The structure (I) is proposed for the 2 3 constant-boiling mixture. Antimony trifluoride behaves more as a metal fluoride with sulphur trioxide and gives a white solid thoughts5 to be a trisfluorosulphate S b( SO ,F) ,. The behaviour of the halogens in forming fluorosulphates is also inter- esting. Fluorine reacts with sulphur dioxide in the presence of a silver difluoride catalyst to give a compound F.SO,.OF fluorine fluoro- ~ulphate.~~ This compound is related to fluorine perchlorate and contains a fluorine-oxygen bond and it can also be formally regarded as the diacid fluoride of peroxymonosulphuric acid. It is highly reactive and in bulk is liable to explode without warning.Its structure has been confirmed by nuclear magnetic resonance spectros~opy.~~ Chlorine itself appears not to form a fluoro~ulphate,~~ but chloryl fluoride with sulphur trioxide gives a red liquid chloryl fluorosulphate ClO,.SO,F. 94 Bromine forms two 88 Lange Ber. 1927 60 967. 8 9 Goddard Hughes and Ingold J. 1950 2559. Gillespie and Oubridge Proc. Chem. Soc. 1960 308. 91 Dudley Cady and Eggers J. Amer. Chem. SOC. 1956 78 290. 92 Dudley Schoolery and Cady J. Amer. Cltem. SOC. 1956 78 568. 93 Roberts and Cady J. Amer. Chern. SOC. 1960 82 352. 94 Woolf J. 1954,4113; Schmeisser and Ebenoch Angew. Chem. 1954,66 230. 52 QUARTERLY REVIEWS distinct fluorosulphates BrSO,F a red liquid and Br(SO,F), a solid of m.p. 59 O by reaction with peroxydisulphuryl diflu~ride;~~ iodine forms only I(SO,F) in a similar reaction.Another compound IF,(SO,F) is formed from fluorine fluorosulphate and iodine. Thus with fluorine a rather weak 0-F bond is formed with chlorine the 0-CI bond would be thermodynamically unstable with respect to CI, and bromine and iodine both form bonds to the fluorosulphate rather more readily giving structures in which the halogen is positive. Sulphuryl Fluoride. Sulphuryl fluoride the diacid difluoride of sulphuric acid is readily obtained by fluorination of sulphur dioxide with elementary fluorine,s5 by replacement of chlorine in sulphuryl chloride by means of a variety of reagents,78 or by heating barium fluorosulphate.80 A careful studys6 has been made of its thermodynamic properties from 1 2 " ~ to its boiling point at 217.78"~ (-5538"c) and from this study a standard entropy of 62-66 e.u.at the boiling point has been obtained. The structure of sulphuryl fluoride has been established by microwave ~pectro~copy.~~ The bond lengths are YSF = 1.530 & 0.003 A (SF, 1.58 A 0.02 A) and rso = 1.405 & 0.003 I$ (SO, 1.43 & 0.02); the bond angles are LOSO = 123" 58' & 12' and LFSF = 96" 7' & 10'. The structure is therefore a distorted tetrahedron with LOSO greater than the LFSF as would be expected owing to the greater electron density in the S-0 bonds. The S-F and S-0 bonds are significantly shorter than in sulphur hexafluoride and trioxide respectively. The infrared and Raman spectra are consistent with this ~tructure.~~ The spectroscopic entropy at the boiling point is 63.24 e.u.which is greater than the calorimetric value and it is suggested that this difference may be due to disorder in the crystals at O'K owing to the close similarity in size between the bonded oxygen and the fluorine The heat of formation of gaseous sulphuryl fluoride has been estimated as 205 kcal. mole-' by combining the appearance potentials of the ions SO2+ and S02F2+ with the known heat of formation of sulphur dioxide and the dissociation energy of molecular fluorine.s8 If the S-F bond energy derived from sulphur hexafluoride (76 kcal. mole-l) and the S-0 bond energy from sulphur trioxide 2o (104 kcal. mole-l) were used the heat of formation would be only 147 kcal. mole-'. The actual bond energies are higher than this as would be expected from the observed shortening of the S-F and S-0 bonds in sulphuryl fluoride compared with those in sulphur hexafluoride and trioxide respectively.Chemically sulphuryl fluoride is inert. It is hydrolysed slowly by aqueous sodium hydroxide to sodium fluorosulphate and sodium fluoride SO,F + 2NaOH + NaS0,F + NaF + H,O O5 Moissan and Lebeau Compt. rend. 1901 132 374. O6 Bockhoff Petrella and Pace J . Chem. Phys. 1960 82 799. O7 Hunt and Wilson Spectruchim. Acta 1960 16 570. sg Reese DibeIer and Franklin J. Chem. Phys. 1958 29 880. ROBERTS COMPOUNDS CONTAINING S-F BONDS 53 It does not react further with fluorine even at 400° whereas sulphur trioxide gives fluorine fluorosulphate under similar conditions. Fluorodisulphuric Acid and Disulphuryl Dijluoride. There are no reports in the literature of fluorodisulphuric acid but a calcium salt of this acid is thought to be an intermediate in formation of disulphuryl difluoride from calcium fluoride and sulphur trioxide :85 + 2s0 CaF + 2S0 -+ Ca(SO,F) - -f Ca(S,O,Fd H,O in cow.hSSO -+ CaSO + H2S04 + S2O,F2 Disulphuryl difluoride is also produced by decomposition of certain fluorosulphates reaction of fluorosulphuric acid with arsenious and fluorination of the appropriate It is a colourless liquid which hydrolyses slowly to fluorosulphuric acid and is therefore formally the anhydride of this acid. Peroxydisulphuryl Dzyuoride. Peroxydisulphuryl difluoride may be regarded as the diacid fluoride of peroxydisulphuric acid. It is obtainedgg by reaction of fluorine with an excess of sulphur trioxide at 250° or at a lower temperature over a silver difluoride catalyst 2S0 + F -+ FSO,.O*O*SO,F The liquid (b.p.67") is quite stable and can be distilled without decom- position. Its reactions with the halogens have already been described. It also reacts with sulphur dioxide to give trisulphuryl fluoride:91 (FSO,*O) + SO -+ FS02*O*S02*O*S02F Thionyl Tetrafluoride. In addition to the sulphur(vr) oxfluorides containing the group SOzF there exists a compound thionyl tetrafluoride SOF4 which is unique in that the sulphur atom has a co-ordination number of five. It is obtained by the reaction of fluorine on thionyl fluoride :SO SOF + F -f SOF or by oxidation of sulphur tetrafluoride by nitrogen dioxide:27 SF + NO -+ SOF + NO It is probable that its structure is similar to that of sulphur tetrafluoride being a trigonal bipyramid with the oxygen in an equatorial position similar to that occupied by the lone pair of electrons in sulphur tetrafluoride.The nuclear magnetic resonance spectrum of thionyl tetrafluoride shows only a single peak at the temperature at which it was studied but it would probably show a similar pattern to that of sulphur tetrafluoride16J7 at lower temperatures as fluorine exchange would then be slower. O 9 See1 and Simon Angew. Chem. 1960,72 709. 54 QUARTERLY REVIEWS It is hydrolysedjby aqueous media to sulphuryl fluoride SOF + H,O -+ SO,F + 2HF but less rapidly than sulphur tetrafluoride and it can be handled in glass apparatus with only the normal precautions to exclude moisture. With ammoniag9 it reacts to give the ammonium salt NH FSN [:I which when heated affords ammonium fluoride and a polymeric sulpha- murylfluoride ( - := N ) which is isoelectronic with phosphonitrilic \ / n fluoride (PNF,),.Like sulphur tetrafluoride it forms complexes with boron trifluoride and with arsenic and antimony pentafluoride,26 but these are much weaker. The boron trifluoride complex has a vapour pressure of one atmosphere below O" whereas the arsenic and the antimony pentafluoride complex have measurable decomposition pressures at room temperature. See1 and Detmer2' regard these complexes as ionic e.g. SOF,+AsF,-. If this is the case the ion formed SOF,+ is isoelectronic with the very stable sulphuryl fluoride S02F,. No other reactions of thionyl tetrafluoride have been reported. Sulphur-Nitrogen-Fluorine Compounds In contrast to the other compounds containing sulphur-fluorine bonds no compounds in which sulphur is sexivalent and bonded to both nitrogen and fluorine have been fully characterised but it is probable that this is due more to the lack of experimental work on this type of compound than to any chemical principle.The first compound of this type to be prepared was tetrathiazyl tetrafluoride S4N4F4 which is obtainedlOO by cautious fluorination of tetrasulphur tetranitride with silver difluoride. The SIN ring remains loo Glemser Schroder and Haeseler Z. anorg. Chem. 1955 279 28. ROBERTS COMPOUNDS CONTAINING S-F BONDS 55 intact and the fluorine atoms are bonded to sulphur. This is shown by its hydrolysis by aqueous sodium hydroxide to give sodium sulphite which means that the sulphur atom is quadrivalent. The equivalent trimeric compound has been obtained by fluorination of the trimeric chloride.lol A more vigorous fluorination of tetrasulphur tetranitride leads to a mixture of two compounds SN,F and SNF whose structures have not been determined but probably do not involve S-F bonds; this mixture reacts furtherlo with silver difluoride to give the compound SF,=NF fluoroiminosulphur difluoride.This compound is similar in physical and chemical properties to thionyl fluoride. It is a colourless gas b.p. -23" and with dilute sodium hydroxide gives ammonia sodium sulphite and sodium fluoride FN=SF + 5NaOH -+ Na,SO + 3NaF + NH + H,O A series of compounds the iminosulphur difluorides derived from this by replacement of the fluorine bonded to nitrogen can be obtained by reaction of sulphur tetrafluoride with nitriles isocyanates and thiocyan- ates :lo3 R*C= N + SF +- RCF,*N=SF R.N=C=O + SF + R*N=SF + COFa NaOCN + ZSF -+ CFa*N=SF + NaF + SOF The compound trifluoromethyliminosulphur difluoride is also obtained by fluorination of methyl thiocyanate,lo3 although it was originally thought that an isomer pentafluorosulphur cyanide was formed.lo4 On further fluorination this compound gave a product thought then to be SF,CF,-NF2 but more likely to be CF,.NF.SF,.If this is the case it is the only known compound containing a pentafluorosulphur group bonded to nitrogen. Rather surprisingly this compound is unstable.lo4 The unstable compound SN2F mentioned above decomposes on being heated; it gives the com- pound NSF isoelectric with sulphur dioxide which it resembles closely in its chemical and physical properties.lo5 It boils at 4.8" and is hydrolysed by sodium hydroxide to ammonia sodium sulphite and sodium fluoride which indicates that its structure must be NGS-F.In addition to these compounds fluorination of tetrasulphur tetra- nitride produces a variety of unstable compounds which have not been isolated and further study of this reaction is needed if it is to be com- pletely understood. It is also probable that a range of compounds con- taining the group SF,.N could be prepared. lol Schroder and Glemser 2. anorg. Chem. 1959 298 78. lo2 Glemser and Schroder 2. anorg. Chem. 1956,284 97. Io3 W. C. Smith Tullock R. D. Smith and Engelhardt J. Amer. Chem. SOC. 1960 l o 4 Attaway Groth and Bigelow J. Amer. Chem. Soc. 1959 81 3599. lo5 Glemser and Haeseler Z. anorg. Chem. 1956,287 54. 82 551.