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Tungsten and molybdenum fluorides by metal explosions
J. inorg,nucl.Chem.,1969, Vol. 31, pp.955 to 963. PergamonPress. Printedin GreatBritain
TUNGSTEN AND MOLYBDENUM FLUORIDES BY METAL EXPLOSIONS RICHARD
L. J O H N S O N
and BERNARD
SIEGEL
Laboratories Division, Aerospace Corporation, E1 Segundo, Calif. 90045
(Received 20 May 1968) A l ~ ' a e t - H i g h yields of the volatile hexafluorides of tungsten and molybdenum can be obtained by the electrical explosion of these metals into the relatively inert SFe. This constitutes an extremely facile synthesis route for the metal hexafluorides, because the fluorination of the metals by conventional techniques requires the use of elemental fluorine. Carbon tetrafluoride can also be used for the facile synthesis of WFe by the metal explosion technique, but is less efficacious than SFe. The reaction path of metal explosions into SFe depends upon reactant stoichiometry, but the yield of metal hexafluoride is essentially independent o f this stoichiometry. Explosion of molybdenum into PF5 leads to the formation of molybdenum trifluoride and new fluorides M o F n , in w h i c h n ~< 1. The relative proportions o f M o F s and lower fluorides that form in these reactions depend strongly upon the imparted electrical energy level. Tungsten is less efficacious than molybdenum in abstracting fluorine from PFs. A t comparable energy levels tungsten forms only fluorides of low fluorine content. It is indicated that the fluorides in which n < 1 are mixtures of monofluorides with still lower fluorides. The applicability of these results to other metals is discussed. INTRODUCTION
tungsten metal requires the action of elemental fluorine [1,2], and metathetical conversions of W vI compounds to W F 6 require strong fluorinating agents, such as HF[2] or BrFa[3]. The reaction between tungsten metal and F~ proceeds directly to W F 6, and lower fluorides must be prepared by subsequent reduction of W F s. The lowest tungsten fluoride that is presently known is WF4, which is prepared by prolonged reaction of W F 6 with benzene [4]. Similar considerations apply to the preparation of the molybdenum fluorides, of which the lowest member presently known is MoFa [5]. Since it was shown recently[6] that the electrical explosion of several metals into S F 6, which is ordinarily considered to be extremely inert, produces SF4 and metal fluorides, we considered the possible applicability of this technique toward the synthesis of tungsten and molybdenum fluorides. Firstly, we anticipated the direct fluorination of the metals to the hexafluorides by the use of "inert" reactants rather than elemental fluorine. Secondly, we envisaged the possibility of preparing lower fluorides directly from the metals, opening up the possibility of synthesizing new lower fluorides of low fluorine content. Such experimentation with the VI-A metals, tungsten and molybdenum, is described below, along with suggestions pertaining to the applicability of our results to the synthesis of analogous fluorides of Group VII-A and VIII metals. FLUORINATION
I. 2. 3. 4. 5. 6.
of
H. F. Priest, lnorg. Synth. 3, 171 (1950). J. W. Melior, lnorg, theor. Chem. Vol. 11. Longmans-Greene, London (1931). N. S~ Nikolaev and V. F. Sukhoverkhov, Chem. A bstr. 53, 9871 (1959). H. F, Priest and W. C. Schumb,J.Am. chem. Soc. 70, 3378 (1948). H.J. Emeleus and V. Gutmann, J. chem. Soc. 2979 (1949);A cta CrystaUogr. 4, 244 (1951). E. Cook and B. Siegel,J. inorg, nucl. Chem. 29, 2739 (1967). 955
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R . L . J O H N S O N and B. S I E G E L
The exploding metal technique superficially resembles cathode sputtering in that there is volatilization of metal under very high energy conditions. However, cathode sputtering is a continuous process, whereas the metal explosion is a highly transient process. There is extremely rapid attainment of high temperature, followed by rapid temperature quenching. Under appropriate energy conditions this can lead to an extremely efficient abstraction of the imparted energy by gaseous reactants, with initiation of processes that more closely resemble shock tube reactions than electrical discharge processes [7]. Another advantageous factor in the use of the exploding metal technique for metal fluoride synthesis is the negligible attack on the reactor wall, if an excess of gaseous reactant is used. The pulsed nature of the process and the extremely rapid transfer of energy from exploding metal to the surrounding gas can essentially restrict the reaction to an inner zone of gas: This has been established by the present authors in a study of wire explosions in methane[7]. Thus, outer regions of the surrounding gas in effect form a substitute wall that shields the actual reactor wall from contact with hot reactive intermediate species formed from the inert gaseous fluoride used as a reactant. EXPERIMENTAL
PROCEDURE
The apparatus and general technique used in this laboratory for studying the chemistry of metal explosions have been described in our previous publicationsl6-9]. In the present experiments the metals, which were 20 to 30 ml dia. wires, were exploded exclusively by an ignitron trigger switch, using a reactor volume of 833 cc. This permitted the explosion of sufficient quantifies of metal by the use of a single loop of wire suspended between the electrodes. Analytical data were obtained by the following procedure. In SF8 reactions, unreacted SFe and volatile products were distilled under high vacuum from the reactor into a trap containing a concentrated aqueous solution of NaOH. The volatile products, which were SE4 and metal hexafluorides, were completely hydrolyzed by the alkali solution, with conversion of all fluorine to NaF. Sulfur hexafluoride is inert, however, and the quantity of unreacted SF6 could thus be determined by PVT relationships. The quantities of SF4 and metal hexafluorides were determined by titration of the hydrolysate for fluorine by the thorium nitrate method, and by gravimetric determination of the metal (Mo as silver molybdate and W as the oxide[10/. It was established by infra-red absorption spectra that SF4 and metal hexafluorides were the only volatile products. Because both WFe and MoFe are considerably less volatile than SFe and SFo one can efficiently separate the metal hexafluoride from the sulfur fluorides by trap to trap distillation under high vacuum. However this is tedious and it was convenient to determine the products by the analytical procedure described above. In the carbon tetrafluoride reactions, the volatile products were WF8 and various fluorocarbons, mainly CeF4. The fluorocarbons are unaffected by hydrolysis of WFe, and were analyzed by gas chromatography; a temperature-programmed 12 ft Porapak Q column was used. The noB-volatile products adhering to the reactor wall were recovered by scraping, and subjected to elemental and X-ray diffraction analysis. These residues were then extracted with water and/or concentrated NaOH solutions. Soluble and insoluble portions were than analyzed as above. Additionally, the reactor wall was generally extracted with water and/or concentrated NaOH solutions, to obtain products that could not be recovered by scraping. Elemental analysis was then performed on the extracts. Non-soluble residues were fused with Na2COa at 1100°C, prior to analysis. 7. B. Siegel and R. L. Johnson, A Thermal Model of Wire Explosions in Methane, Exploding Wires (Edited by W. G. Chace and H. K. Moore), Vol. 4, pp. 253-267. Plenum Press, New York (1968). 8. R. L. Johnson and B. Siegel,J. electrochem. Soc. 115, 24 (1968). 9. Eileen Cook and B. Siegel, J. inorg, nucl. Chem. 30, 1699 (1968). 10. N. Furman, Editor, Scott's Standard Methods of Chemical Analysis, 5th Edn. Van Nostrand New York (1939).
Tungsten and molybdenumfluoridesby metal explosions
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EXPERIMENTAL RESULTS
Tungsten explosions in CF4 and SF6 When 220 to 240 mg of tungsten were exploded into 450 torr. of CF~ at an electrical input energy level of 1260 J, 25 per cent of the tungsten reacted to form WFe. From the product data we could discern two principal stoichiometrics, given by Equations (1) and (2), each accounting for about 50 per cent of the tungsten conversion. Hexafluoroethane was also found but it was a very CF4(g) + W(s) --> WF0(g) +~ C(s) 3 CF4(g)+ W(s) --~ WFt(g) + ~ C~F4(g)
(1) (2)
minor product; the C2FJC2Fo ratio was 10. The yield of WFe increased to 31 per cent when the input energy was increased to 2190 J and the initial CF4 pressure was reduced to 100 torr. This was accompanied by a drastic change in product ratios, Equation (1) now constituting the predominant path of the reaction. The non-volatile residues, which could be readily scraped from the "reactor wall, consisted of carbon and tungsten. There were no detectable fluorine contents. Two types of reaction between exploding tungsten and SF6 were observed, depending upon the ratio of SF6 to tungsten. In all runs which provided a molar SFe/W ratio exceeding 3, the reaction proceeded exclusively by Equation (3). This condition was met by exploding 220 to 240 mg of tungsten into SFo at initial 3 SFe(g)+ W(s) -~ WFe(g)+ 3 SF4(g)
(3)
pressures in excess of 100 torr. When the input energy was 2190 J, the conversion of W to WFe was in the range of 80 to 90 per cent. In some of these runs the SFe was diluted with argon to an Ar/SFo ratio of 6.5, maintaining the partial pressure of SFs in excess of 100 torr. However this did not significantly affect the yield of WFe. We scaled up the magnitude of the above experiments by exploding tungsten wires of 1-0g, with a corresponding increase in the SFe pressure. At the same energy level of 2190 J, this led to 78 to 83 per cent yields of WFt. When the molar SFo/W ratio was reduced to 2.0 at 2190J, the explosion of 0-5 g of tungsten produced WFo yields of 80 to 90 per cent, but 1/6 of the WFo had formed by Equation (4) and 5/6 by Equation (3). Upon reducing SFs(g) + W(s) --> WFe(g) + S(s)
(4)
the S F J W ratio further to 1.8, a comparable yield of WFe was obtained, but the fraction formed by Equation (4) was 42 per cent, and only 58 per cent had formed by Equation (3). Unlike the CF4 reactions, the SFs reactions did not produce granular deposits of non-volatiles on the reactor wall, and products could not be recovered by scraping. Extraction of the wall with hot concentrated N a O H solutions yielded only a trace of extracted fluorine.
Molybdenum explosions in S F6 When 430 mg Mo were exploded into SF~ at a molar S F J M o ratio of 4.0 and an input energy of 2190J, 79-84 per cent of the metal was converted to MoFe,
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R . L . J O H N S O N and B. S I E G E L
exclusively by Equation (5). Upon reducing the S F J M o ratio 3SFr(g) + Mo(s) ~ MoFr(g) + 3SF4(g)
(5)
to 2"0, we obtained a 79 per cent conversion to MoFn, but 12 per cent of the latter had formed by Equation (6). At a SFn/Mo ratio of 1.60, there was a 75 per SFn(g) + Mo(s) ~ MoFs(g) + S(s)
(6)
cent conversion to MoF6, with 52 per cent of the latter formed by Equation 6. Unlike the tungsten explosions in SF~, a thin colored film that could not be recovered by scraping was always observed on the reactor wall in these molybdenum explosions, after the volatiles had been distilled. In the absence of air these films were yellow-brown, but upon exposure to air they rapidly became blue with visible indication of hydrolysis by evolution of a white vapor (presumably HF). These films were readily soluble in water at room temperature. Analysis indicated that the molybdenum contents of these aqueous solutions accounted for the conversion of 6 to 8-5 per cent of the exploded metal. The F/Mo ratios of these products were generally 3.0__-0.2. Sulfur was absent from the aqueous solutions. These data indicate that MoFa is the minor non-volatile product of the molybdenum explosions in SFr. In the massive state MoFa has been reported to be insoluble in water[5], but the present product is a fine film whose solubility could be expected to be different from that of a bulk powder. Although MoFa is quite non-volatile, a somewhat volatile MoF5 has been reported[11], and we attempted to sublime the molybdenum fluoride films in an effort to ascertain whether small amounts of MoF5 had formed. However, with the metal reactor heated to 100°C, under high vacuum, no evidence for MoF5 could be found.
Explosions in PF5 For reasons discussed in the next section, efforts were made to synthesize lower fluorides of tungsten and molybdenum by exploding these metals in PFs. Upon exploding 440mg of Mo into 515 torr PF5 at 530J, 32-35 per cent of the metal was converted to a non-volatile fluoride. This material was deposited on the reactor wall as a readily scrapable brown powder. Prior to extraction with alkali this product exhibited an X-ray diffraction pattern that was primarily identical to that of molybdenum, along with a weak line that was identical to the principal line of the MoF3 spectrum[5]. This indicated the presence of traces of MoFn. Upon extraction with a hot concentrated N a O H solution, traces of fluoride appeared in the extract, and the X-ray diffraction pattern of the insoluble residue no longer exhibited the MoF3 line. Elemental analyses of the insoluble residues (the bulk of the product) from runs carried out under nominally identical conditions were consistent with the empirical formulae MoFo.r4 to MoF1.0o. A pronounced change in non-volatile products was observed when the imparted energy was increased to 840J. Whereas at 530J only traces of MoFn had formed, at 840J a substantial I 1. R. D. Peacock, Proc. chem. Soc. 59 (1957).
Tungsten and molybdenum fluorides by metal explosions
959
quantity of MoF8 was synthesized. Prior to alkali extraction, the X-ray diffraction pattern exhibited all of the lines of MoFa, and the most intense of these lines were quite strong. Upon alkali extraction, elemental analysis of the extract indicated that 18 per cent of the exploded metal had been converted to soluble MoFa. Furthermore, the quantity of metal converted to non-soluble fluoride was also greater than at 530 J. We now found that about 54 per cent of the exploded molybdenum had been converted to a non-soluble fluoride. However, the fluorine content of the latter was considerably lower than had been found at 530 J. We now found compositions ranging from MoF0.aa to MoF0.49. As before, the X-ray diffraction patterns of the latter were free of MoFa, and were virtually identical to that of molybdenum metal. A single molybdenum explosion was performed at 2190 J. The data for this run confirmed the trend that shows an increasing proportion of MoF3 among the products as the imparted energy level is increased. Forty-three per cent of the exploded metal was converted to MoF a. However only 19 per cent of the metal had been converted to a non-soluble fluoride; the empirical composition of the latter was MoF0.25. The trend of decreasing fluorine content in the non-soluble fluoride, with increasing level of imparted energy, was maintained. In the formation of lower fluorides by metal explosions in PFs, tungsten proved less efficacious than molybdenum. In the tungsten reactions, 240-250 mg were exploded into PF5 at initial PFs/W molar ratios of 4-0-4.3. At 530J, only 43-75 mg of black powder could be recovered by scraping the reactor wall after the volatiles had been distilled. These powders were insoluble in boiling concentrated N a O H solutions; the extracts contained trivial amounts of fluorine. The empirical composition of the fluoride was WF0.1s. At 2190 J, 132-138 mg of black powder were recovered. However in these experiments the material recovered by scraping had been augmented by additional powder obtained by washing the reactor with water. Particles that could not be removed by scraping were suspended in the wash liquid, and were readily recoverable. The bulk of the product was insoluble in boiling concentrated N a O H solutions and had an empirical formula that varied from WF0.~ to WFo.ss in different runs. The X-ray diffraction patterns of these samples were substantially identical to that of tungsten metal. DISCUSSION AND CONCLUSIONS
H exafluoride syntheses The data demonstrate that the exploding metal method can be used for the extremely facile syntheses of WF6 and MoF6, in very high yield. The great efficacy of SF6 as a fluorinating agent in this process is very fortunate because of its extreme inertness at ordinary conditions, and thus its ideal handling properties. The only volatile byproduct of metal hexafluoride formation, when SF6 is the fluorinating agent, is SF4, which can be efficiently separated from the desired products by distillation, as can unreacted SF6. We would also like to point out that the exploding metal method can be used to synthesize quite appreciable amounts of the metal hexafluorides. The per cent yield was only slightly affected by scale-up experiments, and we were able to synthesize 1.3 g of WF6 by using only three 14-microfarad capacitors charged to 15 kV. In principle much larger
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R . L . J O H N S O N and B. S I E G E L
quantities could be synthesized with appropriately higher levels of imparted electrical energy. The present data show, moreover, that despite extremely high transient temperatures attained by the explosion process, there must be a judicious selection of gaseous fluorinating agent. One cannot select a gaseous source of fluorine at random. In this selection thermochemical energetics are more important than the usually encountered rate processes at ordinary or moderately high temperatures. Thus SFe, which is kinetically inert toward most metals at quite elevated temperatures, is a more efficacious fluorination agent toward exploding Mo and W than many fluorides that are normally considered to be quite reactive. Qualitatively, the important parameter is the degree of exothermicity of the reaction [6, 8]. The higher yields of WFe from explosions in SF~, as compared to CF4, are explained on the basis that Equation (3) is quite exothermic, whereas Equation (2) is somewhat endothermic; and Equation (4) is considerably more exothermic than Equation (1). The drastic shift in product ratios when the imparted energy varies by a factor of nearly 2 in tungsten explosions into CF4, is also attributable to this cause. The similar behavior of exploding tungsten and molybdenum toward SF6 is reflected not only in yields of metal hexafluorides, but also in the product distributions as a function of reactant stoichiometry. In both cases there is negligible reduction in MFe yield as the S F J M ratio is reduced to levels insufficient to completely support Equations (3) or (5). Instead Equations (4) and (6) provide appreciable portions of the overall reaction. The relative importance of these latter reactions increases sharply at S F J M ratios below 2.0. This shift in reaction path without reduction in yield of MFe is indicative of the high reactivity of the exploding metal (Mo or W) toward SFe. Nearly all of the latter molecules react successfully with the exploding metal in this extremely transient process when the S F J M ratio is sufficiently low. From a preparative viewpoint this is an advantage, because a greater fraction of the byproducts become non-volatile, without appreciable degradation of the metal to lower fluorides.
Lower fluorides Although only minor amounts of MoFs had formed when molybdenum was exploded in SFr, and although WF~ was the exclusive tungsten fluoride product of tungsten explosions in SFe or CF4, we believed that substantial yields of lower fluorides of these metals could be obtained with the proper gaseous fluoride reactant. It was felt that the high exothermicity of WFe and MoFe formation from the metal and SFe prevented lower fluoride formation, because the latter process was probably endothermic and could not compete successfully with the exothermic processes. Thermochemical data are not available for the quantitative evaluation of the energies of lower fluoride formation, but it is reasonable to assume that reactions of Mo and W with SFe to form lower fluorides would be far less exothermic than MFs formation, and that the processes would be highly endothermic for low values of n in MFn. This is a general phenomenon that is based on a fixed value for the endothermic heat of atomization of M in MFn, and the fact that considerable exothermic intrinsic bond energy is lost from the heat of formation as n decreases[12]. The possibility of lower fluoride formation would 12. B. Siegel, J. Chem. Educ. 40, 143, 308 (1963).
Tungsten and molybdenumfluorides by metal explosions
961
therefore be enhanced if we selected a gaseous fluoride reactant that reacts with the metal in a highly endothermic manner to form a metal hexafluoride. This should force a stepwise process, commencing with low values of n, that would consume the imparted energy, at our present levels of electrically imparted energy, before the formation of hexafluorides. By this reasoning hexafluorides should not form at all at such energy levels. A gaseous fluoride that fits the above criterion, and is reasonably convenient in experimental manipulation, is PFs. The reaction of Equation (7) is 3 PFs(g) + W(s) --* WFe (g) + 3 PFa(g)
(7)
highly endothermic. Our hypothesis is amply proved by our experimental results with this fluorinating agent. In the present study it was assumed, but not proved, that PF3 is the byproduct of our fluorination reactions. Since phosphorous was absent from the non-volatile products, this is a very reasonable assumption. We now discuss the identities of the lower fluorides formed by the metal explosions in PFs. One conclusion is unequivocal. By increasing the imparted electrical energy levels, one obtains increasingly higher yields of the trifluoride MoF3. The identification of the latter was substantiated by X-ray diffraction data, solubility behavior, and elemental analysis. The trifluoride is always accompanied by an insoluble fluoride, from which it is readily separable by solubility. At 530J the tendency toward MoF3 formation is at a minimum, and only traces of the trifluoride were observed. The primary product at this energy level is a still lower fluoride of overall composition MoF0.74 to MoF1.00, evidently approaching the composition of a monofluoride. At higher energy levels, MoF3 is formed at the expense of the fluorine content of the monofluoride; at 840J we obtained MoF3 and an insoluble fluoride MoF0.~3 to MoF0.49, whereas at 2190joules MoF3 and the insoluble MoF0.25 were formed. An obvious point of consideration is the homogeneity of the new lower fluorides produced in this study. On the basis of the data reported herein for tungsten fluorides we can be certain only that WF4 and even higher tungsten fluorides must be absent from our non-volatile tungsten fluorides, because such higher fluorides are soluble in alkali solutions. However, data obtained in a study of the oxidation rates of a WF0.31 compound produced by the method described herein, demonstrated that 10 per cent of the compound is a non-crystalline monofluoride (WF), whereas 90 per cent is a still lower fluoride of overall composition WF0.24[13]. Therefore, the monofluoride component does not contribute to the X-ray diffraction pattern of the composite product, and the observed diffraction pattern arises only from the major component of very low fluorine content. Since the diffraction pattern resembles that of tungsten metal, we conclude that in the fluorides of very low fluorine content the fluorines are substituted at metal lattice sites. On the basis of the solubility and X-ray diffraction data presented in the 13. B. Siegel and R. L. Johnson, Effect of Tungsten-Fluorine Bonds on Tungsten Oxidation, To be published.
962
R . E . J O H N S O N and B. S I E G E L
present paper, we can be certain that MoFa is not present in the non-soluble molybdenum fluoride products, because of the disappearance of the MoFa diffraction pattern after extraction of the products with hot alkali solutions; nor could even higher molybdenum fluorides be present because these are also highly soluble in alkali solutions. It appears, therefore, that similarly to the tungsten fluorides, we have generally obtained mixtures of a non-crystalline molybdenum monofluoride with molybdenum fluorides of very low fluorine contents. It is the latter that contribute to the observed X-ray diffraction patterns, which are probably attributable to fluorine substitution in the metal lattice. We speculate that the non-crystallinity of the tungsten and molybdenum monofluorides is attributable to spatial bonding difficulties in forming lattices of such MF compounds, because crystalline products are formed frequently in metal explosion reactions, and the MoFa product found in this study was always crystalline. The fact that the overall empirical formula of the non-soluble molybdenum fluorides approaches that of a monofluoride under conditions of minimum imparted electrical energy indicates that under such reaction conditions the product is predominantly a monofluoride, whereas at higher energy levels the monofluoride is accompanied by increasing amounts of much lower fluorides. The molybdenum diffraction pattern found for the MoF1.0 composition probably arises from a slight admixture with a lower fluoride, since a several per cent component is readily detectible by X-ray diffraction, and this is within our standard analytical deviation for combined elemental analyses. Note that the highest fluoride formed in the tungsten reactions was WF0.aa, and the fluorine content of the fluoride increases somewhat with increasing level of the imparted energy. This behavior is completely different from that of molybdenum. It must be concluded that tungsten is considerably less efficacious than molybdenum with regard to the abstraction of fluorine from PFs. This is probably attributable to striking differences in the volatilities of these metals; the melting point of molybdenum is 760°K less than that of tungsten[14]. The lower volatility of tungsten tends to increase the speed of condensation of explosively vaporized and liquified metal, and one might predict that molybdenum would be in an active state for longer periods than tungsten. It was shown in [9] that explosively liquified metal droplets react readily to form carbides under appropriate conditions. Since endothermic reactions in which the metal participates chemically are very inefficient in explosion reactions [6], one might predict that differences between the chemistry of molybdenum and tungsten would be more readily observed in endothermic processes, such as lower fluoride formation, than exothermic ones, such as the reaction of these exploding metals with SFe to form hexafluorides. The frequent variability of n in experiments carded out under nominally identical conditions requires discussion. We believe that these variations arise from non-uniform diameters in the metals to be exploded. Regions of smaller diameter tend to develop hot spots that vaporize preferentially to regions of wider diameter. It is currently believed that explosions of real wires occur via a series of constrictions or "pinches". If the wire is of very small diameter and nearly uniform, nodes between constrictions are numerous and close together, 14. D. R. Stull aad G. C. Sinke, Thermodynamic Properties of the Elements. Am. Chem. Soc., Wash. D.C. (1956).
Tungsten and molybdenum fluorides by metal explosions
963
leading to instantaneous vaporization of the metal. Otherwise, explosion occurs mainly at vaporized sites at constrictions, carrying along liquid droplets formed at wider sections. The ideal explosion conditions are not compatible with our objective of synthesizing large amounts of transition metal compounds, which requires the use of long strands of wires of considerable diameters. It is particularly difficult to obtain uniform wires of highly refractory metals. One should thus expect random variations in the proportions of vaporized or liquified metal, and perhaps random variations in the size of liquid droplets. These variations could easily affect the efficacy of abstraction of fluorine from the gaseous reactant. However, as our data show, product fluctuations are more pronounced in the inefficient endothermic processes involving PFs, than in the exothermic reactions of SFr. The fluctuations in yield of metal hexafluoride are small by comparison to fluctuations in fluorine content of lower fluorides. Application to other metals The preparation of volatile transition metal hexafluorides directly from the metal with relatively inert fluorinating agents should be possible for metals other than the VI-A elements described in the present paper. Most of the Group VII-A and VIII metals of the second and third transition series form such compounds. HoWever one must consider the thermal stability of the hexafluorides, because we[8] have shown previously that thermally labile compounds do not form by the exploding metal technique. Among the hexafluorides of the relevant metals, only those of technetium, rhenium, osmium and iridium can be classified as reasonably stable; it is known that PtF6 is thermally quite unstable and that RuF6 and RhF~ are even less stable [15]. Since it has already been shown that PtFn does not form by platinum explosions in SF816], we predict that the Ru and Rh hexafluorides also cannot be formed by this technique, and that it is unlikely that one could prepare a comparable fluoride of palladium in this manner. However it does seem likely that the hexafluorides of Tc, Re, Os and Ir could be prepared by metal explosions. In the case of rhenium, ReF6 can be converted to a volatile heptafluoride by heating the hexafluoride in fluorine at high temperatures [ 16]. It would be interesting to determine whether the exploding metal technique can be used at higher energy levels for the direct synthesis of the heptafluoride, to the exclusion of the hexafluoride. Even those metals that form thermally labile hexafluorides should react readily by the metal explosion technique to form lower fluorides, perhaps more effectively with SF6 than PFs. Acknowledgements-The authors are indebted to Miss Cassandra Johnson for the elemental and gas chromatographic analyses and to Mr. E. S. Elliott for obtaining the X-ray diffraction data. 15. J. H. Canterford, R. Colton and T. A. O'Donneli, Rev. Pure appl. Chem. 17,123 (1967). 16. J. G. Maim and H. Selig. J. inorg, nucl. Chem. 20, 189 (1961).
TUNGSTEN AND MOLYBDENUM FLUORIDES BY METAL EXPLOSIONS RICHARD
L. J O H N S O N
and BERNARD
SIEGEL
Laboratories Division, Aerospace Corporation, E1 Segundo, Calif. 90045
(Received 20 May 1968) A l ~ ' a e t - H i g h yields of the volatile hexafluorides of tungsten and molybdenum can be obtained by the electrical explosion of these metals into the relatively inert SFe. This constitutes an extremely facile synthesis route for the metal hexafluorides, because the fluorination of the metals by conventional techniques requires the use of elemental fluorine. Carbon tetrafluoride can also be used for the facile synthesis of WFe by the metal explosion technique, but is less efficacious than SFe. The reaction path of metal explosions into SFe depends upon reactant stoichiometry, but the yield of metal hexafluoride is essentially independent o f this stoichiometry. Explosion of molybdenum into PF5 leads to the formation of molybdenum trifluoride and new fluorides M o F n , in w h i c h n ~< 1. The relative proportions o f M o F s and lower fluorides that form in these reactions depend strongly upon the imparted electrical energy level. Tungsten is less efficacious than molybdenum in abstracting fluorine from PFs. A t comparable energy levels tungsten forms only fluorides of low fluorine content. It is indicated that the fluorides in which n < 1 are mixtures of monofluorides with still lower fluorides. The applicability of these results to other metals is discussed. INTRODUCTION
tungsten metal requires the action of elemental fluorine [1,2], and metathetical conversions of W vI compounds to W F 6 require strong fluorinating agents, such as HF[2] or BrFa[3]. The reaction between tungsten metal and F~ proceeds directly to W F 6, and lower fluorides must be prepared by subsequent reduction of W F s. The lowest tungsten fluoride that is presently known is WF4, which is prepared by prolonged reaction of W F 6 with benzene [4]. Similar considerations apply to the preparation of the molybdenum fluorides, of which the lowest member presently known is MoFa [5]. Since it was shown recently[6] that the electrical explosion of several metals into S F 6, which is ordinarily considered to be extremely inert, produces SF4 and metal fluorides, we considered the possible applicability of this technique toward the synthesis of tungsten and molybdenum fluorides. Firstly, we anticipated the direct fluorination of the metals to the hexafluorides by the use of "inert" reactants rather than elemental fluorine. Secondly, we envisaged the possibility of preparing lower fluorides directly from the metals, opening up the possibility of synthesizing new lower fluorides of low fluorine content. Such experimentation with the VI-A metals, tungsten and molybdenum, is described below, along with suggestions pertaining to the applicability of our results to the synthesis of analogous fluorides of Group VII-A and VIII metals. FLUORINATION
I. 2. 3. 4. 5. 6.
of
H. F. Priest, lnorg. Synth. 3, 171 (1950). J. W. Melior, lnorg, theor. Chem. Vol. 11. Longmans-Greene, London (1931). N. S~ Nikolaev and V. F. Sukhoverkhov, Chem. A bstr. 53, 9871 (1959). H. F, Priest and W. C. Schumb,J.Am. chem. Soc. 70, 3378 (1948). H.J. Emeleus and V. Gutmann, J. chem. Soc. 2979 (1949);A cta CrystaUogr. 4, 244 (1951). E. Cook and B. Siegel,J. inorg, nucl. Chem. 29, 2739 (1967). 955
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The exploding metal technique superficially resembles cathode sputtering in that there is volatilization of metal under very high energy conditions. However, cathode sputtering is a continuous process, whereas the metal explosion is a highly transient process. There is extremely rapid attainment of high temperature, followed by rapid temperature quenching. Under appropriate energy conditions this can lead to an extremely efficient abstraction of the imparted energy by gaseous reactants, with initiation of processes that more closely resemble shock tube reactions than electrical discharge processes [7]. Another advantageous factor in the use of the exploding metal technique for metal fluoride synthesis is the negligible attack on the reactor wall, if an excess of gaseous reactant is used. The pulsed nature of the process and the extremely rapid transfer of energy from exploding metal to the surrounding gas can essentially restrict the reaction to an inner zone of gas: This has been established by the present authors in a study of wire explosions in methane[7]. Thus, outer regions of the surrounding gas in effect form a substitute wall that shields the actual reactor wall from contact with hot reactive intermediate species formed from the inert gaseous fluoride used as a reactant. EXPERIMENTAL
PROCEDURE
The apparatus and general technique used in this laboratory for studying the chemistry of metal explosions have been described in our previous publicationsl6-9]. In the present experiments the metals, which were 20 to 30 ml dia. wires, were exploded exclusively by an ignitron trigger switch, using a reactor volume of 833 cc. This permitted the explosion of sufficient quantifies of metal by the use of a single loop of wire suspended between the electrodes. Analytical data were obtained by the following procedure. In SF8 reactions, unreacted SFe and volatile products were distilled under high vacuum from the reactor into a trap containing a concentrated aqueous solution of NaOH. The volatile products, which were SE4 and metal hexafluorides, were completely hydrolyzed by the alkali solution, with conversion of all fluorine to NaF. Sulfur hexafluoride is inert, however, and the quantity of unreacted SF6 could thus be determined by PVT relationships. The quantities of SF4 and metal hexafluorides were determined by titration of the hydrolysate for fluorine by the thorium nitrate method, and by gravimetric determination of the metal (Mo as silver molybdate and W as the oxide[10/. It was established by infra-red absorption spectra that SF4 and metal hexafluorides were the only volatile products. Because both WFe and MoFe are considerably less volatile than SFe and SFo one can efficiently separate the metal hexafluoride from the sulfur fluorides by trap to trap distillation under high vacuum. However this is tedious and it was convenient to determine the products by the analytical procedure described above. In the carbon tetrafluoride reactions, the volatile products were WF8 and various fluorocarbons, mainly CeF4. The fluorocarbons are unaffected by hydrolysis of WFe, and were analyzed by gas chromatography; a temperature-programmed 12 ft Porapak Q column was used. The noB-volatile products adhering to the reactor wall were recovered by scraping, and subjected to elemental and X-ray diffraction analysis. These residues were then extracted with water and/or concentrated NaOH solutions. Soluble and insoluble portions were than analyzed as above. Additionally, the reactor wall was generally extracted with water and/or concentrated NaOH solutions, to obtain products that could not be recovered by scraping. Elemental analysis was then performed on the extracts. Non-soluble residues were fused with Na2COa at 1100°C, prior to analysis. 7. B. Siegel and R. L. Johnson, A Thermal Model of Wire Explosions in Methane, Exploding Wires (Edited by W. G. Chace and H. K. Moore), Vol. 4, pp. 253-267. Plenum Press, New York (1968). 8. R. L. Johnson and B. Siegel,J. electrochem. Soc. 115, 24 (1968). 9. Eileen Cook and B. Siegel, J. inorg, nucl. Chem. 30, 1699 (1968). 10. N. Furman, Editor, Scott's Standard Methods of Chemical Analysis, 5th Edn. Van Nostrand New York (1939).
Tungsten and molybdenumfluoridesby metal explosions
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EXPERIMENTAL RESULTS
Tungsten explosions in CF4 and SF6 When 220 to 240 mg of tungsten were exploded into 450 torr. of CF~ at an electrical input energy level of 1260 J, 25 per cent of the tungsten reacted to form WFe. From the product data we could discern two principal stoichiometrics, given by Equations (1) and (2), each accounting for about 50 per cent of the tungsten conversion. Hexafluoroethane was also found but it was a very CF4(g) + W(s) --> WF0(g) +~ C(s) 3 CF4(g)+ W(s) --~ WFt(g) + ~ C~F4(g)
(1) (2)
minor product; the C2FJC2Fo ratio was 10. The yield of WFe increased to 31 per cent when the input energy was increased to 2190 J and the initial CF4 pressure was reduced to 100 torr. This was accompanied by a drastic change in product ratios, Equation (1) now constituting the predominant path of the reaction. The non-volatile residues, which could be readily scraped from the "reactor wall, consisted of carbon and tungsten. There were no detectable fluorine contents. Two types of reaction between exploding tungsten and SF6 were observed, depending upon the ratio of SF6 to tungsten. In all runs which provided a molar SFe/W ratio exceeding 3, the reaction proceeded exclusively by Equation (3). This condition was met by exploding 220 to 240 mg of tungsten into SFo at initial 3 SFe(g)+ W(s) -~ WFe(g)+ 3 SF4(g)
(3)
pressures in excess of 100 torr. When the input energy was 2190 J, the conversion of W to WFe was in the range of 80 to 90 per cent. In some of these runs the SFe was diluted with argon to an Ar/SFo ratio of 6.5, maintaining the partial pressure of SFs in excess of 100 torr. However this did not significantly affect the yield of WFe. We scaled up the magnitude of the above experiments by exploding tungsten wires of 1-0g, with a corresponding increase in the SFe pressure. At the same energy level of 2190 J, this led to 78 to 83 per cent yields of WFt. When the molar SFo/W ratio was reduced to 2.0 at 2190J, the explosion of 0-5 g of tungsten produced WFo yields of 80 to 90 per cent, but 1/6 of the WFo had formed by Equation (4) and 5/6 by Equation (3). Upon reducing SFs(g) + W(s) --> WFe(g) + S(s)
(4)
the S F J W ratio further to 1.8, a comparable yield of WFe was obtained, but the fraction formed by Equation (4) was 42 per cent, and only 58 per cent had formed by Equation (3). Unlike the CF4 reactions, the SFs reactions did not produce granular deposits of non-volatiles on the reactor wall, and products could not be recovered by scraping. Extraction of the wall with hot concentrated N a O H solutions yielded only a trace of extracted fluorine.
Molybdenum explosions in S F6 When 430 mg Mo were exploded into SF~ at a molar S F J M o ratio of 4.0 and an input energy of 2190J, 79-84 per cent of the metal was converted to MoFe,
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exclusively by Equation (5). Upon reducing the S F J M o ratio 3SFr(g) + Mo(s) ~ MoFr(g) + 3SF4(g)
(5)
to 2"0, we obtained a 79 per cent conversion to MoFn, but 12 per cent of the latter had formed by Equation (6). At a SFn/Mo ratio of 1.60, there was a 75 per SFn(g) + Mo(s) ~ MoFs(g) + S(s)
(6)
cent conversion to MoF6, with 52 per cent of the latter formed by Equation 6. Unlike the tungsten explosions in SF~, a thin colored film that could not be recovered by scraping was always observed on the reactor wall in these molybdenum explosions, after the volatiles had been distilled. In the absence of air these films were yellow-brown, but upon exposure to air they rapidly became blue with visible indication of hydrolysis by evolution of a white vapor (presumably HF). These films were readily soluble in water at room temperature. Analysis indicated that the molybdenum contents of these aqueous solutions accounted for the conversion of 6 to 8-5 per cent of the exploded metal. The F/Mo ratios of these products were generally 3.0__-0.2. Sulfur was absent from the aqueous solutions. These data indicate that MoFa is the minor non-volatile product of the molybdenum explosions in SFr. In the massive state MoFa has been reported to be insoluble in water[5], but the present product is a fine film whose solubility could be expected to be different from that of a bulk powder. Although MoFa is quite non-volatile, a somewhat volatile MoF5 has been reported[11], and we attempted to sublime the molybdenum fluoride films in an effort to ascertain whether small amounts of MoF5 had formed. However, with the metal reactor heated to 100°C, under high vacuum, no evidence for MoF5 could be found.
Explosions in PF5 For reasons discussed in the next section, efforts were made to synthesize lower fluorides of tungsten and molybdenum by exploding these metals in PFs. Upon exploding 440mg of Mo into 515 torr PF5 at 530J, 32-35 per cent of the metal was converted to a non-volatile fluoride. This material was deposited on the reactor wall as a readily scrapable brown powder. Prior to extraction with alkali this product exhibited an X-ray diffraction pattern that was primarily identical to that of molybdenum, along with a weak line that was identical to the principal line of the MoF3 spectrum[5]. This indicated the presence of traces of MoFn. Upon extraction with a hot concentrated N a O H solution, traces of fluoride appeared in the extract, and the X-ray diffraction pattern of the insoluble residue no longer exhibited the MoF3 line. Elemental analyses of the insoluble residues (the bulk of the product) from runs carried out under nominally identical conditions were consistent with the empirical formulae MoFo.r4 to MoF1.0o. A pronounced change in non-volatile products was observed when the imparted energy was increased to 840J. Whereas at 530J only traces of MoFn had formed, at 840J a substantial I 1. R. D. Peacock, Proc. chem. Soc. 59 (1957).
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quantity of MoF8 was synthesized. Prior to alkali extraction, the X-ray diffraction pattern exhibited all of the lines of MoFa, and the most intense of these lines were quite strong. Upon alkali extraction, elemental analysis of the extract indicated that 18 per cent of the exploded metal had been converted to soluble MoFa. Furthermore, the quantity of metal converted to non-soluble fluoride was also greater than at 530 J. We now found that about 54 per cent of the exploded molybdenum had been converted to a non-soluble fluoride. However, the fluorine content of the latter was considerably lower than had been found at 530 J. We now found compositions ranging from MoF0.aa to MoF0.49. As before, the X-ray diffraction patterns of the latter were free of MoFa, and were virtually identical to that of molybdenum metal. A single molybdenum explosion was performed at 2190 J. The data for this run confirmed the trend that shows an increasing proportion of MoF3 among the products as the imparted energy level is increased. Forty-three per cent of the exploded metal was converted to MoF a. However only 19 per cent of the metal had been converted to a non-soluble fluoride; the empirical composition of the latter was MoF0.25. The trend of decreasing fluorine content in the non-soluble fluoride, with increasing level of imparted energy, was maintained. In the formation of lower fluorides by metal explosions in PFs, tungsten proved less efficacious than molybdenum. In the tungsten reactions, 240-250 mg were exploded into PF5 at initial PFs/W molar ratios of 4-0-4.3. At 530J, only 43-75 mg of black powder could be recovered by scraping the reactor wall after the volatiles had been distilled. These powders were insoluble in boiling concentrated N a O H solutions; the extracts contained trivial amounts of fluorine. The empirical composition of the fluoride was WF0.1s. At 2190 J, 132-138 mg of black powder were recovered. However in these experiments the material recovered by scraping had been augmented by additional powder obtained by washing the reactor with water. Particles that could not be removed by scraping were suspended in the wash liquid, and were readily recoverable. The bulk of the product was insoluble in boiling concentrated N a O H solutions and had an empirical formula that varied from WF0.~ to WFo.ss in different runs. The X-ray diffraction patterns of these samples were substantially identical to that of tungsten metal. DISCUSSION AND CONCLUSIONS
H exafluoride syntheses The data demonstrate that the exploding metal method can be used for the extremely facile syntheses of WF6 and MoF6, in very high yield. The great efficacy of SF6 as a fluorinating agent in this process is very fortunate because of its extreme inertness at ordinary conditions, and thus its ideal handling properties. The only volatile byproduct of metal hexafluoride formation, when SF6 is the fluorinating agent, is SF4, which can be efficiently separated from the desired products by distillation, as can unreacted SF6. We would also like to point out that the exploding metal method can be used to synthesize quite appreciable amounts of the metal hexafluorides. The per cent yield was only slightly affected by scale-up experiments, and we were able to synthesize 1.3 g of WF6 by using only three 14-microfarad capacitors charged to 15 kV. In principle much larger
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quantities could be synthesized with appropriately higher levels of imparted electrical energy. The present data show, moreover, that despite extremely high transient temperatures attained by the explosion process, there must be a judicious selection of gaseous fluorinating agent. One cannot select a gaseous source of fluorine at random. In this selection thermochemical energetics are more important than the usually encountered rate processes at ordinary or moderately high temperatures. Thus SFe, which is kinetically inert toward most metals at quite elevated temperatures, is a more efficacious fluorination agent toward exploding Mo and W than many fluorides that are normally considered to be quite reactive. Qualitatively, the important parameter is the degree of exothermicity of the reaction [6, 8]. The higher yields of WFe from explosions in SF~, as compared to CF4, are explained on the basis that Equation (3) is quite exothermic, whereas Equation (2) is somewhat endothermic; and Equation (4) is considerably more exothermic than Equation (1). The drastic shift in product ratios when the imparted energy varies by a factor of nearly 2 in tungsten explosions into CF4, is also attributable to this cause. The similar behavior of exploding tungsten and molybdenum toward SF6 is reflected not only in yields of metal hexafluorides, but also in the product distributions as a function of reactant stoichiometry. In both cases there is negligible reduction in MFe yield as the S F J M ratio is reduced to levels insufficient to completely support Equations (3) or (5). Instead Equations (4) and (6) provide appreciable portions of the overall reaction. The relative importance of these latter reactions increases sharply at S F J M ratios below 2.0. This shift in reaction path without reduction in yield of MFe is indicative of the high reactivity of the exploding metal (Mo or W) toward SFe. Nearly all of the latter molecules react successfully with the exploding metal in this extremely transient process when the S F J M ratio is sufficiently low. From a preparative viewpoint this is an advantage, because a greater fraction of the byproducts become non-volatile, without appreciable degradation of the metal to lower fluorides.
Lower fluorides Although only minor amounts of MoFs had formed when molybdenum was exploded in SFr, and although WF~ was the exclusive tungsten fluoride product of tungsten explosions in SFe or CF4, we believed that substantial yields of lower fluorides of these metals could be obtained with the proper gaseous fluoride reactant. It was felt that the high exothermicity of WFe and MoFe formation from the metal and SFe prevented lower fluoride formation, because the latter process was probably endothermic and could not compete successfully with the exothermic processes. Thermochemical data are not available for the quantitative evaluation of the energies of lower fluoride formation, but it is reasonable to assume that reactions of Mo and W with SFe to form lower fluorides would be far less exothermic than MFs formation, and that the processes would be highly endothermic for low values of n in MFn. This is a general phenomenon that is based on a fixed value for the endothermic heat of atomization of M in MFn, and the fact that considerable exothermic intrinsic bond energy is lost from the heat of formation as n decreases[12]. The possibility of lower fluoride formation would 12. B. Siegel, J. Chem. Educ. 40, 143, 308 (1963).
Tungsten and molybdenumfluorides by metal explosions
961
therefore be enhanced if we selected a gaseous fluoride reactant that reacts with the metal in a highly endothermic manner to form a metal hexafluoride. This should force a stepwise process, commencing with low values of n, that would consume the imparted energy, at our present levels of electrically imparted energy, before the formation of hexafluorides. By this reasoning hexafluorides should not form at all at such energy levels. A gaseous fluoride that fits the above criterion, and is reasonably convenient in experimental manipulation, is PFs. The reaction of Equation (7) is 3 PFs(g) + W(s) --* WFe (g) + 3 PFa(g)
(7)
highly endothermic. Our hypothesis is amply proved by our experimental results with this fluorinating agent. In the present study it was assumed, but not proved, that PF3 is the byproduct of our fluorination reactions. Since phosphorous was absent from the non-volatile products, this is a very reasonable assumption. We now discuss the identities of the lower fluorides formed by the metal explosions in PFs. One conclusion is unequivocal. By increasing the imparted electrical energy levels, one obtains increasingly higher yields of the trifluoride MoF3. The identification of the latter was substantiated by X-ray diffraction data, solubility behavior, and elemental analysis. The trifluoride is always accompanied by an insoluble fluoride, from which it is readily separable by solubility. At 530J the tendency toward MoF3 formation is at a minimum, and only traces of the trifluoride were observed. The primary product at this energy level is a still lower fluoride of overall composition MoF0.74 to MoF1.00, evidently approaching the composition of a monofluoride. At higher energy levels, MoF3 is formed at the expense of the fluorine content of the monofluoride; at 840J we obtained MoF3 and an insoluble fluoride MoF0.~3 to MoF0.49, whereas at 2190joules MoF3 and the insoluble MoF0.25 were formed. An obvious point of consideration is the homogeneity of the new lower fluorides produced in this study. On the basis of the data reported herein for tungsten fluorides we can be certain only that WF4 and even higher tungsten fluorides must be absent from our non-volatile tungsten fluorides, because such higher fluorides are soluble in alkali solutions. However, data obtained in a study of the oxidation rates of a WF0.31 compound produced by the method described herein, demonstrated that 10 per cent of the compound is a non-crystalline monofluoride (WF), whereas 90 per cent is a still lower fluoride of overall composition WF0.24[13]. Therefore, the monofluoride component does not contribute to the X-ray diffraction pattern of the composite product, and the observed diffraction pattern arises only from the major component of very low fluorine content. Since the diffraction pattern resembles that of tungsten metal, we conclude that in the fluorides of very low fluorine content the fluorines are substituted at metal lattice sites. On the basis of the solubility and X-ray diffraction data presented in the 13. B. Siegel and R. L. Johnson, Effect of Tungsten-Fluorine Bonds on Tungsten Oxidation, To be published.
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present paper, we can be certain that MoFa is not present in the non-soluble molybdenum fluoride products, because of the disappearance of the MoFa diffraction pattern after extraction of the products with hot alkali solutions; nor could even higher molybdenum fluorides be present because these are also highly soluble in alkali solutions. It appears, therefore, that similarly to the tungsten fluorides, we have generally obtained mixtures of a non-crystalline molybdenum monofluoride with molybdenum fluorides of very low fluorine contents. It is the latter that contribute to the observed X-ray diffraction patterns, which are probably attributable to fluorine substitution in the metal lattice. We speculate that the non-crystallinity of the tungsten and molybdenum monofluorides is attributable to spatial bonding difficulties in forming lattices of such MF compounds, because crystalline products are formed frequently in metal explosion reactions, and the MoFa product found in this study was always crystalline. The fact that the overall empirical formula of the non-soluble molybdenum fluorides approaches that of a monofluoride under conditions of minimum imparted electrical energy indicates that under such reaction conditions the product is predominantly a monofluoride, whereas at higher energy levels the monofluoride is accompanied by increasing amounts of much lower fluorides. The molybdenum diffraction pattern found for the MoF1.0 composition probably arises from a slight admixture with a lower fluoride, since a several per cent component is readily detectible by X-ray diffraction, and this is within our standard analytical deviation for combined elemental analyses. Note that the highest fluoride formed in the tungsten reactions was WF0.aa, and the fluorine content of the fluoride increases somewhat with increasing level of the imparted energy. This behavior is completely different from that of molybdenum. It must be concluded that tungsten is considerably less efficacious than molybdenum with regard to the abstraction of fluorine from PFs. This is probably attributable to striking differences in the volatilities of these metals; the melting point of molybdenum is 760°K less than that of tungsten[14]. The lower volatility of tungsten tends to increase the speed of condensation of explosively vaporized and liquified metal, and one might predict that molybdenum would be in an active state for longer periods than tungsten. It was shown in [9] that explosively liquified metal droplets react readily to form carbides under appropriate conditions. Since endothermic reactions in which the metal participates chemically are very inefficient in explosion reactions [6], one might predict that differences between the chemistry of molybdenum and tungsten would be more readily observed in endothermic processes, such as lower fluoride formation, than exothermic ones, such as the reaction of these exploding metals with SFe to form hexafluorides. The frequent variability of n in experiments carded out under nominally identical conditions requires discussion. We believe that these variations arise from non-uniform diameters in the metals to be exploded. Regions of smaller diameter tend to develop hot spots that vaporize preferentially to regions of wider diameter. It is currently believed that explosions of real wires occur via a series of constrictions or "pinches". If the wire is of very small diameter and nearly uniform, nodes between constrictions are numerous and close together, 14. D. R. Stull aad G. C. Sinke, Thermodynamic Properties of the Elements. Am. Chem. Soc., Wash. D.C. (1956).
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leading to instantaneous vaporization of the metal. Otherwise, explosion occurs mainly at vaporized sites at constrictions, carrying along liquid droplets formed at wider sections. The ideal explosion conditions are not compatible with our objective of synthesizing large amounts of transition metal compounds, which requires the use of long strands of wires of considerable diameters. It is particularly difficult to obtain uniform wires of highly refractory metals. One should thus expect random variations in the proportions of vaporized or liquified metal, and perhaps random variations in the size of liquid droplets. These variations could easily affect the efficacy of abstraction of fluorine from the gaseous reactant. However, as our data show, product fluctuations are more pronounced in the inefficient endothermic processes involving PFs, than in the exothermic reactions of SFr. The fluctuations in yield of metal hexafluoride are small by comparison to fluctuations in fluorine content of lower fluorides. Application to other metals The preparation of volatile transition metal hexafluorides directly from the metal with relatively inert fluorinating agents should be possible for metals other than the VI-A elements described in the present paper. Most of the Group VII-A and VIII metals of the second and third transition series form such compounds. HoWever one must consider the thermal stability of the hexafluorides, because we[8] have shown previously that thermally labile compounds do not form by the exploding metal technique. Among the hexafluorides of the relevant metals, only those of technetium, rhenium, osmium and iridium can be classified as reasonably stable; it is known that PtF6 is thermally quite unstable and that RuF6 and RhF~ are even less stable [15]. Since it has already been shown that PtFn does not form by platinum explosions in SF816], we predict that the Ru and Rh hexafluorides also cannot be formed by this technique, and that it is unlikely that one could prepare a comparable fluoride of palladium in this manner. However it does seem likely that the hexafluorides of Tc, Re, Os and Ir could be prepared by metal explosions. In the case of rhenium, ReF6 can be converted to a volatile heptafluoride by heating the hexafluoride in fluorine at high temperatures [ 16]. It would be interesting to determine whether the exploding metal technique can be used at higher energy levels for the direct synthesis of the heptafluoride, to the exclusion of the hexafluoride. Even those metals that form thermally labile hexafluorides should react readily by the metal explosion technique to form lower fluorides, perhaps more effectively with SF6 than PFs. Acknowledgements-The authors are indebted to Miss Cassandra Johnson for the elemental and gas chromatographic analyses and to Mr. E. S. Elliott for obtaining the X-ray diffraction data. 15. J. H. Canterford, R. Colton and T. A. O'Donneli, Rev. Pure appl. Chem. 17,123 (1967). 16. J. G. Maim and H. Selig. J. inorg, nucl. Chem. 20, 189 (1961).