Pentafluorides

Apart from that of the recently synthesized gold pentafluoride (2), which was reported to be amorphous, the structures of all the transition metal pentafluorides are known. For all the compounds, cell dimensions have been derived, either from X-ray powder photographs or from single-crystal data. For seven of them structural analyses, at various levels of accuracy, have been performed on single-crystal data, and the remainder have unit-cell dimensions closely similar to a compound of known structure (see Table V). 

Although the third structure type was established for ruthenium pentafluoride (88), the results were based on restricted single-crystal data, giving less accurate molecular dimensions. Full data were used for the structure analysis of the osmium compound (89 ) but the results were still unsatisfactory, because of the predominant scattering by the 

Compound Space group (symmetries) Measurement temperature (°C) a (A) b (A) c (A) Bond distances (A) Transition temperature (°C) Formula unit volume (A3) Reference 

The V—F—V angle of 150° in the vanadium pentafluoride structure lies between those for the niobium and rhodium compounds. Thus, the structure cannot be described in terms of either of the close-packed arrangements. Although in the structure some approach toward close-packed planes of fluorine atoms can be seen, a comparison of volume per formula unit for the pentafluorides shows that for the second and 

If allowance is made for these anomalies, the volume per formula unit tends to decrease along each series, as would be expected for the decrease in size of the metal atom (see Table IV). The small variations, for example, for the tantalum and tungsten compounds, may reflect inaccuracies in some of the reported unit-cell dimensions. 

Submitted by R. T. PAINE and L. B. ASPREVt Checked by L. GRAHAM and N. BARTLETT  

The syntheses described here involve simple one-electron reduction reactions of metal hexafluorides. The procedures have been found to be conveniently applicable to the preparation of MoFs,s ReFS)6 OsFs,6 and UFS.7 High-purity hexafluorides can be obtained relatively easily by known methods,2 and the reducing agents are ordinary reagents. 

It should be pointed out that the procedures and conditions described here do not result in the reduction of SF6 or WF6. 

In a typical experiment the metal vacuum line and connections to the hexafluoride and HF storage tubes are pumped to at least KT4 torr over several hours. A 30-mL Kel-F reaction tube9 fitted with a metal valve closure is charged with silicon powder and a Teflon-covered stirring bar. The reaction tube is attached to the vacuum system and evacuated. 

The pentafluoride products are easily recovered by vacuum evaporation of the HF, SiF4, and excess MF6. These volatile substances are passed through soda-lime and charcoal traps to scrub the HF and MF6. For each hexafluoride a 

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Phosphorus pentafluoride PF will readily accept an electron pair from a fluoride ion F to form the stable hexafluorophosphate(V) anion PF C. This ion is isoelectronic with SF. and neither SF nor PF show any notable tendency to accept further electron pairs, though there is some evidence for the existence of an SF ion. 

Arsenic forms only the pentafluoride AsFj, a colourless liquid, b.p. 326 K. This resembles phosphorus pentafluoride. 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

In eontrast, dialkylhalonium salts sueh as dimethylbromonium and dimethyliodonium fluoroantimonate, whieh we prepared from excess alkyl halides with antimony pentafluoride or fluoroantimonie acid and isolated as stable salts (the less-stable chloronium salts were obtained only in solution), are very effective alkylating agents for heteroatom eompounds (Nu = R2O, R2S, R3N, R3P, ete.) and for C-alkylation (arenes, alkenes). 

Iodine Acetaldehyde, acetylene, aluminum, ammonia (aqueous or anhydrous), antimony, bromine pentafluoride, carbides, cesium oxide, chlorine, ethanol, fluorine, formamide, lithium, magnesium, phosphorus, pyridine, silver azide, sulfur trioxide... 

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