Antimony Pentafluoride, SbF₅

Antimony Pentafluoride, SbF₅, was first reported by Berzelius in 1824, who obtained it by the action of hydrofluoric acid on antimonic acid; for some time the existence of this compound was denied, until in 1867 Marignac confirmed the results of Berzelius, including the preparation of a number of double compounds. In 1891 Moissan prepared the compound by direct union of the elements.

Antimony pentafluoride is most conveniently prepared either by the method of Berzelius or by the action of anhydrous hydrogen fluoride upon antimony pentachloride. The latter method is carried out by heating a mixture of hydrogen fluoride and antimony pentachloride at 25° to 30° C. until no more hydrochloric acid is evolved; after a considerable time the boiling point of the mixture rises to 150° to 155° C., at which temperature antimony pentafluoride distils over.

The pentafluoride is a colourless, thick, oily liquid which boils at 149° to 150° C. Its density at 22.7° C. is 2.993. It is soluble in water and is hygroscopic. It has a drastic action upon the skin.

The dihydrate, SbF₅.2H₂O, has also been prepared.

Chlorine appears to have no action upon antimony pentafluoride; bromine reacts to form a viscid, dark brown mass of indefinite composition, which may contain the compound SbF₅Br. Iodine forms two compounds. With excess of antimony pentafluoride at 160° to 220° C. a dark, bluish-green substance, (SbF₅)₂I, is formed, melting at 110° to 115° C. It does not lose iodine when heated to 240° C., but it is readily decomposed by water. When antimony pentafluoride is heated with excess of iodine at a temperature above the boiling point of iodine, a dark brown compound, SbF₅I, is formed, melting at 80° C. This compound is decomposed when heated above 260° C. with evolution of iodine, but it is not so readily decomposed by water as the former compound.

Sulphur dissolves in antimony pentafluoride to form a dark blue solution from which the compound SbF₅S can be separated. This melts at 230° C., is very hygroscopic, and is decomposed by water and moist air. The decomposition by water is probably represented by the equations:

2SbF₅S + H₂O = 2SbF₃ + S + SOF₂ + 2HF
SOF₂ + H₂O = SO₂ + 2HF

Antimony pentafluoride is decomposed when a current of hydrogen sulphide is passed over it, the products being sulphur, hydrogen fluoride and antimony thio-fluoride. An aqueous solution reacts with hydrogen sulphide only when it is warmed.

The pentafluoride reacts with nitrogen sulphide, sulphur dichloride and chromyl chloride; also with molybdenum pentachloride and tungsten hexachloride forming respectively a molybdenum fluoride and tungsten hexafluoride, together with double compounds with antimony pentafluoride.

When dry ammonia is passed over the pentafluoride the latter becomes coated with a yellowish-red crust of indefinite composition; the action is vigorous until the protective coat is formed, which then prevents further action. When heated with liquid ammonia at 100° C. in a platinum tube a white powder is formed which is probably the complex substance NH(SbF₃.NH₂.HF)₂. It is readily decomposed by moist air; its solution is acid towards litmus and it is slowly acted upon by water with the formation of antimonic acid.

Antimony pentafluoride reacts with phosphorus forming a yellow vapour, with phosphorus trichloride forming phosphorus trifluoride, and with phosphorus pentoxide forming phosphorus oxyfluoride.

With arsenic trifluoride a series of crystalline compounds is formed; and with antimony trifluoride, a series of compounds ranging from SbF₅.2SbF₃ to SbF₅.5SbF₃. These may be prepared by distilling a mixture of antimony pentafluoride and antimony trifluoride. The compound SbF₅.2SbF₃ is a colourless, transparent, crystalline substance; its density at 21° C. is 4.188; it boils at 390° C. and it is hygroscopic. The compound SbF₅.5SbF₃ boils at about 384° C.

Antimony pentafluoride reacts with the tetrachlorides of tin, titanium and silicon with evolution of hydrogen chloride in each case. When warmed with colloidal silicic acid, antimonic acid and silicon tetrafluoride are formed.

Many carbonaceous materials are attacked by the pentafluoride, including filter-paper, cork, wood, india-rubber, benzene, ether, alcohol, acetone, glacial acetic acid, ethyl acetate, carbon disulphide, light petroleum and chloroform. With chloroform an easily liquefiable gas (probably CCl₃F) is formed.

Antimony pentafluoride when dry does not react with the majority of metals. When heated with sodium a violent reaction occurs and a white vapour is formed. It is reduced to the trifluoride when heated with powdered antimony.

The pentafluoride can be converted quantitatively into sodium hydrogen pyroantimonate by the addition of sodium hydroxide or sodium carbonate.

A double compound of antimony pentafluoride and antimony pentachloride, 2SbF₅.SbCl₅, is obtained by the action of chlorine on antimony trifluoride. Several other double compounds of these two pentahalides are indicated by the results of an investigation of the freezing point curves of mixtures of the two. Four of these compounds have been isolated: 2SbF₅.SbCl₅(D₄²⁰ 3.08), SbF₅.SbCl₅, 2SbF₅.3SbCl₅ (D₄²⁰ 2.79), and SbF₅.3SbCl₅(D₄²⁰ 2.73); two other compounds, 3SbF₅.SbCl₅ and SbF₅.2SbCl₅, are also believed to exist. They are all decomposed on fusion. Molecular weight determinations have been made in solutions in sulphuryl chloride: the compounds 2SbF₅.3SbCl₅ and SbF₅.3SbCl₅ give values which are only about one-third of the theoretical molecular weight, while the compound 2SbF₅.SbCl₅ does not appear to dissociate in 10 per cent, or more concentrated solution. The molecular volumes of some of these compounds indicate that there is a considerable decrease in volume when the compounds are formed from their constituents. Chemically the compounds closely resemble corresponding mixtures of the two binary compounds. Bromine reacts with all of them. When the compound 2SbF₅.SbCl₅ is mixed with nitrosyl fluoride at -80° C., and the mixture allowed to warm up, the compound SbF₅.NOF is obtained as slender, colourless, needle-shaped crystals. This compound may also be obtained by the action of nitrosyl fluoride on antimony pentafluoride, and by the action of antimony pentafluoride on the corresponding arsenic compound, AsF₅.NOF. The compound SbF₅.NOF sublimes below red heat without decomposition; it is very hygroscopic and is decomposed by water and by alcohol. It reacts with potassium fluoride with the formation of a double compound of potassium fluoride and antimony pentafluoride and separation of nitrosyl fluoride.

Antimony pentafluoride forms a number of double compounds with alkali chlorides and fluorides of the general type SbF₅.MF and SbF₅.2MF. These may be prepared by the addition of alkali hydroxide to an acid solution of antimony pentafluoride, or by the solution of alkali antimonate in hydrofluoric acid. They are all deliquescent and soluble in water, but are stable when dry. Their solutions in water evolve hydrogen fluoride, and on evaporation yield oxyfluorides. The solutions will react with hydrogen sulphide only after prolonged treatment.

Certain compounds of antimony pentafluoride with organic bases have also been obtained.