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Antimony Pentachloride, SbCl5

Antimony Pentachloride, SbCl5 was first obtained by Rose by the direct union of the elements, powdered antimony combining spontaneously with chlorine, with incandescence. No antimony trichloride is formed. It is usually prepared by saturating molten antimony trichloride with chlorine, and distilling under reduced pressure.

Antimony pentachloride is a colourless or faintly yellow liquid, possessing a sharp odour; it fumes strongly in air. It melts at 3° C., and yields, at lower temperatures, needle-shaped crystals. Between 15° and 68° C. the density is stated to be given by

Dt4 = 2.392 – 0.00204t

Under reduced pressure it can be boiled without decomposition, the boiling point at different pressures being indicated in the following table:

Pressure (mm.).142268 (°C.)6879102-103

The dipole moment, calculated from values obtained for the dielectric constant, is 1.14×10-18 e.s.u. (In connection with this the molecular structure has been discussed.)

The Raman effect has been investigated, both for antimony pentachloride and for chlorantimonic acid HSbCl6.

Investigation of the boiling points of solutions of antimony pentachloride in carbon tetrachloride and in chloroform indicates that some dissociation occurs in dilute solutions.

The chemiluminescence produced when antimony powder is dropped into chlorine has been investigated. Two continuous bands were observed, at 6250-5200 A. (max. 5950 A.) and 4950-3930 A. (max. 4650 A.). No antimony lines could be detected.

The behaviour of antimony pentachloride as a solvent has been investigated. Chlorine, bromine, stannic chloride, chromyl chloride and chromic acid dissolve in normal fashion; iodine trichloride and gold trichloride undergo dissociation, and iodine, stannic bromide and stannic iodide react with the solvent.

The heat of formation of antimony pentachloride is as follows:

Sb (solid) + 5Cl (gas) = SbCl5 (liquid) + 104,900 gram-calories
SbCl3 (solid) + 2Cl (gas) = SbCl5 (liquid) + 13,800 gram-calories

By exposure to moist air, or by the addition of the required amount of water, a crystalline monohydrate, SbCl5.H2O, is obtained; this hydrate may be purified by recrystallisation from chloroform. Its melting point lies between 87° and 92° C.; under reduced pressure it can be distilled with decomposition, the pentachloride being evolved at first, followed by a little trichloride; a non-volatile residue is left. When heated under atmospheric pressure chlorine is evolved. The hydrate is decomposed by water and by a solution of sodium carbonate. When heated with chloroform or carbon tetrachloride, phosgene is formed.

A tetrahydrate, SbCl5.4H2O, has been obtained either by evaporating a saturated aqueous solution of the pentachloride over concentrated sulphuric acid, or by precipitation from a solution in chloroform.

With excess of water hydrolysis of the pentachloride occurs, with formation of antimonic and hydrochloric acids; this hydrolysis can be prevented by the addition of concentrated hydrochloric acid, or of a solution of tartaric acid; in the former case, however, chlorantimonic acid is formed.

Antimony pentachloride dissociates on heating, forming the trichloride and chlorine, the dissociation being slow at 120° C. but complete just above 300° C. Between 120° and 260° C. the dissociation constant is given by

log K = 9.74 – 3570/T

The heat of the reaction is given as 16,320 gram-calories.

With iodine, three main reactions can take place, all of which are probably complex. When less than 1.5 per cent, of iodine is dissolved in antimony pentachloride the reaction may provisionally be represented by

SbCl5 + 2I = SbCl3 + 2ICl

but iodine trichloride, antimony triiodide and a chloroiodide of quinquevalent antimony may also be formed. There is, however, no evidence for the formation of the compound SbCl5I. Two double compounds, SbCl5.2ICl and SbCl5.3ICl, may also be obtained according to the equations

2SbCl5 + 2I = SbCl5.2ICl + SbCl3
3SbCl5 + 4I = SbCl5.3ICl + 2SbCl3 + ICl

These reactions may be carried out by heating the equivalent proportions of antimony pentachloride and iodine at a temperature of 30° to 35° C. under a pressure of 15 mm. The existence of these double compounds has been confirmed by thermal examination.

When dry hydrogen sulphide acts upon antimony pentachloride a white crystalline compound, SbSCl3, is formed. It has a low melting point and is stable in dry air; it is decomposed by water into antimony oxychloride and sulphur, and by strongly heating into antimony trichloride and sulphur. It is attacked slowly by concentrated sulphuric acid with evolution of hydrogen chloride; it reacts also with chlor-sulphonic acid.

Antimony pentachloride will react with sulphur monochloride, forming the compound SbCl5.SCl4. A mixture of equal volumes of antimony pentachloride and sulphur monochloride is dissolved in sulphuryl chloride and the solution dried over phosphorus pentoxide. The equation is

5SbCl5 + S2Cl2 = 2(SbCl5.SCl4) + 3SbCl3

The compound is obtained in the form of yellow-amber cubes with octahedral facets; it fumes strongly on exposure to air, becoming liquid with separation of sulphur. It melts at 157° to 163° C. It has no salt-like properties and is non-polar. A similar compound has been prepared by the action of sulphur tetrachloride on antimony pentachloride, and by chlorine on antimony trisulphide.

The corresponding compounds with selenium tetrachloride, SbCl5.SeCl4, and with tellurium tetrachloride, SbCl5.TeCl4, have been obtained by the action of dry chlorine upon a fused mixture of antimony and selenium, or antimony and tellurium. The compound SbCl5.SeOCl2 has also been described.

Several addition compounds are formed with ammonia, including a red triammoniate, SbCl5.3NH3, a white, volatile tetrammoniate, SbCl5.4NH3, and a hexammoniate, SbCl5.6NH3. The first two of these ammoniates react with hydrochloric acid to form the corresponding ammonium chloride compound. The compound SbCl5.NH4Cl may be obtained by heating together equivalent proportions of the binary compounds in a sealed tube. The monohydrate, SbCl5.NH4Cl.H2O, has also been obtained. It crystallises in the rhombic system, and may be regarded as ammonium metachlorantimonate. Its crystal elements are: a:b:c = 0.8909:1:0.7748.

Antimony pentachloride reacts with nitric oxide and with nitrogen tetroxide forming the yellow, crystalline compounds 2SbCl5.NO and 3SbCl5.2NO2 respectively. In each case the reactions are accompanied by considerable evolution of heat.

With nitrosyl chloride, the compounds SbCl5.NOCl and 2SbCl5.5NOCl have been obtained.

Phosphine reacts with antimony pentachloride, hydrogen chloride being evolved according to the equation

PH3 + 4SbCl5 = 4SbCl3 + PCl5 + 3HCl

A mixture of the pentachloride and phosphonium iodide explodes violently in air, but when heated in a sealed tube a complex reaction takes place which may be represented by the equation

3SbCl5 + 3PH4I = Sbl3 + 2SbCl3 + 9HClPH3 + P2

Antimony pentachloride combines directly with phosphorus pentachloride, and with phosphorus oxychloride, forming the double compounds SbCl5.PCl5 and SbCl5.POCl3 respectively.

With carbon disulphide a vigorous reaction takes place, which may be represented by the equation

2SbCl5 + CS2 = CCl4 + 2SbCl3 + 2S

If, however, the mixture is kept cold, the compound SbSCl3 is obtained. Antimony pentachloride is reduced to the trichloride by the action of silicomethane.

When antimony pentachloride is heated with germanium tetraiodide a reaction occurs which results in the formation of antimony triiodide and the liberation of iodine:

4SbCl5 + 5GeI4 = 4SbI3 + 4I2 + 5GeCl,

Thermal investigation of the binary system TiCl4 - SbCl5 indicates the formation of a eutectic containing approximately 40 per cent, antimony pentachloride ( -50° C.).

Gold chloride will dissolve in antimony pentachloride, but no compound appears to be formed.

The parachor for antimony pentachloride is 26 units below the sum of the atomic constants; it is assumed, therefore, that there are two singlet linkages, and the formula,

is analogous to that ascribed to phosphorus pentachloride. In this formula each bond indicates a shared electron, while the superscript figures give the number of unshared electrons. In certain organic compounds antimony pentachloride is thought to exhibit an asymmetrical configuration.

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