Hydrogen Phosphides

The analysis of this compound shows that it is hydrogen hemiphosphide, (PH₂)_x, and the molecular weight corresponds to tetrahydrogen diphosphide, P₂H₄.

This liquid, which is the cause of the spontaneous inflammability of phosphine prepared by methods (a), (b), (c), was isolated by passing through a freezing mixture the gas resulting from the action of water upon calcium phosphide. The condensate was a mobile liquid having a density of about 1.01 and vaporising between 30° and 40° C. The boiling-point at 735 mm. was found to be 57° to 58° C., and on careful distillation there is no residue. The vapour density (probably of the products of decomposition) is 74.73 to 77.0 and the empirical formula is PH₂; hence, on the analogy of hydrazine the constitutional formula has been written as H₂=P-P=H₂.

The compound is unstable and decomposes when heated; when kept it changes slowly in the dark and quickly in the light into the gaseous and solid hydrides. Since 100 parts by weight give 38 parts of the solid and 62 parts of the gaseous hydride this decomposition may be represented by the equation

5PH₂ = 3PH₃ + P₂H

Preparation

This is best carried out in a Woulfe bottle having a capacity of about 3 litres and furnished with three necks, through one of which hydrogen is admitted. Through another pieces of calcium phosphide are introduced into water, and the third is fitted with the exit tube which is cooled by a reflux condenser and connected with a spiral tube terminating in a wash-bottle, the whole of which is immersed in a freezing-mixture. Uncondensed gas is ignited in a draught. The bottle is warmed to about 60° C. in a water-bath and the calcium phosphide added as required. 100 grams of Ca₃P₂ give 3 to 4 c.c. of liquid phosphide.

A yellow solid is formed during the decomposition of potassium phosphide by water, and this solid is formed in many other reactions which yield the other hydrides, possibly as a secondary product resulting from the decomposition of the hemiphosphide by water. It is also formed by the exposure of phosphine to light, and is one of the products of the action of chlorine upon phosphine.

Preparation

The mixed hydrogen phosphides generated by the action of water on calcium phosphide, in a 3-litre flask through which is passed a current of carbon dioxide, are passed through a condenser to remove most of the water vapour and then through a series of wide glass tubes packed with granular calcium chloride (which must leave no residue when dissolved in hydrochloric acid). The tubes are protected from the light by rolls of paper. After the whole apparatus has been filled with carbon dioxide, the calcium phosphide is introduced into the flask in amount sufficient to give a layer about 1 cm. deep. A steady stream of phosphine is evolved at 60° C., the spontaneously inflammable part of which is immediately decomposed by the calcium chloride with deposition of the solid hydrogen diphosphide. The escaping gas is led directly into the flue and burnt. The solid spreads through the tubes and appears in the final wash-bottle, which contains hydrochloric acid. The apparatus is then again filled with CO₂ and the last calcium chloride tube detached and used as the first of the next repeat operation. The contents of the calcium chloride tubes are dissolved in cold dilute hydrochloric acid. The hydrogen phosphide, which usually floats, is filtered off, washed with ice-cold water, then with alcohol and with ether which has been redistilled over sodium. This dissolves a little of the compound to give a yellowish colloidal solution. The solid is filtered off, centrifuged, dried for 12 hours in a vacuum desiccator over P₂O₅, and kept in a closed vessel of brown glass in a desiccator.

Properties

The diphosphide is a yellow powder, which is tasteless, odourless and insoluble in most solvents. It decomposes in the light giving phosphine, and is easily decomposed by heating in an atmosphere of hydrogen. It ignites in air at temperatures between 100° and 200° C.

The empirical formula is (P₂H)x. The molar weight, deduced from the lowering of the freezing-point of white phosphorus, corresponds to the formula P₁₂H₆. The density is 1.83 at 19° C. The heat of formation from white phosphorus and hydrogen is given as +35.4 Cals.

Chemical Reactions

P₁₂H₆ is dissociated when heated above 70° C. in an indifferent gas—into its elements at 175° C. in CO₂, or into phosphorus and phosphine at 215° C. The ignition temperature in air is 120° to 150° C. The only liquids which dissolve it without decomposition are phosphorus and P₂H₄. It dissolves in ammonia at -40° C. with evolution of phosphine. After evaporation of the ammonia a black solid is left which appears to be an ammine of a higher hydrogen phosphide. Like the other phosphides it is easily oxidised by halogens, chlorates and nitric acid.

The hydrogen of this phosphide appears to have a slight acidic character, since the phosphide dissolves in alcoholic alkalies giving deep red solutions which contain polyphosphides, similar to those which are formed by the action of alcoholic alkali on finely divided scarlet phosphorus. These compounds are easily hydrolysed by dilution, or by the addition of acids, with the precipitation of a yellow or reddish mixture of solid hydrogen phosphide and scarlet phosphorus (which possibly contains a suboxide or P₄H.OH).

When the diphosphide is gradually heated to 175° C. in vacuo, or when its ammoniate is heated, a reddish powder is formed to which the composition P₉H₂ has been assigned:

5P₁₂H₆ = 6P₉H₂ + 6PH₃

This compound is fairly stable in dry air, but in moist air is converted into phosphine and phosphoric acid. It dissolves in liquid ammonia. A phosphide P₅H₂ has also been reported.

Some hydroxyphosphides are formed also by oxidation and hydrolysis of a mixture of phosphorus and phosphorus trichloride, or by the hydrolysis of tetraphosphorus diiodide, P₄I₂, when this is boiled with water. The solid to which the composition P₄H.OH has been assigned dissolves in alcoholic potash giving hydrogen and the potassium salt, P₄H.OK.