Boron Hydrides

The existence of solid Boron Hydrides or Hydroboronshas been suspected by various chemists. The existence of a volatile hydride was long ago anticipated from analogy with the compounds of other non-metallic elements; but neither Wohler and Deville nor Gustavson succeeded in preparing it. In 1879, F. Jones showed that a volatile boron hydride did exist, and two years later, F. Jones and Taylor studied the hydride and assigned to it the formula BH₃. Their work was confirmed in a qualitative way by Sabatier. Later, Bamsay and Hatfield announced the existence of what were, in their opinion, probably two isomeric hydrides of the formula B₃H₃, but Bamsay was unable subsequently to duplicate the work. No value can be placed upon the scanty experimental data which served to deduce the preceding formulae, since the gaseous boron hydrides were undoubtedly contaminated with silicon hydride.

The hydrides prepared by Jones and Taylor, Sabatier, and Bamsay and Hatfield were obtained, mixed with a very large excess of hydrogen, by the action of dilute acids on magnesium boride, and similar gaseous mixtures can be obtained by the action of acids upon commercial iron and manganese borides. From the mixed gases produced from magnesium boride and hydrochloric acid, Stock and Massenez in 1912 succeeded in isolating two distinct boron hydrides of molecular formulae B₄H₁₀ and B₆H₁₂; and since then Stock and others have obtained several more boron hydrides. Although the compound BH₃ is not yet known, there are, according to Stock, Friederici, and Priess, at least ten boron hydrides: -

1. B₂H₆. Obtained by heating B₄H₁₀. Colourless gas.
2. B₄H₁₀. Obtained from magnesium boride and hydrochloric acid.
3. A colourless liquid, very unstable, obtained like (1).
4. B₆H₁₂. Obtained like (2).
5. B₁₀H₁₄. Obtained like (1); also by heating (1).
6. Non-volatile solid, soluble in CS₂. Obtained like (1).
7. Non-volatile yellow solid, insoluble in CS₂. Obtained like (1), and by heating (6).
8. Difficultly volatile liquid, obtained by heating (6).
9. Colourless, non-volatile solid, insoluble in CS₂. Obtained by heating (1).
10. Brown hydride or hydrides, resembling boron and poor in hydrogen content. Obtained by heating (7).

The starting-point in the preparation of the boron hydrides is crude magnesium boride, made by mixing 1 part of very finely powdered boron sesqui-oxide with 3 parts of magnesium powder and rapidly heating the mixture, 10 grams at a time, in a thin sheet-iron crucible in a stream of hydrogen. The product, quickly cooled in hydrogen, is finely powdered, sifted, and slowly dropped into 15 per cent, hydrochloric acid at 50° to 80°, a slow stream of hydrogen being passed through the apparatus. The evolved gases, dried over calcium chloride and phosphoric anhydride, are passed through U-tubes cooled in liquid air; in them the boron hydrides B₄H₁₀ and B₆H₁₂ solidify, together with a little carbon dioxide and silicon hydrides.

The distillation of this crude product furnishes in order the following fractions: (i.) silicon hydride, SiH₄, (ii.) carbon dioxide, (iii.) silicon hydride, Si₂H₆, (iv.) boron hydride, B₄H₁₀, (v.) boron hydride, B₆H₁₂, (vi.) residue of less volatile boron and silicon hydrides. The two hydrides B₄H₁₀ and B₆H₁₂ are obtained as follows: - At the temperature of liquid air the pressure above the solid is reduced to zero. Traces of hydrogen and silicon hydride are thus removed. The temperature is then raised to - 80°, when the solid melts. The pressure is quickly lowered to a few millimetres, and the evolved gas rejected as long as it contains silicon hydride. The gas then evolved at - 80° and 3 mm. consists of the hydride B₄H₁₀, and, unless the room temperature is above 20°, partly condenses in the mercury pump. The residue is distilled at -40° until the pressure falls to 1.5 mm., the temperature raised to 0°, and distillation continued until the pressure is only 10 mm. All the hydride B₄H₁₀ has then been removed, and the residue, on further distillation, gives another hydride, B₆H₁₂, the pressure falling to less than 5 mm.

From 200 grams of magnesium boride, 100 c.c. (at N.T.P.) of pure hydride B₄H₁₀ and 60 milligrams of hydride B₆H₁₂ may be obtained.