Diborane, B2H6

Diborane, B₂H₆

The simplest boron hydride known, is diborane. It is prepared by treating boron trifluoride with LiAlH₄ in diethyl ether.

4BF₃   +   3LiAlH₄    →   2B₂H₆   +   3LiF   +   3AlF₃

A convenient laboratory method for the preparation of diborane involves the oxidation of sodium borohydride with iodine.

2NaBH₄   +   I₂    →   B₂H₆   +   2NaI   +   H₂

Diborane is produced on an industrial scale by the reaction of BF3 with sodium hydride.

2BF₃   +   6NaH   +  (Heat, 450 K)    →    B₂H₆   +   6NaF

Diborane is a colorless, highly toxic gas with a b.p. of 180 K. Diborane catches fire spontaneously upon exposure to air. It burns in oxygen releasing an enormous amount of energy.

B₂H₆   +   3O₂    →    B₂O₃   +   3H₂O;              △_cH^-  =   -1976 kJ mol^-1

Most of the higher boranes are also spontaneously flammable in air. Boranes are readily hydrolysed by water to give boric acid.

B₂H₆ (g)   +   6H₂O (I)   →    2B(OH)₃ (aq)   +   6H₂ (g)

Diborane undergoes cleavage reactions with Lewis based (L) to give borane adducts, BH₃.L

B₂H₆   +   2NMe₃    →   2BH₃.NMe₃

B₂H₆   +   2CO   →    2BH₃.CO

Reaction of ammonia with diborane gives initially B₂H₆.2NH₃ which is formulated as [BH₂(NH₃)₂ ]⁺ [BH₄]^-; further heating gives borazine, B₃N₃H₆ known as “inorganic benzene” in view of its ring structure with alternate BH and NH group.

3B₂H₆  +  6NH₃   →   3[BH₂(NH₃)₂ ]⁺ [BH₄]^-  (Heat)   →   2B₃N₃H₆   +   12H₂

The structure of diborane is shown in below figure.

The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four terminal B-H bonds are regular two centre-two electron bonds while the two bridge (B-H-B) bonds are different and can be described in terms of three centre-two electron bonds shown in below figure.

Boron also forms a series of hydridoborates; the most important one is the tetrahedral (BH₄)^- ion. Tetrahydridoborates of several metals are known. Lithium and sodium tetra-hydridoborates, also known as borohydrides, are prepared by the reaction of metal hydrides with B₂H₆ in diethyl ether.

2MH   +   B₂H₆    →   2M⁺ [BH₄]^-            (M= Li or Na)

Both LiBH₄ and NaBH₄ are used as reducing agents in organic synthesis. They are useful starting materials for preparing other metal borohydrides.