Write preparation of diborane and give physical and chemical properties of it.

Preparation :

- The simplest boron hydride known, is diborane. It is prepared by treating boron trifluoride with $\mathrm{LiAlH}_{4}$ in diethyl ether.

$4 \mathrm{BF}_{3}+3 \mathrm{LiAlH}_{4} \rightarrow 2 \mathrm{~B}_{2} \mathrm{H}_{6}+3 \mathrm{LiF}+3 \mathrm{AlF}_{3}$

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

$\mathrm{I}_{2}+2 \mathrm{NaBH}_{4} \rightarrow \mathrm{B}_{2} \mathrm{H}_{6}+2 \mathrm{NaI}+\mathrm{H}_{2}$

- Diborane is produced on an industrial scale by the reaction of $\mathrm{BF}_{3}$ with sodium hydride. $2 \mathrm{BF}_{3}+6 \mathrm{NaH} \stackrel{450 \mathrm{~K}}{\longrightarrow} \mathrm{B}_{2} \mathrm{H}_{6}+6 \mathrm{NaF}$

Physical properties :

$\Rightarrow$ Diborane is a colourless, highly toxic gas with a b.p. of $180 \mathrm{~K}$.

- Diborane catches fire spontaneously upon exposure to air.

- It burns in oxygen releasing an enormous amount of energy.

Chemical properties :

- Boranes are readily hydrolyzed by water to give boric acid.

$\mathrm{B}_{2} \mathrm{H}_{6(\mathrm{~g})}+6 \mathrm{H}_{2} \mathrm{O}_{(l)} \rightarrow 2 \mathrm{~B}(\mathrm{OH})_{3(\mathrm{aq})}+6 \mathrm{H}_{2(\mathrm{~g})}$

- Diborane undergoes cleavage reactions with Lewis bases (L) to give borane adducts, $\mathrm{BH}_{3} \times \mathrm{L}$.

$\mathrm{B}_{2} \mathrm{H}_{6}+2 \mathrm{NMe}_{3} \rightarrow 2 \mathrm{BH}_{3} \cdot \mathrm{NMe}_{3} \mathrm{~B}_{2} \mathrm{H}_{6}+2 \mathrm{CO} \rightarrow 2 \mathrm{BH}_{3} \cdot \mathrm{CO}$

Reaction of ammonia with diborane gives initially $\mathrm{B}_{2} \mathrm{H}_{6} \cdot 2 \mathrm{NH}_{3}$ which is formulated as $\left[\mathrm{BH}_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\left[\mathrm{BH}_{4}\right]^{-}$; further heating gives borazine, $\mathrm{B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6}$ known as "inorganic benzene" in view of its ring structure with alternate $\mathrm{BH}$ and $\mathrm{NH}$ groups.

$3 \mathrm{~B}_{2} \mathrm{H}_{6}+6 \mathrm{NH}_{3} \rightarrow 2\left[\mathrm{BH}_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}\left[\mathrm{BH}_{4}\right]^{-} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{~B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6}+12 \mathrm{H}_{2}$