Modern inorganic chemistry

146 THE ELEMENTS OF GROUP III

Borane, BH 3

Borane does not exist as such, but a donor molecule can break up

diborane and form an adduct, thus :

B 2 H 6 + 2N(CH 3 ) 3 -» 2(CH 3 ) 3 N -» BH 3

trimethylamine

In this case the covalency of boron is brought up to four because

the donor molecule supplies the necessary electrons. The adduct

formed, trimethylamine-borane, is a stable white solid. Other

compounds of a similar kind are known, all derived from the simple

structure H 3 N -> BH 3. This compound is isoelectronic with ethane,

i.e. it contains the same number of electrons and has the same shape :

H H HH

H:N-*B:H H:C:C:H

H H H H

ammonia-borine ethane

There are similar analogues to other aliphatic hydrocarbons, for

example H 2 N -> BH 2 , which is isoelectronic with ethene, and a most

interesting compound called borazine, B 3 N 3 H 6 , which possesses

physical properties remarkably like those of the aromatic analogue

benzene, C 6 H 6. Borazine has, in fact, a ring structure like benzene :

N C

H H

borazine benzene

There is the possibility of building up an extensive systematic
chemistry of compounds containing boron-nitrogen bonds, analog-
ous to the chemistry of carbon-carbon bonds ; but the reactivity
of the B— N bond is much greater than that of the C — C bond, so
that we get physical but not chemical, resemblances between
analogous compounds.
There is one other important way in which borane can be stabil-
ised. Diborane reacts with a suspension of lithium hydride in dry
ether thus

2L1H + B 2 H 6 -> 2LiBH 4

lithium tetrahydridoborate

(borohydride)