Modern inorganic chemistry

THE ELEMENTS OF GROUP III 147

Here, the essential reaction is the formation of the tetrahvdridoborate

ion and again the covalency of boron is brought up to four, i.e.:

H ,A H |"H H"

2H: + B[ B -* 2 **B*

hvdridc ion / %%* \ *

H H H [H H^

i.e. 2H~ tetrahydridoborate ion

The alkali metal tetrahydridoborates are salts; those of sodium

and potassium are stable in aqueous solution, but yield hydrogen in

the presence of a catalyst. They are excellent reducing agents,

reducing for example ion(III) to iron(II). and silver ions to the

metal; their reducing power is used in organic chemistry, for example

to reduce aldehydes to alcohols. They can undergo metathetic

reactions to produce other borohydrides, for example

3LiBH 4 + AlCl 3 e- A1(BH 4 ) 3 + 3LiCl

aluminium

tetrahydridoborate

Aluminium tetrahydridoborate is a volatile liquid. It is the most

volatile aluminium compound known. It is covalent and does not

contain ions but has a 'hydrogen-bridge' structure like that of

diborane, i.e. each boron atom is attached to the aluminium by two

hydrogen bridges:

H H

\/

/A \

/ \

H\ H /

\ /

H --- Al' --- H

H H

Other boron hydrides are known, most of them having the general
formula BwHw + 4 , for example pentaborane, B 5 H 9 , decaborane,
B 10 H 14. Each can be made by heating diborane in suitable condi-
tions ; for example at 420 K, decaborane is obtained. Boron hydrides
have been tried as rocket fuels.

Aluminium hydride, (AIH 3 )n

When lithium hydride is allowed to react with aluminium chloride

in ether solution, two reactions occur: