Modern inorganic chemistry

THE ELEMENTS OF GROUP Ml 145

COMPOUNDS OF BORON AND ALUMINIUM

HYDRIDES

Boron forms a whole series of hydrides. The simplest of these is

diborane, B 2 H 6. It may be prepared by the reduction of boron

trichloride in ether by lithium aluminium hydride. This is a general

method for the preparation of non-metallic hydrides.

4BC1 3 + 3LiAlH 4 -* 2B 2 H 6 + 3LiCl + 3A1C1 3

Diborane has a geometric structure similar to that of dimeric alu-

minium chloride, namely

This is known as a 'hydrogen-bridge' structure. There are not
enough electrons to make all the dotted-line bonds electron-pairs
and hence it is an example of an electron-deficient compound. The
structure of diborane may be alternatively shown as drawn in
FUjure 7.2(a) and {b\

H. H.. H

B

•* H H

(a) (b)

Figure 7.2. The structure of diborane

All the available valency electrons, including those of the bridge

hydrogens, are used as shown in (a), leaving the bridge hydrogens

as protons, H+. The orbitals linking the boron atoms are not like

those in ethylene but form two banana-shaped 'clouds', as shown in

(b); and the protons are embedded in these 'clouds'. (There is no

tendency for diborane to act as an acid by losing these protons, as

they are too firmly held.) Diborane is an inflammable gas which is

immediately decomposed by water:

B 2 H 6 + 6H 2 O -» 2H 3 BO 3 4- 6H 2

boric acid