Modern inorganic chemistry

42 STRUCTURE AND BONDING

( /

Q — B— OC

ChU CH 3 / \ CH 3

Cl CL CH 3

This compound, which contains atoms arranged tetrahedrally

around the boron atom, can readily be isolated from a mixture of

dimethyl ether and boron trichloride. On occasions a chlorine atom,

in spite of its high electron affinity, will donate an electron pair, an

example being found in the dimerisation of gaseous monomeric

aluminium chloride to give the more stable A1 2 C1 6 in which each

aluminium has a tetrahedral configuration:

CL Cl Cl CL Cl

2

Y - XX

Cl CL Cl Cl

In Group III, boron, having no available d orbitals, is unable to fill
its outer quantum level above eight and hence has a maximum
covalency of 4. Other Group III elements, however, are able to form
more than four covalent bonds, the number depending partly on
the nature of the attached atoms or groups.
The ability to act as a lone pair acceptor is not confined to
Group III, and can occur wherever a quantum level is incomplete.
This ability to accept electrons explains why covalent chlorides,
with the exception of carbon tetrachloride, are readily hydrolysed,
the apparently anomalous behaviour of carbon tetrachloride being
readily explained by the fact that the carbon has a completed
quantum level and is unable to form an Intermediate complex' with
water.

'COVALENT IONS'

Covalent bonding, in all the cases so far quoted, produces molecules
not ions, and enables us to explain the inability of the compounds
formed to conduct electricity. Covalently bonded groups of atoms
can, however, also be ions. When ammonia and hydrogen chloride
are brought together in the gaseous state proton transfer occurs as
follows: