Modern inorganic chemistry

140 THE ELEMENTS OF GROUP III

thallium. Unipositive compounds of aluminium, gallium and indium

(unlike those of thallium which are stabilised because of insolubility)

disproportionate in water:

3M+ -^M3+ 4-2M

The tendency of elements of higher atomic number to retain the s

electrons as an inert pair is also encountered in Group IV, and in

this case it is found that for lead the most stable oxidation state is

•f 2, achieved by loss of two p electrons.

COORDINATION NUMBER

Boron achieves a covalency of three by sharing its three outer
electrons, for example BF 3 (p. 153). By accepting an electron pair
from a donor molecule or ion, boron can achieve a noble gas con-
figuration whilst increasing its covalency to four, for example
H 3 N—>BC1 3. K^BF^. This is the maximum for boron and the
second quantum level is now complete; these 4-coordinate species
are tetrahedral (p. 38).
Aluminium also has a strong tendency to achieve a noble gas
configuration by electron pair acceptance as shown in dimeric
aluminium chloride,

Cl Cl Cl

\ / \ /

Al Al

Cl^i \ a / L,I ^^Cl

in the adduct H 3 N-»A1C1 3. and in Li^AlH^. in a similar manner
to boron. In the case of aluminium, however, the third quantum
level is not full since there are unfilled 3d orbitals available, and
aluminium is able to coordinate up to a maximum of six ligands
(molecules or ions) depending upon their size and shape, for example
[A1F 6 ]^3 -. [A1(OH) 6 ]^3 -. [A1(H 2 O) 6 ]3+. The metal-ligand bonding
in these complexes may be partly ionic and partly covalent in nature.
Gallium, indium and thallium resemble aluminium and form
compounds with 3, 4 and 6 ligands. The increase in coordination
number, maximum between the first and second elements in a group,
is characteristic of Groups III to VII: but the maximum coordina-
tion (6) of the second element, in purely inorganic compounds, is
usually only seen with ligands that are small and electronegative,
for example H 2 O, F~, OH~. Thus, owing to its greater size, there
are no corresponding stable compounds with the chloride ion, e.g.
aluminium forms [A1C1 4 ]~ but not [A1C1 6 ]^3 ~.