GROUP IV 163
four but, with the exception of carbon monoxide, double or triple
bonds are formed in such a way as to make the covalency of carbon
always four. The exceptional structure of carbon monoxide makes
the molecule an electron donor (pp. 178, 179). Silicon does not form
equivalent double- or triple-bonded molecules.
Silicon, germanium, tin and lead can make use of unfilled d
orbitals to expand their covalency beyond four and each of these
elements is able (but only with a few ligands) to increase its covalency
to six. Hence silicon in oxidation state +4 forms the octahedral
hexafluorosilicate complex ion [SiF 6 ]^2 ~ (but not [SiCl]^2 "). Tin
and lead in oxidation state 4-4 form the hexahydroxo complex ions,
hexahydroxostannate(IV), [Sn(OH) 6 ]^2 ~ and hexahydroxoplum-
bate(IV) respectively when excess alkali is added to an aqueous
solution containing hydrated tin(IV) and lead(IV) ions.
Carbon, however, is unable to form similar complexes since the
energy required to promote electrons to the next higher energy
level, the 3s, is too great (or since carbon has no available d orbitals
in its outer quantum level).OCCURRENCE AND EXTRACTION OF THE ELEMENTS
CARBON
Pure carbon occurs naturally in two modifications, diamond and
graphite. In both these forms the carbon atoms are linked by
covalent bonds to give giant molecules (Figure 8.2).(a) (b)
Figure 8.2, (a) Carbon symmetry—tetrahedral (sp^3 ); C C bond length 15,4 nm,
(b) Carbon symmetry trigonal planar (sp^2 ); C C bond length 14.2 nm; interplanar
distance 33.5 nm