Modern inorganic chemistry

176 GROUP IV

in silicon, 226kJmol~^1 ) and catenation (the phenomenon of self-

linkage between atoms of the same element) is consequently less

marked with silicon than with carbon; the higher silanes decompose

slowly even at room temperature. Silanes are far more sensitive to

oxygen than alkanes and all the silanes are spontaneously inflam-

mable in air, for example

SiH 4 + 2O 2 -> SiO 2 + 2H 2 O

This greater reactivity of the silanes may be due to several factors,

for example, the easier approach of an oxygen molecule (which may

attach initially to the silane by use of the vacant silicon d orbitals)

and the formation of strong Si—O bonds (stronger than C—O).

Halogen derivatives of silanes can be obtained but direct halogena-

tion often occurs with explosive violence; the halogen derivatives

are usually prepared by reacting the silane at low temperature with

a carbon compound such as tetrachloromethane, in the presence of

the corresponding aluminium halide which acts as a catalyst.

Silanes are very sensitive to attack by alkalis and will even react

with water made alkaline by contact with glass; this reaction is in

marked contrast to the reactions shown by alkanes. Unlike alkanes,

silanes are found to have marked reducing properties and will reduce,

for example, potassium manganate(VII) to manganese(IV) oxide,

and iron(III) to iron(II).

In addition to the volatile silanes, silicon also forms non-volatile

hydrides with formulae (SiH 2 )x but little is known about their struc-

ture. Silicon, however, does not form unsaturated hydrides corre-

sponding to the simple alkenes.

Germanium

Germanium forms a series of hydrides of general formula GenH2n+2

which are quite similar to the corresponding silanes. Only a small

number of germanes have so far been prepared. Germanes are not

as inflammable as the corresponding silanes (the Ge—O bond is

not as strong as the Si—O bond) and they are also less reactive to-

wards alkalis, monogermane being resistant to quite concentrated

alkali.

Tin

The greater metallic nature of tin is clearly indicated here for tin

forms only one hydride, stannane, SnH 4. It is best prepared by the