GROUP IV 177
reaction of lithium aluminium hydride and tin(IV) chloride in ether:LiAlH 4 + SnCl 4 -> SnHJ -h LiCl| -h A1C1 3
It is a colourless gas which decomposes on heating above 420 K to
give metallic tin, often deposited as a mirror, and hydrogen. It is a
reducing agent and will reduce silver ions to silver and mercury(II)
ions to mercury. SnSn bonding is unknown in hydrides but does
exist in alkyl and aryl compounds, for example (CH 3 ) 3 Sn-Sn(CH 3 ) 3.LeadLead, like tin, forms only one hydride, plumbane. This hydride is
very unstable, dissociating into lead and hydrogen with great
rapidity. It has not been possible to analyse it rigorously or determine
any of its physical properties, but it is probably PbH 4. Although this
hydride is unstable, some of its derivatives are stable; thus, for
example, tetraethyllead, Pb(C 2 H 5 ) 4 , is one of the most stable
compounds with lead in a formal oxidation state of + 4. It is used
as an "antiknock' in petrol.
OXIDES OF GROUP IV ELEMENTS
All Group IV elements form both a monoxide, MO, and a dioxide,
MO 2. The stability of the monoxide increases with atomic weight
of the Group IV elements from silicon to lead, and lead(II) oxide,
PbO, is the most stable oxide of lead. The monoxide becomes more
basic as the atomic mass of the Group IV elements increases, but
no oxide in this Group is truly basic and even lead(II) oxide is
amphoteric. Carbon monoxide has unusual properties and empha-
sises the different properties of the group head element and its
compounds.
The dioxides are all predominantly acidic but again acidity
decreases with increasing atomic mass of the Group IV element
and lead(IV) oxide, PbO 2 , is amphoteric. The stability of the dioxides
decreases with increasing atomic mass of the Group IV elements
and although tin(IV) oxide, SnO 2 , is the most stable oxide of tin,
lead(IV) oxide is less stable than lead(II) oxide.
Oxides of carbon
Carbon monoxide, CO. Carbon monoxide is a colourless, odourless
gas. It is extremely poisonous, since the haemoglobin of the blood