Page 207 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

16 Main Group Elements

3
form with a diamond structure (density, 5.75 g/cm ). This D. Oxides
transformation from the shiny metallic allotrope to the
All three elements, lead, tin, and germanium, form both a
crumbly form was known in the past as “tin pest” or “tin
monoxide and a dioxide. The order of stability for monox-
disease.”
ides is GeO < SnO < PbO; that for dioxides is GeO 2 >
Because tin is rather expensive, it is rarely used alone;
SnO 2 > PbO 2 . PbO 2 functions as the positive plate of lead
however, it finds wide application in coatings, in platings,
storage batteries. A mixed oxide of lead, Pb 3 O 4 , is called
and in solders and alloys (bronze and pewter), in which it
red lead. It is used as a pigment and primer and as an
can be extended by use with a less expensive metal.
additive to leaded glass.
Lead, like tin, is a metal with which the ancient world
was familiar. The rather simple pyrometallurgy permitted
easy recovery of lead from the mineral galena, PbS. E. Organometallic Compounds
2PbS (s) + 3O 2 (g) → 2PbO (s) + 2SO 2 (g) Tetraethyllead, Pb(C 2 H 5 ) 4 , is added to gasoline to reduce
knocking; however, its use will continue to decline as a
PbO (s) + C (s) → Pb (l) + CO (g) switch is made to other, less environmentally hazardous
PbO (s) + CO (g) → Pb (l) + CO 2 (g) antiknock agents.
In terms of the number and variety of industrial ap-
Thelargestuseofleadisinstoragebatteries,fromwhich plications, organotin derivatives are the most valuable of
more than 80% is recovered and recycled. Lead is also any organometallic compounds. Polyvinyl chloride plas-
used in ethyl gasoline and in paints, but both uses are tics are readily degraded by heat and light unless stabilized
being phased out. Lead is also found in certain types of by the addition of dioctyltin compounds. Other organotin
solders and pipes. The toxicity of lead is well known, but compounds function as curing agents for another impor-
its congeners, tin and germanium, have not been shown to tant type of polymer, the silicones. Certain triorganotin
be poisonous. derivatives have been found to function as highly selec-
tive, versatile pesticides. These pesticides have additional
B. Hydrides advantages; they are safe and readily biodegradable.
Germanes, the hydrides of germanium, are somewhat sim-
ilar to silanes in their chemical and physical properties. VI. MULTIPLE-BOND COMPOUNDS
The general formula Ge n He 2n+2 (n = 1–5) recalls that of
C n H 2n+2 for alkanes and that of Si n H 2n+2 for silanes. Ger- Considerable progress in the determination of quantitative
manes are less volatile and flammable than silanes and are values for bond strengths in silicon compounds has been
somewhat less reactive toward alkali. The two known stan- realized. These thermochemical data gain additional sig-
nanes, SnH 4 and Sn 2 H 6 , are much less stable than silanes; nificance in light of the continuing success with syntheses
and the one plumbane, PbH 4 , decomposes readily. In fact, of stable silicon compounds containing multiple bonds. In
its existence is still questioned. fact, startlingly rapid advances in the double-bond chem-
istry of silicon and phosphorus, as well as other members
of Groups 14 and 15 (IVA and VA), have created a need
C. Halides
for increasingly reliable thermodynamic data to provide
Lead, tin, and germanium form two distinct series of coherence and direction for further work.
halides, MX 2 and MX 4 . The dihalides are more ionic and Only a few years ago, students were taught that sil-
have higher melting points and boiling points; the tetra- icon and germanium formed no multiple bonds to any-
halides are generally covalent with a tetrahedral geometry. thing. Textbooks emphasized that these elements, as well
SnCl 2 is a colorless, crystalline solid that dissolves in wa- as other, heavier main group elements, were simply not
ter, melts at 246 C, and boils at 623 C, whereas SnCl 4 is a able to self-link through double bonds to yield stable com-
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colorless volatile liquid that dissolves in nonpolar hydro- poundsanalogoustothoseofalkenes,theimportantfamily
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carbon solvents, melts at −33 C, and boils at only 114 C. of unsaturated hydrocarbons with C C bonds. Now the
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The stability of dihalides follows the order GeX 2 < facile synthesis of multiple bonds in heavier main group
SnX 2 < PbX 2 ; that of tetrahalides is GeX 4 > SnX 4 > elements is routinely reported, and doubtless the detailed
PbX 4 . The lead dihalide PbCl 2 is much more stable than thermochemistry will keep apace.
PbCl 4 . The latter is explosive when warmed to 105 C; One of the reasons for the lack of success in early at-
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however, PbCl 2 can be heated to 500 C, its melting point, tempts to synthesize multiple-bond compounds of heavier
and then up to 953 C, at which temperature it boils without elements can be attributed to the failure to recognize that
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decomposition. two single bonds (two σ bonds) are more stable than one