Page 220 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Main Group Elements 29

discussions regarding the bonding between these ele- gen and sulfur. Selenium ranks sixty-sixth in abundance
ments. After S 4 N 4 , the most important of these binary among elements (0.05 ppm in crustal rock); tellurium
compounds is doubtless S 2 N 2 , which is best prepared ranks a lowly seventy-second (0.001 ppm). Sulfur has ∼30
by the catalyzed (silver wool) thermal decomposition of allotropic forms; selenium has only 6 (3 red forms, each
S 4 N 4 . Like its parent, this compound is unstable with re- consisting of Se 8 rings but differing slightly in stacking
spect to the elements, decomposing explosively to the ele- patterns; 1 gray, metallic forms consisting of extended
◦
ments on being heated above 30 C or when struck. Interest helical chains of Se atoms; and 1 black, brittle vitre-
in S 2 N 2 arises from the observation that it spontaneously ous form). Tellurium has but one crystalline allotrope; it
polymerizes when allowed to stand in the solid phase to consists of helical polymeric chains like those of gray
give (SN) x , poly(sulfur nitride). The mechanism of this selenium.
polymerization has been the object of much study, which Selenium and tellurium both occur free in nature and in
shows that polymerization is facilitated by the nearly combination in minerals called selenides and tellurides.
square-planer geometry of the monomer. The resulting The native elements as well as the minerals are found
polymer retains a nearly planar array of alternating sulfur closely associated with sulfur and sulfur minerals. Inas-
and nitrogen atoms arranged in a zigzag pattern in which muchasseleniumandtelluriummineralsarenotabundant,
◦
the bond angles have opened from about 90 to 111–113 . no commercially exploitable ore deposits are known. The
Poly(sulfur nitride) forms golden crystals with a metal- elements are usually obtained from the anodic sludge re-
lic sheen and is the first “nonmetallic metal” to have been sulting from the electrochemical refining of copper.
described. The fibrous crystals exhibit true metallic con-
ductivity, which increases with decreasing temperatures
until superconductivity is reached below 0.33 K. The con- A. Hydrides
ductivity is, however, anisotropic. The conductivity along
Both H 2 Se and H 2 Te are known. Like H 2 S, they are ex-
the fibers is ∼50 times that perpendicular to them, but
tremely toxic, highly malodorous gases.
this may be due more to the nature of the crystals than
to anisotropy at the molecular level. There is disagree-
ment regarding the exact bonding structure that permits B. Oxides
thispolymertobehaveasametal.Partofthisdisagreement
lies in some remaining uncertainty in the exact structure of Both SeO 2 and TeO 2 are known; however, they do not
the crystals. Qualitatively, the structure may be regarded as exist as discrete, V-shaped gas molecules like those of
SO 2 . Both SeO 2 and TeO 2 occur as polymeric solids and
having alternating π bonds along the length of the chains,
can be formed by direct combination of the elements with
with one “extra” electron from each sulfur atom occupying
oxygen.
a delocalized π antibonding orbital or band of the polymer
The trioxides, SeO 3 and TeO 3 , are also known, but they
metal.
are not obtained by oxidation of the dioxide as is the case
Poly(sulfur nitride) apparently oxidizes slowly in air
with SO 3 . The selenium and tellurium trioxides are usu-
andhasnotachievedsignificantutility.Halogenated(SN) x
ally prepared by dehydration of their respective oxoacids,
has been shown to have even higher conductivity than
H 2 SeO 4 and Te(OH) 6 or (H 6 TeO 6 ).
the parent polymer, and these derivatives continue to be
the subject of intense study. The remarkable properties
of poly(sulfur nitride) have spurred extensive research in
C. Halides
the general area of conducting polymeric materials as
a consequence of the obvious potential utility of such A variety of halides of selenium and tellurium has been
materials. synthesized, but their somewhat specialized chemistries
and structures are not considered.
XII. SELENIUM AND TELLURIUM
D. Oxoacids
The last two chalcogens from Group 16 (VIA) to be con- Unlike H 2 SO 3 , both H 2 SeO 3 and H 2 TeO 3 can be isolated
sidered are selenium and tellurium, both of which resem- in stable form. Both are white solids that can be easily
ble sulfur to some extent. Radioactive polonium not is converted to the respective dioxide by dehydration in a jet
considered since it is radioactive and extremely rare. of dry air.
Both selenium and tellurium are also rather rare, and The higher acids, H 2 SeO 4 and H 6 TeO 6 , are also known;
their commercial values and chemical uses are insignifi- these correspond to the +6 oxidation state for the sulfur
cant compared with those of the exciting congeners oxy- atom in sulfuric acid, H 2 SO 4 .