365Oxides and Oxyanions
The principal sulfur oxides are obtained by burning sulfur:
S + O 2 → SO 22 SO 2 + O 2 → 2 SO 3Other oxides are known, e.g. sulfur monoxide and disulfur mono- and dioxides, but they
are unstable.
The sulfur oxides form numerous oxyanions with the formula SOn2–. Sulfur dioxide and
sulfites (SO2−3) are related to the unstable sulfurous acid (H 2 SO 3 ). Sulfur trioxide and
sulfates (SO2−4) are related to sulfuric acid. Sulfuric acid and SO 3 combine to give oleum,
a solution of pyrosulfuric acid (H 2 S 2 O 7 ) in sulfuric acid.
Peroxides convert sulfur into unstable such as S 8 O, a sulfoxide. Peroxymonosulfuric acid
(H 2 SO 5 ) and peroxydisulfuric acids (H 2 S 2 O 8 ), made from the action of SO 3 on concentrated
H 2 O 2 , and H 2 SO 4 on concentrated H 2 O 2 respectively.
Thiosulfate salts (S 2 O2−3), sometimes referred as "hyposulfites", used in photographic
fixing (HYPO) and as reducing agents, feature sulfur in two oxidation states. Sodium
dithionite, (S 2 O2−4), contains the more highly reducing dithionite anion. Sodium dithionate
(Na 2 S 2 O 6 ) is the first member of the polythionic acids (H 2 SnO 6 ), where n can range from 3
to many.
Halides and Oxyhalides
The two main sulfur fluorides are sulfur hexafluoride, a dense gas used as nonreactive
and nontoxic propellant, and sulfur tetrafluoride, a rarely used organic reagent that is
highly toxic. Their chlorinated analogs are sulfur dichloride and sulfur monochloride.
Sulfuryl chloride and chlorosulfuric acid are derivatives of sulfuric acid; thionyl chloride
(SOCl 2 ) is a common reagent in organic synthesis.
Pnictides
An important S–N compound is the cage tetrasulfur tetranitride (S 4 N 4 ). Heating this
compound gives polymeric sulfur nitride ((SN)x), which has metallic properties even
though it does not contain any metal atoms. Thiocyanates contain the SCN− group.
Oxidation of thiocyanate gives thiocyanogen, (SCN) 2 with the connectivity NCS-SCN.
Phosphorus sulfides are numerous, the most important commercially being the cages
P 4 S 10 and P 4 S 3.
Metal Sulfides
The principal ores of copper, zinc, nickel, cobalt, molybdenum, and other metals are
sulfides. These materials tend to be dark-colored semiconductors that are not readily
attacked by water or even many acids. They are formed, both geochemically and in the
laboratory, by the reaction of hydrogen sulfide with metal salts.
The mineral galena (PbS) was the first demonstrated semiconductor and found a use as
a signal rectifier in the cat's whiskers of early crystal radios.