Page 237 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.6 HIGHER-VALENT STATES: OXIDES AND OXOACIDS 217

REVIEW PROBLEM 6.6
−
Consistent with the nucleophilic character of the sulfur in HSO , one might expect
3
that the S-protonated tautomer should also be reasonably stable. 17 Oxygen NMR
studies strongly suggest the presence of both species in equilibrium in aqueous
solutions:

H
+
− 2+ −
S − S
O O
HO O
O − O −

Suggest a mechanism for the interconversion.

A variety of sulfur oxoacids and oxoanions are known that contain more than one sulfur
per molecule or ion. Although a systematic account is outside the scope of this book, the
thiosulfate anion, S O 2− , on account of its long-standing role as a “fixer” in traditional
2 3
photography, is worth mentioning. The corresponding free acid, thiosulfuric acid, H S O ,
2 2 3
can be obtained by the reaction of chlorosulfonic acid, ClSO H, and H S at low temperature
3 2
and under anhydrous conditions.
ClSO H + H S → H S O + HCl (6.53)
3 2 2 2 3
The structure of the product, H S O (see below), provides the key clue: an S–S bond has
2 2 3
been formed. The sulfur in H S then is a plausible nucleophile, and the sulfur in ClSO H,
2 3
bearing a Cl, a potential leaving group, is a plausible electrophilic site:
− −
H O H O
2+ 2+ −
S S Cl + S S + Cl (6.54)
H HO O − H − O OH

Chloride then picks up a proton, producing HCl and H S O as the final products:
2 2
3
− −
H O H O
2+ 2+
+ S S − HCl S S (6.55)
− OH
H − O OH
Cl − O
+
The thiosulfate anion, a soft base, strongly coordinates Ag , a soft Lewis acid through the
terminal sulfur atom. Aqueous sodium thiosulfate (“hypo”), Na S O , is therefore used to
3
2 2
remove unchanged AgBr from exposed photographic film. The process is called fixing,as