Thiosulfoxide

A thiosulfoxide or thiothionyl compound is a chemical compound containing a sulfur to sulfur double bond, with the formula {{chem2|(R\s)(R'\s)S\dS}}, where R and R' represent any group (typically fluorine, chlorine, alkoxy, alkyl, aryl or other organyl residues. The thiosulfoxide has a molecular shape known as trigonal pyramidal. Its coordination is also trigonal pyramidal. The point group of the thiosulfoxide is C_s. A 1982 review concluded that there was as yet no definitive evidence for the existence of stable thiosulfoxides {{cite journal|title=Compounds containing the sulfur-sulfur double bond|author1=Gerald W. Kutney|author2=Kenneth Turnbull|journal=Chem. Rev.|year=1982|volume=82|issue=4|pages=333–357|doi=10.1021/cr00050a001}} which can be attributed to the double bond rule which states that elements of period 3 and beyond do not form multiple bonds. The related sulfoxides of the type {{chem2|(R\s)(R'\s)S\dO}} are very common. Many compounds containing a sulfur-sulfur double bond have been reported in the past although only a few verified classes of actually stable compounds exist, closely related to thiosulfoxides.

File:Thiothionylfluoride-2D-dimensions.png

Sulfur-sulfur double bonds can be stabilized with electron-withdrawing groups in so-called thionosulfites of the type {{chem2|(R\sO\s)(R'\sO\s)S\dS}}. These compounds can be prepared by reaction of diols with disulfur dichloride. Sulfur halides such as disulfur dichloride, {{chem2|Cl\sS\sS\sCl}}, can convert to the branched isomer thiothionyl chloride, {{chem2|Cl2S\dS}}; disulfur difluoride exists as an equilibrium mixture with thiothionyl fluoride, {{chem2|F2S\dS}}, which is thermodynamically more stable. These disulfide isomerizations are occasionally studied in silico.{{cite journal|title=How Unstable are Thiosulfoxides? An ab Initio MO Study of Various Disulfanes RSSR (R = H, Me, Pr, All), Their Branched Isomers R₂SS, and the Related Transition States|author1=Ralf Steudel|author2=Yana Drozdova|author3=Karol Miaskiewicz|author4=Roland H. Hertwig|author5=Wolfram Koch|journal=J. Am. Chem. Soc.|year=1997|volume=119|issue=8|pages=1990–1996|doi=10.1021/ja9624026}}

N-(Thiosulfinyl)amines of the type {{chem2|R\sN\dS\dS}} are another group of stable compounds containing a S=S bond. The first such compound was prepared in 1974 reaction of the nitroso compound N,N-dimethyl-p-nitrosoaniline with tetraphosphorus decasulfide. Heating to 200 °C extrudes sulfur in this compound and forms the corresponding azo compound. Disulfur monoxide S=S=O is stable at 20 °C for several days.

Occasionally thiosulfates are depicted as having a S=S unit but the sulfur-sulfur bond in it is in fact a single bond.