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thiosulfoxide
A thiosulfoxide is a chemical compound containing a sulfur to sulfur double bond of the type RR'S=S with R and R' both alkyl or aryl residues. The thiosulfoxide has a molecular shape known as trigonal pyramidal. Its coordination is also trigonal pyramidal. The point group of the thiosulfoxide is Cs. A 1982 review concluded that there was as yet no definitive evidence for the existence of stable thiosulfoxides 〔''Compounds containing the sulfur-sulfur double bond'' Gerald W. Kutney, and Kenneth Turnbull ''Chem. Rev.'', 1982, 82 (4), 333-357 〕 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 RR'S=O 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.
Sulfur sulfur double bonds can be stabilized with electron-withdrawing groups in so-called thionosulfites of the type RO(RO)S=S. These compounds can be prepared by reaction of diols with disulfur dichloride. Sulfur halides such as disulfur dichloride Cl-S-S-Cl can convert to the branched isomer Cl2S=S. The related disulfur difluoride exists as an equilibrium mixture with thiothionyl fluoride F2S=S thermodynamically more stable. These disulfide isomerizations are occasionally studied in silico.〔"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" Ralf Steudel, Yana Drozdova, Karol Miaskiewicz, Roland H. Hertwig, and Wolfram Koch ''J. Am. Chem. Soc.'', 1997, 119 (8), 1990-1996• 〕
N-(Thiosulfinyl)amines of the type R-N=S=S 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.
==References==
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