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. A 1982 review concluded that there was as yet no definitive evidence for the existence of stable thiosulfoxides  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.
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.
- Compounds containing the sulfur-sulfur double bond Gerald W. Kutney, and Kenneth Turnbull Chem. Rev., 1982, 82 (4), 333-357 doi:10.1021/cr00050a001
- 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• doi:10.1021/ja9624026