Athiosulfoxide orthiothionyl compound is achemical compound containing asulfur tosulfurdouble bond, with the formula(R−)(R'−)S=S, where R and R' represent any group (typicallyfluorine,chlorine,alkoxy,alkyl,aryl or otherorganyl residues. The thiosulfoxide has a molecular shape known astrigonal 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[1] which can be attributed to thedouble bond rule which states that elements of period 3 and beyond do not form multiple bonds. The relatedsulfoxides of the type(R−)(R'−)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 withelectron-withdrawing groups in so-calledthionosulfites of the type(R−O−)(R'−O−)S=S. These compounds can be prepared by reaction ofdiols withdisulfur dichloride. Sulfur halides such as disulfur dichloride,Cl−S−S−Cl, can convert to the branchedisomerthiothionyl chloride,Cl2S=S;disulfur difluoride exists as anequilibrium mixture withthiothionyl fluoride,F2S=S, which is thermodynamically more stable. These disulfide isomerizations are occasionally studiedin silico.[2]
N-(Thiosulfinyl)amines of the typeR−N=S=S are another group of stable compounds containing a S=S bond. The first such compound was prepared in 1974 reaction of thenitroso compoundN,N-dimethyl-p-nitrosoaniline withtetraphosphorus decasulfide. Heating to 200 °C extrudes sulfur in this compound and forms the correspondingazo compound.Disulfur monoxide S=S=O is stable at 20 °C for several days.
Occasionallythiosulfates are depicted as having a S=S unit but the sulfur-sulfur bond in it is in fact a single bond.