A thiosulfoxide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

containing a

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

of the type RR'S=S, with R and R' both

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...

residues. The thiosulfoxide has a molecular shape known as

trigonal pyramidal In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base, resembling a tetrahedron (not to be confused with the tetrahedral geometry). When all three atoms at the corne ...

. 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 which can be attributed to the

double bond rule In chemistry, the double bond rule states that elements with a principal quantum number greater than 2 for their valence electrons ( period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double ...

which states that elements of period 3 and beyond do not form multiple bonds. The related

sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

s 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. Thiothionylfluoride-2D-dimensions

Sulfur-sulfur double bonds can be stabilized with

electron-withdrawing group In chemistry, an electron-withdrawing group (EWG) is a substituent that has some of the following kinetic and thermodynamic implications: *with regards to electron transfer, electron-withdrawing groups enhance the oxidizing power tendency of the ...

s in so-called thionosulfites of the type RO(RO)S=S. These compounds can be prepared by reaction of

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

s with

disulfur dichloride Disulfur dichloride is the inorganic compound of sulfur and chlorine with the formula S2Cl2. Some alternative names for this compound are ''sulfur monochloride'' (the name implied by its empirical formula, SCl), ''disulphur dichloride'' (Britis ...

. Sulfur halides such as disulfur dichloride, Cl-S-S-Cl, can convert to the branched

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Is ...

Cl₂S=S; disulfur difluoride exists as an equilibrium mixture with thiothionyl fluoride, F₂S=S, which is thermodynamically more stable. These disulfide isomerizations are occasionally studied ''

in silico In biology and other experimental sciences, an ''in silico'' experiment is one performed on computer or via computer simulation. The phrase is pseudo-Latin for 'in silicon' (correct la, in silicio), referring to silicon in computer chips. It ...

''. 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 In organic chemistry, nitroso refers to a functional group in which the nitric oxide () group is attached to an organic moiety (chemistry), moiety. As such, various nitroso groups can be categorized as ''C''-nitroso compounds (e.g., nitrosoalkane ...

compound N,N-dimethyl-p-nitrosoaniline with tetraphosphorus decasulfide. Heating to 200 °C extrudes sulfur in this compound and forms the corresponding

azo compound Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups). IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...

. Disulfur monoxide S=S=O is stable at 20 °C for several days. Occasionally

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e ...

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

References

{{Reflist Sulfur compounds