1,1,1,2-tetrafluorodisulfane, also known as 1,2-difluorodisulfane 1,1-difluoride or just difluorodisulfanedifluoride (FSSF₃) is an unstable molecular compound of

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

and

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 formula ...

. The molecule has a pair of sulfur atoms, with one fluorine atom on one sulfur, and three fluorine atoms on the other. It has the uncommon property that all the

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...

s are different. The

bond strength In chemistry, bond energy (''BE''), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually a ...

is not correlated with bond length but is inversely correlated with the

force constant In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of th ...

( Badger's rule). The molecule can be considered as

sulfur tetrafluoride Sulfur tetrafluoride is the chemical compound with the formula S F4. It is a colorless corrosive gas that releases dangerous HF upon exposure to water or moisture. Despite these unwelcome characteristics, this compound is a useful reagent for t ...

in which a sulfur atom is inserted into a S-F bond. Atoms are labelled with the sulfur atom connected to three fluorine atoms as S_''hyp'' (for hypervalent) and S_''top''. The fluorine atoms are labelled F_''top'' attached to S_''top'', and on the hypervalent S atom: F_''cis'', the closest F atom to F_''top'', F_''trans'' the furthest away F atom from F_''top'', and F_''eq'' Carlowitz first determined the structure in 1983. F_''eq'' is 90° from F_''trans'', and 84° from F_''cis'', and the torsion compared to F_''top'' is about 95°.

Reactions

The dimerization reaction 2SF₂ FSSF₃ is reversible. It also disproportionates: SF₂ + FSSF₃ → FSSF + SF₄. A side reaction also produces the intermediate F₃SSSF₃.

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

catalyses disproportionation to sulfur and sulfur tetrafluoride by forming a reactive intermediate HSF molecule. When FSSF₃ dissociates, the F_''cis'' atom forms a new bond to the S_''top'' atom, and the S-S bond breaks. As a gas, at ambient and totally clean conditions, FSSF₃ decomposes with a half life of about 10 hours. Disproportionation to SSF₂ and SF₄ catalysed by metal fluorides can take place in under one second. However it is indefinitely stable at -196 °C. A symmetrical molecule F₂SSF₂ is calculated to be 15.1 kcal/mol higher in energy than FSSF₃. FSSF₃ is easily hydrolysed with water. FSSF₃ spontaneously reacts with oxygen gas to make

thionyl fluoride Thionyl fluoride is the inorganic compound with the formula . This colourless gas is mainly of theoretical interest, but it is a product of the degradation of sulfur hexafluoride, an insulator in electrical equipment. The molecule adopts a dis ...

, the only sulfur fluoride that does not need any assistance to do this. FSSF₃ reacts with copper at high temperatures producing copper fluoride and copper sulfide.

Formation

SF₃SF can be made in the laboratory when low pressure (10 mm Hg) SCl₂ vapour is passed over

potassium fluoride Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali halide and occurs naturally as the rare m ...

or mercuric fluoride heated to 150 °C. Byproducts include FSSF, SSF₂, SF₄, SF₃SCl, and FSSCl. SF₃SCl can be removed from this mixture in a reaction with mercury. Separation of the sulfur fluorides can be achieved by low temperature distillation. SF₃SF distills just above -50 °C. SF₃SF is also made in small amounts by reacting sulfur with

silver fluoride Silver fluoride can refer to: * Silver subfluoride (disilver monofluoride), Ag2F * Silver(I) fluoride (silver monofluoride, argentous fluoride), AgF * Silver(II) fluoride (silver difluoride, argentic fluoride), AgF2 * Silver(III) fluoride (silver t ...

, or

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

disulfur difluoride Disulfur difluoride is a halide of sulfur, with the chemical formula S2F2. Structure Disulfur difluoride will undergo intramolecular rearrangement in the presence of alkali elements' fluorides, yielding the isomer S=SF2: ::FS-SF -> \atop \ce\u ...

and SSF₂. The molecule is formed by the dimerization of

sulfur difluoride Sulfur difluoride is an inorganic compound with the chemical formula SF2. It can be generated by the reaction of sulfur dichloride and potassium fluoride or mercury(II) fluoride at low pressures: :SCl2 + 2 KF → SF2 + 2 KCl :SCl2 + HgF2 → SF2 + ...

Properties

The

nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

spectrum of FSSF₃ shows four bands, each of eight lines at -53.2, -5.7, 26.3 and 204.1 ppm. FSSF₃ is stable as a solid, as a liquid below -74 °C and dissolved in other sulfur fluoride liquids. This is in contrast to SF₂ which is only stable as a dilute gas. Infrared vibration bands for FSSF₃ are at 810, 678, 530, 725, and 618(S-S) cm⁻¹.

The related compound FSSSF₃ has a similar structure, but with an extra sulfur atom in the chain. Thiothionyltetrafluoride, S=SF₄ may exist as a gas. It is less energetically favourable to FSSF₃ by 37 kJ/mol, but has a high

energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

of 267 kJ/mol. However it may disproportionate rapidly to sulfur and sulfur tetrafluoride. The other known sulfur fluorides are

sulfur hexafluoride Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...

disulfur decafluoride Disulfur decafluoride is a chemical compound with the formula . It was discovered in 1934 by Denbigh and Whytlaw-Gray. Each sulfur atom of the molecule is octahedral, and surrounded by five fluorine atoms and one sulfur atom. The two sulfur ato ...

and

thiothionyl fluoride Thiothionyl fluoride is a chemical compound of fluorine and sulfur and an isomer of disulfur difluoride. Preparation Thiothionyl fluoride can be obtained from the reaction between disulfur dichloride with potassium fluoride at about 150 °C or ...

, difluorotrisulfane, and difluorotetrasulfane. The F_''top'' atom can be substituted with Cl to yield ClSSF₃ (2-chloro-1,1,1-trifluorodisulfane).

References

Extra reading

* * What happens when irradiated by ultraviolet photons. * animation of dissociation *{{cite web, last1=Extance, first1=Andy, title=Sulfur difluoride dimer exposes bonding strangeness, url=https://www.chemistryworld.com/research/sulfur-difluoride-dimer-exposes-bonding-strangeness/6577.article, website=Chemistry World, accessdate=9 November 2016, date=12 September 2013 Sulfur fluorides Hypervalent molecules

Reactions

Formation

Properties

Related

References

Extra reading