# Difluorodisulfanedifluoride

1,1,1,2-tetrafluorodisulfane, also known as 1,2-difluorodisulfane 1,1-difluoride or just difluorodisulfanedifluoride (FSSF$_3$) is an unstable molecular compound of fluorine and sulfur. 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 lengths are different. The bond strength is not correlated with bond length but is inversely correlated with the force constant (Badger's rule). The molecule can be considered as sulfur tetrafluoride 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.
| atom 1 | atom 2 | bond length Å | bond dissociation energy kcal/mol | bond angle to S-S axis ° |
| F$_{top}$ | S$_{top}$ | 1.62 | 86.4 | 105 |
| F$_{cis}$ | S$_{hyp}$ | 1.67 | 102.1 | 76 |
| F$_{trans}$ | S$_{hyp}$ | 1.77 | 97.8 | 92 |
| F$_{eq}$ | S$_{hyp}$ | 1.60 | 86.7 | 106 |
| S$_{top}$ | S$_{hyp}$ | 2.08 | | |
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 2SF$_2$ ⇌ FSSF$_3$ is reversible. It also disproportionates: SF$_2$ + FSSF$_3$ → FSSF + SF$_4$. A side reaction also produces the intermediate F$_3$SSSF$_3$. hydrogen fluoride catalyses disproportionation to sulfur and sulfur tetrafluoride by forming a reactive intermediate HSF molecule. When FSSF$_3$ 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, FSSF$_3$ decomposes with a half life of about 10 hours. Disproportionation to SSF$_2$ and SF$_4$ catalysed by metal fluorides can take place in under one second. However it is indefinitely stable at -196 °C.
A symmetrical molecule F$_2$SSF$_2$ is calculated to be 15.1 kcal/mol higher in energy than FSSF$_3$.
FSSF$_3$ is easily hydrolysed with water.
FSSF$_3$ spontaneously reacts with oxygen gas to make thionyl fluoride, the only sulfur fluoride that does not need any assistance to do this. FSSF$_3$ reacts with copper at high temperatures producing copper fluoride and copper sulfide.

## Formation

SF$_3$SF can be made in the laboratory when low pressure (10 mm Hg) SCl$_2$ vapour is passed over potassium fluoride or mercuric fluoride heated to 150 °C. Byproducts include FSSF, SSF$_2$, SF$_4$, SF$_3$SCl, and FSSCl. SF$_3$SCl can be removed from this mixture in a reaction with mercury. Separation of the sulfur fluorides can be achieved by low temperature distillation. SF$_3$SF distills just above -50 °C.
SF$_3$SF is also made in small amounts by reacting sulfur with silver fluoride, or photolysis of disulfur difluoride and SSF$_2$. The molecule is formed by the dimerization of sulfur difluoride.

## Properties

The nuclear magnetic resonance spectrum of FSSF$_3$ shows four bands, each of eight lines at -53.2, -5.7, 26.3 and 204.1 ppm.
FSSF$_3$ is stable as a solid, as a liquid below -74 °C and dissolved in other sulfur fluoride liquids. This is in contrast to SF$_2$ which is only stable as a dilute gas.
Infrared vibration bands for FSSF$_3$ are at 810, 678, 530, 725, and 618(S-S) cm$^{−1}$.

## Related

The related compound FSSSF$_3$ has a similar structure, but with an extra sulfur atom in the chain. Thiothionyltetrafluoride, S=SF$_4$ may exist as a gas. It is less energetically favourable to FSSF$_3$ by 37 kJ/mol, but has a high energy barrier of 267 kJ/mol. However it may disproportionate rapidly to sulfur and sulfur tetrafluoride. The other known sulfur fluorides are sulfur difluoride, sulfur tetrafluoride, sulfur hexafluoride, disulfur decafluoride, disulfur difluoride and thiothionyl fluoride, difluorotrisulfane, and difluorotetrasulfane. The F$_{top}$ atom can be substituted with Cl to yield ClSSF$_3$ (2-chloro-1,1,1-trifluorodisulfane).