Disulfur diiodide

Disulfur diiodide is an unstable inorganic chemical compound with the chemical formula S₂I₂. It is a red-brown solid that decomposes above −30 °C to elemental sulfur and iodine.^[1]

Disulfur diiodide

Sulfur, S   Iodine, I
Names
IUPAC name Diiododisulfane
Systematic IUPAC name Iodosulfanyl thiohypoiodite
Other names Sulfur monoiodide (incorrect name)
Identifiers
CAS Number	53280-15-2 Y
3D model (JSmol)	Interactive image
ChemSpider	31043639
ECHA InfoCard	100.053.127
EC Number	258-458-4
PubChem CID	71774781
CompTox Dashboard (EPA)	DTXSID101315384
InChI InChI=1S/I2S2/c1-3-4-2 Key: NJLGSHIGTKGJLG-UHFFFAOYSA-N
SMILES S(SI)I
Properties
Chemical formula	S2I2
Molar mass	317.93 g·mol−1
Appearance	Reddish-brown solid
Melting point	−30 °C (−22 °F; 243 K) (decomposes)
Solubility	Soluble in carbon tetrachloride, slightly soluble in pentane
Structure
Point group	C2
Coordination geometry	2 at sulfur atoms
Molecular shape	gauche
Related compounds
Related compounds	Hydrogen disulfide Disulfur difluoride Disulfur dichloride Disulfur dibromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Production

Reaction of sulfur and iodine

The first attempt and claim to produce a sulfur iodide were made in 1813 by Bernard Courtois when exploring the properties of his newly discovered element, iodine. He reacted to sulfur and iodine, claiming they had made a compound. However, this production was doubted by Gay-Lussac. Between 1827 and 1896, more attempts were made to make sulfur iodide by combining the elements; however, all were inconclusive on the existence of the compound or failed. Later, when thermal analysis was developed, it was shown that when the elements were combined, it only resulted in a mixture, not a compound.^[2]

Production by double replacement

When attempts to produce sulfur iodide by the direct combination of the elements failed to overcome the low thermodynamic stability of the compound, production by double replacement was attempted between 1833 and 1886. Some reactions that were attempted was the reaction of disulfur dichloride and hydroiodic acid:^[2]

S₂Cl₂ + 2 HI → S₂I₂ + 2 HCl

The reaction of hydrogen sulfide and iodine trichloride:

3 H₂S + 2 ICl₃ → S₃I₂ + 6 HCl

The reaction of hydroiodic acid and sulfur:

2 HI + 3 S → H₂S + S₂I₂

The reaction of disulfur dichloride and potassium iodide:

S₂Cl₂ + 2 KI → S₂I₂ + 2 KCl

and more, all assumed to have failed to produce sulfur iodide. However, the reaction between S₂Cl₂ and HI attempted in 1835 was later proven to have produced disulfur diiodide.^[2]

In 1940, another production was attempted with the fourth reaction and was reported to have detected various sulfur iodides, such as disulfur diiodide and sulfur diiodide (SI₂). When observing the reaction of very dilute disulfur dichloride in carbon tetrachloride and potassium iodide:^[2][3]

S₂Cl₂ + 2 KI → 2 S + I₂ + 2 KCl

they observed a color change from yellow to reddish-brown to finally violet, which was assumed to be evidence for the formation of sulfur iodides. The compound was found to decompose at room temperature slowly in a solution, with the decomposition rate increasing with increasing temperature.^[3]

Isolation

Disulfur diiodide was first isolated by the reaction of disulfur dichloride and potassium iodide, sodium iodide, or hydrogen iodide in pentane at −90 °C, and verified by infrared spectroscopy.^[4][5]

Properties

Disulfur diiodide is light-sensitive and is soluble in various haloalkanes, such as carbon tetrachloride.^[3]

Other sulfur iodides

Sulfur diiodide (SI₂) has finally been reported in an argon matrix at 9 K by the reaction of sulfur dichloride and iodine; however, this has been disputed.^[6]

Sulfur and iodine react in antimony pentafluoride or arsenic pentafluoride to form the S₇I⁺ ion, which is stable at room temperature, unlike other sulfur-iodine compounds.^[7]

The empirical formula of disulfur diiodide is SI. It is therefore sometimes called sulfur monoiodide, but S₂I₂ is a covalently bound molecule and is not the same as a SI molecule or an ionic salt having 1:1 stoichiometry. The true sulfur monoiodide molecule is a radical.^[8]

References

1. Wiberg, Egon; Nils Wiberg; Arnold Frederick Holleman (2001). Inorganic chemistry. Academic Press. p. 529. ISBN 978-0-12-352651-9.
2. William B. Jensen (2016). The Search for Sulfur Iodide : Notes from the Oesper Collections, No. 37. University of Cincinnati. Archived from the original on 2022-12-25. Retrieved 2022-12-23.
3. M. R. Aswathanarayana Rao (1940). "Investigations on the Iodides of Sulphur". Proceedings of the Indian Academy of Sciences. 11 (3): 162–174.
4. Gisela Vahl; Priv.-Doz. Dr. Rolf Minkwitz (1978). "Beiträge zur Chemie der Schwefelhalogenide. IV. Über Versuche zur Darstellung von festem Dijoddisulfan bei tiefen Temperaturen" [Contributions of the Chemistry of Sulfur Halides. IV. On the Preparation of Solid Disulfurdiiodide at Low Temperatures]. Zeitschrift für anorganische und allgemeine Chemie (in German). 443 (1). Wiley: 217–224. doi:10.1002/zaac.19784430124.
5. Gisela Krummel; Rolf Minkwitz (1977). "Infrared-spectroscopic investigations on solid disulfurdiiodine". Inorganic and Nuclear Chemistry Letters. 13 (5): 213–215. doi:10.1016/0020-1650(77)80096-2.
6. Martin Feuerhahn; Gisela Vahl (1980). "Infrared spectra of matrix isolated sulfur dibromide and sulfur diiodide". Inorganic and Nuclear Chemistry Letters. 16 (1): 5-8. doi:10.1016/0020-1650(80)80082-1.
7. T. Klapoetke; J. Passmore (1989). "Sulfur and selenium iodine compounds: from non-existence to significance". Accounts of Chemical Research. 22 (7). ACS Publications: 234–240. doi:10.1021/ar00163a002.
8. See SciFinder CAS No 1312-15-8 for main refs