Sulfur monoxide explained

Sulfur monoxide is an inorganic compound with formula . It is only found as a dilute gas phase. When concentrated or condensed, it converts to S₂O₂ (disulfur dioxide). It has been detected in space but is rarely encountered intact otherwise.

Structure and bonding

The SO molecule has a triplet ground state similar to O₂ and S₂, that is, each molecule has two unpaired electrons. The S−O bond length of 148.1 pm is similar to that found in lower sulfur oxides (e.g. S₈O, S−O = 148 pm) but is longer than the S−O bond in gaseous S₂O (146 pm), SO₂ (143.1 pm) and SO₃ (142 pm).

The molecule is excited with near infrared radiation to the singlet state (with no unpaired electrons). The singlet state is believed to be more reactive than the ground triplet state, in the same way that singlet oxygen is more reactive than triplet oxygen.^[1]

Production and reactions

Production of SO as a reagent in organic syntheses has centred on using compounds that "extrude" SO. Examples include the decomposition of the relatively simple molecule ethylene episulfoxide:^[2] as well as more complex examples, such as a trisulfide oxide, C₁₀H₆S₃O.^[3]

C₂H₄SO → C₂H₄ + SO

The SO molecule is thermodynamically unstable, converting initially to S₂O₂. SO inserts into alkenes, alkynes and dienes producing thiiranes, molecules with three-membered rings containing sulfur.^[4]

Generation under extreme conditions

In the laboratory, sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapor in a glow discharge. It has been detected in single-bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas.^[5]

Benner and Stedman developed a chemiluminescence detector for sulfur via the reaction between sulfur monoxide and ozone:^[6]

SO + O₃ → SO₂* + O₂

SO₂* → SO₂ + hν

Occurrence

Ligand for transition metals

As a ligand SO can bond in a number different ways:^{[7] [8]}

• a terminal ligand, with a bent M−O−S arrangement, for example with titanium oxyfluoride^[9]
• a terminal ligand, with a bent M−S−O arrangement, analogous to bent nitrosyl
• bridging across two or three metal centres (via sulfur), as in Fe₃(μ³-S)(μ³-SO)(CO)₉
• η² sideways-on (d–π interaction) with vanadium, niobium, and tantalum.^[10]

Astrochemistry

Sulfur monoxide has been detected around Io, one of Jupiter's moons, both in the atmosphere^[11] and in the plasma torus.^[12] It has also been found in the atmosphere of Venus,^[13] in Comet Hale–Bopp,^[14] in 67P/Churyumov–Gerasimenko,^[15] and in the interstellar medium.^[16]

On Io, SO is thought to be produced both by volcanic and photochemical routes. The principal photochemical reactions are proposed as follows:^[17]

O + S₂ → S + SO

SO₂ → SO + O

Sulfur monoxide has been found in NML Cygni.^[18]

Biological chemistry

Sulfur monoxide may have some biological activity. The formation of transient SO in the coronary artery of pigs has been inferred from the reaction products, carbonyl sulfide and sulfur dioxide.^[19]

Safety measures

Because of sulfur monoxide's rare occurrence in our atmosphere and poor stability, it is difficult to fully determine its hazards. But when condensed and compacted, it forms disulfur dioxide, which is relatively toxic and corrosive. This compound is also highly flammable (similar flammability to methane) and when burned produces sulfur dioxide, a poisonous gas.

Sulfur monoxide dication

Sulfur dioxide SO₂ in presence of hexamethylbenzene C₆(CH₃)₆ can be protonated under superacidic conditions (HF·AsF₅) to give the non-rigid π-complex C₆(CH₃)₆SO²⁺. The SO²⁺ moiety can essentially move barrierless over the benzene ring. The S−O bond length is 142.4(2) pm.^[20]

C₆(CH₃)₆ + SO₂ + 3 HF·AsF₅ → [C₆(CH₃)₆SO][AsF₆]₂ + [H₃O][AsF₆]

Disulfur dioxide

See main article: Disulfur dioxide. SO converts to disulfur dioxide (S₂O₂).^[21] Disulfur dioxide is a planar molecule with C_2v symmetry. The S−O bond length is 145.8 pm, shorter than in the monomer, and the S−S bond length is 202.45 pm. The O−S−S angle is 112.7°. S₂O₂ has a dipole moment of 3.17 D.

Notes and References

1. Near-Infrared-Light-Induced Reaction of Singlet SO with Allene and Dimethylacetylene in a Rare Gas Matrix. Infrared Spectra of Two Novel Episulfoxides. Salama. F.. Frei. H. J.. Journal of Physical Chemistry. 1989. 93. 1285–1292. 10.1021/j100341a023.
2. Sulfur Monoxide Chemistry. The Nature of SO from Thiirane Oxide and the Mechanism of Its Reaction with Dienes. Chao. P.. Lemal. D. M.. Journal of the American Chemical Society. 95. 3. 1973. 920. 10.1021/ja00784a049.
3. 10.1021/ol016678g. 11678709. A Novel Recyclable Sulfur Monoxide Transfer Reagent. Organic Letters. 3. 22. 3565–3568. 2001. Grainger. R. S.. Procopio. A.. Steed. J. W..
4. [1+2] Cycloadditions of Sulfur Monoxide (SO) to Alkenes and Alkynes and [1+4]Cycloadditions to Dienes (Polyenes). Generation and Reactions of Singlet SO?. J.. Nakayama. Y.. Tajima. Piao. X.-H.. Y.. Sugihara. Journal of the American Chemical Society. 2007. 129. 23. 7250–7251. 10.1021/ja072044e. 17506566.
5. The temperatures of single-bubble sonoluminescence (A). Suslick. K. S.. Flannigan. D. J.. The Journal of the Acoustical Society of America. 2004. 116. 4. 2540. 10.1121/1.4785135. 2004ASAJ..116.2540S.
6. Chemical Mechanism and Efficiency of the Sulfur Chemiluminescence Detector. Benner. R. L.. Stedman. D. H.. Applied Spectroscopy. 48. 7. 1994. 848–851. 10.1366/0003702944029901. 1994ApSpe..48..848B. 98849015.
7. Schenk. W. A.. Sulfur Oxides as Ligands in Coordination Compounds. Angewandte Chemie International Edition in English. 1987. 26. 98–109. 10.1002/anie.198700981.
8. Encyclopedia: Sulfur: Inorganic Chemistry. Woollins. J. D.. Encyclopedia of Inorganic Chemistry. 1995. John Wiley and Sons. 0-471-93620-0.
9. Wei . R. . Chen . X. . Gong . Y. . End-On Oxygen-Bound Sulfur Monoxide Complex of Titanium Oxyfluoride . Inorganic Chemistry . 2019 . 58 . 17 . 11801–11806 . 10.1021/acs.inorgchem.9b01880. 31441297 . 201617509 .
10. Side-On Sulfur Monoxide Complexes of Tantalum, Niobium, and Vanadium Oxyfluorides. Inorganic Chemistry. R.. Wei. X.. Chen. Y. . Gong. 2019. 58. 6. 3807–3814. 10.1021/acs.inorgchem.8b03411. 30707575. 73438027.
11. Io's atmosphere: Not yet understood. Lellouch. E.. 1996. Icarus. 124. 1–21. 10.1006/icar.1996.0186.
12. Detection of SO in Io's Exosphere. Russell. C. T.. Kivelson. M. G.. Science. 2000. 287. 5460. 1998–1999. 10.1126/science.287.5460.1998. 10720321. 2000Sci...287.1998R.
13. International Ultraviolet Explorer observations of Venus SO₂ and SO. Na. C. Y.. Esposito. L. W.. Skinner. T. E.. Journal of Geophysical Research. 95. 1990. 7485–7491. 10.1029/JD095iD06p07485. 1990JGR....95.7485N.
14. New Molecular Species in Comet C/1995 O1 (Hale–Bopp) Observed with the Caltech S submillimeter Observatory. D. C.. Lis. D. M.. Mehringer. D.. Benford. M.. Gardner. T. G.. Phillips. D.. Bockelée-Morvan. Dominique Bockelée-Morvan. N.. Biver. P.. Colom. J.. Crovisier. D.. Despois. H.. Rauer. Earth, Moon, and Planets. 78. 1–3. 1997. 13–20. 10.1023/A:1006281802554. 1997EM&P...78...13L. 51862359.
15. Web site: Астрономический вестник. T. 54, Номер 2, 2020 .
16. Observations of interstellar sulfur monoxide. Gottlieb. C. A.. Gottlieb. E. W.. Litvak. M. M.. Ball. J. A.. Pennfield. H.. Astrophysical Journal. 1. 219. 1978. 77–94. 10.1086/155757. 1978ApJ...219...77G.
17. Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption. Moses. J. I.. Zolotov. M. Y.. Fegley. B.. Icarus. 156. 76–106. 2002. 1. 10.1006/icar.2001.6758. 2002Icar..156...76M.
18. Book: Marvel, Kevin. The Circumstellar Environment of Evolved Stars As Revealed by Studies of Circumstellar Water Masers. https://books.google.com/books?id=wwx1Gj5wR5QC&pg=PR182. 23 August 2012. 1996. Universal Publishers. 978-1-58112-061-5. 182–212. NML Cygni.
19. Identification of carbonyl sulfide and sulfur dioxide in porcine coronary artery by gas chromatography/mass spectrometry, possible relevance to EDHF. Balazy. M.. Abu-Yousef. I. A.. Harpp. D. N.. Park. J.. Biochemical and Biophysical Research Communications. 2003. 311. 3. 728–734. 10.1016/j.bbrc.2003.10.055. 14623333.
20. Malischewski. Moritz. Seppelt. Konrad. 2017. Isolation and Characterization of a Non-Rigid Hexamethylbenzene-SO²⁺ Complex. Angewandte Chemie International Edition. en. 56. 52. 16495–16497. 10.1002/anie.201708552. 29084371. 27260554 . 1433-7851.
21. Spectroscopic studies of the SO₂ discharge system. II. Microwave spectrum of the SO dimer. Lovas. F. J.. Tiemann. E.. Johnson. D. R.. The Journal of Chemical Physics. 1974. 60. 12. 5005–5010. 10.1063/1.1681015. 1974JChPh..60.5005L.