Chemistry:Sulfur monoxide

From HandWiki
Sulfur monoxide
Skeletal formula of sulfur monoxide
Spacefill model of sulfur monoxide
Ball and stick model of sulfur monoxide
IUPAC name
Sulfur monoxide
Systematic IUPAC name
3D model (JSmol)
MeSH sulfur+monoxide
Molar mass 48.064 g mol−1
Appearance Colourless gas
log P 0.155
221.94 J K−1 mol−1
5.01 kJ mol−1
Related compounds
Related compounds
Triplet oxygen
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is ☑Y☒N ?)
Infobox references
Tracking categories (test):

Sulfur monoxide is an inorganic compound with formula SO. 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.[2] 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).[2]

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.[3]

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:[4] as well as more complex examples, such as a trisulfide oxide, C10H6S3O.[5]

C2H4SO → C2H4 + SO

The SO molecule is thermodynamically unstable, converting initially to S2O2.[2] SO inserts into alkenes, alkynes and dienes producing thiiranes, molecules with three-membered rings containing sulfur.[6]

Generation under extreme conditions

In the laboratory, sulfur monoxide can be produced by treating sulfur dioxide with sulfur vapor in a glow discharge.[2] It has been detected in single-bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas.[7]

Benner and Stedman developed a chemiluminescence detector for sulfur via the reaction between sulfur monoxide and ozone:[8]

SO + O3 → SO2* + O2
SO2* → SO2 + hν

(* indicates an excited state)


Ligand for transition metals

As a ligand SO can bond in a number different ways:[9][10]

  • a terminal ligand, with a bent M−O−S arrangement, for example with titanium oxyfluoride[11]
  • 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.[12]


Sulfur monoxide has been detected around Io, one of Jupiter's moons, both in the atmosphere[13] and in the plasma torus.[14] It has also been found in the atmosphere of Venus,[15] in Comet Hale–Bopp,[16] in 67P/Churyumov–Gerasimenko,[17] and in the interstellar medium.[18]

On Io, SO is thought to be produced both by volcanic and photochemical routes. The principal photochemical reactions are proposed as follows:[19]

O + S2 → S + SO
SO2 → SO + O

Sulfur monoxide has been found in NML Cygni.[20]

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.[21]

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.[22]

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

Disulfur dioxide

The structure of disulfur dioxide, S2O2
A space-filling model of the disulfur dioxide molecule
Main page: Chemistry:Disulfur dioxide

SO converts to disulfur dioxide (S2O2).[23] Disulfur dioxide is a planar molecule with C2v 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.[23]


  1. "sulfur monoxide (CHEBI:45822)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 
  2. 2.0 2.1 2.2 2.3 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. 
  3. Salama, F.; Frei, H. J. (1989). "Near-Infrared-Light-Induced Reaction of Singlet SO with Allene and Dimethylacetylene in a Rare Gas Matrix. Infrared Spectra of Two Novel Episulfoxides". Journal of Physical Chemistry 93: 1285–1292. doi:10.1021/j100341a023. 
  4. Chao, P.; Lemal, D. M. (1973). "Sulfur Monoxide Chemistry. The Nature of SO from Thiirane Oxide and the Mechanism of Its Reaction with Dienes". Journal of the American Chemical Society 95 (3): 920. doi:10.1021/ja00784a049. 
  5. Grainger, R. S.; Procopio, A.; Steed, J. W. (2001). "A Novel Recyclable Sulfur Monoxide Transfer Reagent". Organic Letters 3 (22): 3565–3568. doi:10.1021/ol016678g. PMID 11678709. 
  6. Nakayama, J.; Tajima, Y.; Piao, X.-H.; Sugihara, Y. (2007). "[1+2] Cycloadditions of Sulfur Monoxide (SO) to Alkenes and Alkynes and [1+4]Cycloadditions to Dienes (Polyenes). Generation and Reactions of Singlet SO?". Journal of the American Chemical Society 129 (23): 7250–7251. doi:10.1021/ja072044e. PMID 17506566. 
  7. Suslick, K. S.; Flannigan, D. J. (2004). "The temperatures of single-bubble sonoluminescence (A)". The Journal of the Acoustical Society of America 116 (4): 2540. doi:10.1121/1.4785135. Bibcode2004ASAJ..116.2540S. 
  8. Benner, R. L.; Stedman, D. H. (1994). "Chemical Mechanism and Efficiency of the Sulfur Chemiluminescence Detector". Applied Spectroscopy 48 (7): 848–851. doi:10.1366/0003702944029901. Bibcode1994ApSpe..48..848B. 
  9. Schenk, W. A. (1987). Sulfur Oxides as Ligands in Coordination Compounds. Angewandte Chemie International Edition in English. 26. pp. 98–109. doi:10.1002/anie.198700981. 
  10. Woollins, J. D. (1995). Encyclopedia of Inorganic Chemistry. John Wiley and Sons. ISBN 0-471-93620-0. 
  11. Wei, R.; Chen, X.; Gong, Y. (2019). "End-On Oxygen-Bound Sulfur Monoxide Complex of Titanium Oxyfluoride". Inorganic Chemistry 58 (17): 11801–11806. doi:10.1021/acs.inorgchem.9b01880. PMID 31441297. 
  12. Wei, R.; Chen, X.; Gong, Y. (2019). "Side-On Sulfur Monoxide Complexes of Tantalum, Niobium, and Vanadium Oxyfluorides". Inorganic Chemistry 58 (6): 3807–3814. doi:10.1021/acs.inorgchem.8b03411. PMID 30707575. 
  13. Lellouch, E. (1996). "Io's atmosphere: Not yet understood". Icarus 124: 1–21. doi:10.1006/icar.1996.0186. 
  14. Russell, C. T.; Kivelson, M. G. (2000). "Detection of SO in Io's Exosphere". Science 287 (5460): 1998–1999. doi:10.1126/science.287.5460.1998. PMID 10720321. Bibcode2000Sci...287.1998R. 
  15. Na, C. Y.; Esposito, L. W.; Skinner, T. E. (1990). "International Ultraviolet Explorer observations of Venus SO2 and SO". Journal of Geophysical Research 95: 7485–7491. doi:10.1029/JD095iD06p07485. Bibcode1990JGR....95.7485N. 
  16. Lis, D. C.; Mehringer, D. M.; Benford, D.; Gardner, M.; Phillips, T. G.; Bockelée-Morvan, D.; Biver, N.; Colom, P. et al. (1997). "New Molecular Species in Comet C/1995 O1 (Hale–Bopp) Observed with the Caltech S submillimeter Observatory". Earth, Moon, and Planets 78 (1–3): 13–20. doi:10.1023/A:1006281802554. Bibcode1997EM&P...78...13L. 
  18. Gottlieb, C. A.; Gottlieb, E. W.; Litvak, M. M.; Ball, J. A.; Pennfield, H. (1978). "Observations of interstellar sulfur monoxide". Astrophysical Journal 1 (219): 77–94. doi:10.1086/155757. Bibcode1978ApJ...219...77G. 
  19. Moses, J. I.; Zolotov, M. Y.; Fegley, B. (2002). "Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption". Icarus 156 (1): 76–106. doi:10.1006/icar.2001.6758. Bibcode2002Icar..156...76M. 
  20. Marvel, Kevin (1996). "NML Cygni". The Circumstellar Environment of Evolved Stars As Revealed by Studies of Circumstellar Water Masers. Universal Publishers. pp. 182–212. ISBN 978-1-58112-061-5. Retrieved 23 August 2012. 
  21. Balazy, M.; Abu-Yousef, I. A.; Harpp, D. N.; Park, J. (2003). "Identification of carbonyl sulfide and sulfur dioxide in porcine coronary artery by gas chromatography/mass spectrometry, possible relevance to EDHF". Biochemical and Biophysical Research Communications 311 (3): 728–734. doi:10.1016/j.bbrc.2003.10.055. PMID 14623333. 
  22. Malischewski, Moritz; Seppelt, Konrad (2017). "Isolation and Characterization of a Non-Rigid Hexamethylbenzene-SO2+ Complex" (in en). Angewandte Chemie International Edition 56 (52): 16495–16497. doi:10.1002/anie.201708552. ISSN 1433-7851. PMID 29084371. 
  23. 23.0 23.1 Lovas, F. J.; Tiemann, E.; Johnson, D. R. (1974). "Spectroscopic studies of the SO2 discharge system. II. Microwave spectrum of the SO dimer". The Journal of Chemical Physics 60 (12): 5005–5010. doi:10.1063/1.1681015. Bibcode1974JChPh..60.5005L.