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Sulfur monoxide
Skeletal formula of sulfur monoxide	Spacefill model of sulfur monoxide
Names
	IUPAC name Sulfur monoxide[citation needed]
	Systematic IUPAC name Oxidosulfur
Identifiers
CAS Number	13827-32-2 ;
3D model (JSmol)	Interactive image;
Beilstein Reference	7577656
ChEBI	CHEBI:45822 ;
ChEMBL	ChEMBL1236102 ;
ChemSpider	102805 ;
Gmelin Reference	666
MeSH	sulfur+monoxide
PubChem CID	114845;
CompTox Dashboard (EPA)	DTXSID60160558 ;
	InChI InChI=1S/OS/c1-2 Key: XTQHKBHJIVJGKJ-UHFFFAOYSA-N ; InChI=1/OS/c1-2 Key: XTQHKBHJIVJGKJ-UHFFFAOYAK;
	SMILES O=S;
Properties
Chemical formula	SO
Molar mass	48.064 g mol−1
Appearance	Colourless gas
Solubility in water	Reacts
log P	0.155
Thermochemistry
Std molar; entropy (S⦵298)	221.94 J K−1 mol−1
Std enthalpy of; formation (ΔfH⦵298)	5.01 kJ mol−1
Related compounds
Related compounds	Triplet oxygen ; Disulfur
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Sulfur monoxide is an inorganic compound with formula S O. 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

The SO molecule is thermodynamically unstable, converting initially to S₂O₂.^[2] Consequently controlled syntheses typically do not detect the presence of SO proper, but instead the reaction of a chemical trap or the terminal decomposition products of S₂O₂ (sulfur and sulfur dioxide).

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]

C₂H₄SO → C₂H₄ + SO

Yields directly from an episulfoxide are poor, and improve only moderately when the carbons are sterically shielded.^[5] A much better approach decomposes a diaryl cyclic trisulfide oxide, C₁₀H₆S₃O, produced from thionyl chloride and the dithiol.^[6]

SO inserts into alkenes, alkynes and dienes producing thiiranes, molecules with three-membered rings containing sulfur.^[7]

Sulfur monoxide may form transiently during the metallic reduction of thionyl bromide.^[8]

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.^[9]

Benner and Stedman developed a chemiluminescence detector for sulfur via the reaction between sulfur monoxide and ozone:^[10]

SO + O₃ → SO₂* + O₂

SO₂* → SO₂ + hν

(* indicates an excited state)

Occurrence

Ligand for transition metals

As a ligand SO can bond in a number different ways:^[11]^[12]

a terminal ligand, with a bent M−O−S arrangement, for example with titanium oxyfluoride^[13]
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.^[14]

Astrochemistry

Sulfur monoxide has been detected around Io, one of Jupiter's moons, both in the atmosphere^[15] and in the plasma torus.^[16] It has also been found in the atmosphere of Venus,^[17] in Comet Hale–Bopp,^[18] in 67P/Churyumov–Gerasimenko,^[19] and in the interstellar medium.^[20]

On Io, SO is thought to be produced both by volcanic and photochemical routes. The principal photochemical reactions are proposed as follows:^[21]

O + S₂ → S + SO

SO₂ → SO + O

Sulfur monoxide has been found in NML Cygni.^[22]

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.^[23]

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.^[24]

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

Disulfur dioxide

SO converts to disulfur dioxide (S₂O₂).^[25] 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.^[25]

References

^ "sulfur monoxide (CHEBI:45822)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute.
^ ^a ^b ^c ^d Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ 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.
^ 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.
^ Harpp, David N. (1997). "The sulfur diatomics". Phosphorus, Sulfur, and Silicon. 120. Amsterdam, NL: Gordon & Breach: 53. doi:10.1080/10426509708545509.
^ 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.
^ 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.
^ Magee, Philip S. (1971). "The Sulfur–Bromine Bond". In Senning, Alexander (ed.). Sulfur in Organic and Inorganic Chemistry. Vol. 1. New York: Marcel Dekker. pp. 271–276. ISBN 0-8247-1615-9. LCCN 70-154612.
^ 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. Bibcode:2004ASAJ..116.2540S. doi:10.1121/1.4785135.
^ Benner, R. L.; Stedman, D. H. (1994). "Chemical Mechanism and Efficiency of the Sulfur Chemiluminescence Detector". Applied Spectroscopy. 48 (7): 848–851. Bibcode:1994ApSpe..48..848B. doi:10.1366/0003702944029901. S2CID 98849015.
^ Schenk, W. A. (1987). "Sulfur Oxides as Ligands in Coordination Compounds. Angewandte Chemie International Edition in English". 26: 98–109. doi:10.1002/anie.198700981. {{cite journal}}: Cite journal requires |journal= (help)
^ Woollins, J. D. (1995). "Sulfur: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry. John Wiley and Sons. ISBN 0-471-93620-0.
^ 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. S2CID 201617509.
^ 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. S2CID 73438027.
^ Lellouch, E. (1996). "Io's atmosphere: Not yet understood". Icarus. 124: 1–21. doi:10.1006/icar.1996.0186.
^ Russell, C. T.; Kivelson, M. G. (2000). "Detection of SO in Io's Exosphere". Science. 287 (5460): 1998–1999. Bibcode:2000Sci...287.1998R. doi:10.1126/science.287.5460.1998. PMID 10720321.
^ 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. Bibcode:1990JGR....95.7485N. doi:10.1029/JD095iD06p07485.
^ Lis, D. C.; Mehringer, D. M.; Benford, D.; Gardner, M.; Phillips, T. G.; Bockelée-Morvan, D.; Biver, N.; Colom, P.; Crovisier, J.; Despois, D.; Rauer, H. (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. Bibcode:1997EM&P...78...13L. doi:10.1023/A:1006281802554. S2CID 51862359.
^ "Астрономический вестник. T. 54, Номер 2, 2020".
^ 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. Bibcode:1978ApJ...219...77G. doi:10.1086/155757.
^ 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. Bibcode:2002Icar..156...76M. doi:10.1006/icar.2001.6758.
^ 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.
^ 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.
^ Malischewski, Moritz; Seppelt, Konrad (2017). "Isolation and Characterization of a Non-Rigid Hexamethylbenzene-SO²⁺ Complex" (PDF). Angewandte Chemie International Edition. 56 (52): 16495–16497. doi:10.1002/anie.201708552. ISSN 1433-7851. PMID 29084371. S2CID 27260554.
^ ^a ^b 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. Bibcode:1974JChPh..60.5005L. doi:10.1063/1.1681015.