Sulfur monoxide is a chemical compound with formulaSO and CAS number 13827-32-2. It is an unstable species only found in the gas phase where it is in equilibrium with a dimeric form, S2O2 (sometimes called disulfur dioxide) which can be represented as OSSO. Sulfur monoxide has been detected around Io, one of Jupiters moons, both in the atmosphere and in the plasma torus. It has also been found in the atmosphere of Venus, in the Hale-Bopp comet and in interstellar space.
The SO molecule has a triplet ground state similar to dioxygen, O2, with two unpaired electrons. The S−O bond length of 148.1pm is similar to that found in lower sulfur oxides (e.g. S8O, S−O =148pm) but is longer than the S−O bond in gaseous S2O (146pm), SO2 (143.1pm) and SO3(142pm).
The S2O2 dimer is planar molecule with C2v symmetry, with two SO units joined via the sulfur atoms, and the oxygen atoms in a cis configuration. The S-O bond length is 145.8pm, shorter than in the monomer, and the S-S bond length is 202.45pm. The OSS angle is 112.7 degrees. It has a dipole moment, μ = 3.17D.
Production and reactions
In the laboratory sulfur monoxide be made reacting sulfur dioxide with sulfur vapour in a glow discharge  and in single bubble sonoluminescence of concentrated sulfuric acid containing some dissolved noble gas.
Production of the transient SO molecule as a reagent in organic syntheses has been investigated. Research has centred on using compounds that decompose under reaction conditions to "extrude" SO. Examples include the decomposition of the relatively simple molecule thiirane 1-oxide:
C2H4OS → C2H4(g) + SO(g)
as well as more complex examples e.g. a trisulfide oxide, C10H6S3O, 
On Io the production of SO is believed to be both volcanic and also photochemical. The principal photochemical reactions proposed  are:
O + S2 → S + SO
SO2 + hν → SO +O
The SO molecule is thermodynamically unstable . The transient SO molecule can be trapped by the formation of transition metal complexes. As a ligand SO can bond in a number different ways:
σ donation forming a bent bond with the metal
bridging across 2 or 3 metal centres via sulfur
edge on bridging
SO inserts into alkenes, alkynes and dienes producing molecules with three membered rings containing sulfur.
The triplet ground state molecule with two unpaired electrons can be excited by near infrared light to the singlet state with no unpaired electrons. The singlet state is believed be more reactive than the ground state triplet state, in the same way that singlet oxygen is more reactive than the triplet ground state.
Sulfur monoxide may have some biological activity, the formation of transient SO in porcine coronary artery has been inferred from the reaction products.
A chemiluminescence detector for sulfur has been reportedthat is based on the reactions:
SO + O3 → SO2(excited) + O2
SO2(excited) → SO2 + hν
^ abSpectroscopic 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 doi:10.1063/1.1681015
^Io’s atmosphere: Not yet understood Lellouch, E. 1996. Icarus 124, 1–21.
^Detection of SO in Io's Exosphere Russell C.T., Kivelson M.G. Science (2000): 287, 5460, 1998 – 1999, doi:10.1126/science.287.5460.1998
^International Ultraviolet Explorer observations of Venus SO2 and SO Na, Chan Y. ; Esposito, L.W. ; Skinner, T.E; Journal of Geophysical Research ; 95 1990, 7485-7491
^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, N. Biver, P. Colom, J. Crovisier, D.Despois and H.Rauer Earth, Moon, and Planets Volume 78, Numbers 1-3 / July, 1997 doi:10.1023/A:1006281802554
^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 doi:10.1086/155757
^ abcd Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
^The temperatures of single-bubble sonoluminescence (A) Suslick K.S. and Flannigan D.J., The Journal of the Acoustical Society of America ( 2004) 116, 4, 2540
^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 doi:10.1021/ja00784a049
^A novel recyclable sulfur monoxide transfer reagent. Grainger RS, Procopio A, Steed JW. Org Lett. 2001 3(22), 3565-8.
^Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type EruptionMoses J.I., Zolotov M.Y., Fegley B.Icarus 156, 76–106 (2002) doi:10.1006/icar.2001.6758
^Sulfur: Inorganic Chemistry Woollins JD, Encyclopedia of Inorganic Chemistry (1995), John Wiley and Sons ISBN 0-471-93620-0
^[1+2] Cycloadditions of Sulfur Monoxide (SO) to Alkenes and Alkynes and [1+4]Cycloadditions to Dienes (Polyenes). Generation and Reactions of Singlet SO? Juzo Nakayama , Yumi Tajima , Piao Xue-Hua , Yoshiaki Sugihara J. Am. Chem. Soc.; 2007; 129(23) pp 7250 - 7251; (Communication)doi:10.1021/ja072044e
^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. Phys. Chem. 1989, 93, 1285-1292
^Identification of carbonyl sulfide and sulfur dioxide in porcine coronary artery by gas chromatography/mass spectrometry, possible relevance to EDHF Balazy M, Abu-Yousef IA, Harpp DN, Park J.Biochem Biophys Res Commun. 2003 Nov 21;311(3):728-34
^Chemical Mechanism and Efficiency of the Sulfur Chemiluminescence Detector Benner, R. L., Stedman, D. H. Applied Spectroscopy, 48, 7, (1994), 848-851doi:10.1366/0003702944029901