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. 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: as well as more complex examples, such as a trisulfide oxide, C10H6S3O. 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. 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. Benner and Stedman developed a chemiluminescence detector for sulfur via the reaction between sulfur monoxide and ozone: SO + O3 → SO2* + O2 SO2* → SO2 + hν Occurrence Ligand for transition metals As a ligand SO can bond in a number different ways: a terminal ligand, with a bent M−O−S arrangement, for example with titanium oxyfluoride 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. Astrochemistry Sulfur monoxide has been detected around Io, one of Jupiter's moons, both in the atmosphere and in the plasma torus. It has also been found in the atmosphere of Venus, in Comet Hale–Bopp, in 67P/Churyumov–Gerasimenko, and in the interstellar medium. On Io, SO is thought to be produced