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Disulfur dioxide

Chemical compound From Wikipedia, the free encyclopedia

Disulfur dioxide
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Disulfur dioxide, dimeric sulfur monoxide or SO dimer is an oxide of sulfur with the formula S2O2.[2] The solid is unstable with a lifetime of a few seconds at room temperature.[3]

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Structure

Disulfur dioxide adopts a cis planar structure with C2v symmetry. The S−O bond length is 145.8 pm, shorter than in sulfur monoxide. The S−S bond length is 202.45 pm and the O−S−S angle is 112.7°. S2O2 has a dipole moment of 3.17 D.[4] It is an asymmetric top molecule.[1][5]

The electronic ground state is a singlet, unlike disulfur or dioxygen.[6]

Formation

Sulfur monoxide (SO) converts to disulfur dioxide (S2O2) spontaneously and reversibly.[4] So the substance can be generated by methods that produce sulfur monoxide. Disulfur dioxide has also been formed by an electric discharge in sulfur dioxide.[5] Another laboratory procedure is to react oxygen atoms with carbonyl sulfide or carbon disulfide vapour.[7]

Although most forms of elemental sulfur (S8 and other rings and chains) do not combine with SO2, atomic sulfur does so to form sulfur monoxide, which dimerizes:[8]

S + SO2 → S2O2 ⇌ 2 SO

Disulfur dioxide is also produced upon a microwave discharge in sulfur dioxide diluted in helium.[9] At a pressure of 0.1 mmHg (13 Pa), five percent of the result is S2O2.[10]

Disulfur dioxide is formed transiently when hydrogen sulfide and oxygen undergo flash photolysis.[11]

A branched isomer isoelectronic to SO3, S=SO2, is believed to form during the thermal decomposition of cyclic vicinal alkyl thiosulfites.[12]

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Properties

The ionization energy of disulfur dioxide is 9.93±0.02 eV.[7]

Disulfur dioxide absorbs at 320–400 nm, as observed of the Venusian atmosphere,[13] and is believed to have contributed to the greenhouse effect on that planet.[14]

Reactions and decomposition

Although disulfur dioxide exists in equilibrium with sulfur monoxide, it also reacts with sulfur monoxide to form sulfur dioxide and disulfur monoxide.[9][15]

Decomposition of S2O2 proceeds via the following disproportionation reaction:

S2O2 → SO2 + 1/8 S8

Complexes

S2O2 can be a ligand with transition metals. It binds in the η2-S–S position with both sulfur atoms linked to the metal atom.[16] This was first shown in 2003. The bis(trimethylphosphine) thiirane S-oxide complex of platinum, when heated in toluene at 110 °C loses ethylene, and forms a complex with S2O2: (Ph3P)2Pt(S2O2).[17] Iridium atoms can also form a complex: cis-[(dppe)2IrS2]Cl with sodium periodate oxidizes to [(dppe)2IrS2O] and then to [(dppe)2IrS2O2], with dppe being 1,2-bis(diphenylphosphino)ethane.[18][19] This substance has the S2O2 in a cis position. The same conditions can make a trans complex, but this contains two separate SO radicals instead. The iridium complex can be decomposed with triphenylphosphine to form triphenylphosphine oxide and triphenylphosphine sulfide.[18]

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Anion

The S
2
O
2
radical anion has been observed in the gas phase. It may adopt a trigonal shape akin to SO3.[20]

Spectrum

Microwave

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In the Solar System

There is some evidence that disulfur dioxide may be a small component in the atmosphere of Venus, and that it may substantially contribute of the planet's severe greenhouse effect.[13] It is not found in any substantive quantity in Earth's atmosphere.

References

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