Sulfoxides feature a pyramidal sulfur center with relatively short S-O distances. In DMSO, the S-O distance is 1.531 Å.
Sulfoxides are generally represented with the structural formula R–S(=O)–R', where R and R' are organic groups. The bond between the sulfur and oxygen atoms is intermediate of a dative bond and a polarized double bond. The double-bond resonance form implies 10 electrons around sulfur (10-S-3 in N-X-L notation). However, as is true for other formally hypervalent species, octet expansion by d orbital participation is no longer believed to be an important contributor to the electronic structure of sulfoxides. Instead, any double bond character of the S–O bond may be accounted for by donation of electron density into C–S antibonding orbitals ('no-bond' resonance forms in valence bond language). Nevertheless, due to its simplicity and lack of ambiguity, the IUPAC recommends use of the expanded octet double-bond structure to depict sulfoxides, rather than the dipolar structure or structures that invoke "no-bond" resonance contributors. The S–O interaction has an electrostatic aspect, resulting in significant dipolar character, with negative charge centered on oxygen.
A lone pair of electrons resides on the sulfur atom giving it tetrahedral electron pair geometry and trigonal pyramidal shape (steric number 4 with one lone pair; see VSEPR theory). When the two organic residues are dissimilar, the sulfur is a chiral center, for example, methyl phenyl sulfoxide.
The energy required to invert this stereocenter is sufficiently high that sulfoxides are optically stable, that is, the rate of racemization is slow at room temperature.
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