Mechanisms of inversion at pyramidal sulfur atoms in three-coordinate sulfur compounds

Pyramidal sulfur atoms in three-coordinate sulfur compounds undergo atomic inversion via various pathways. The unimolecular mechanisms of sulfur inver sion are classified as either a vertex inversion through a planar transitio n state (nondissociative mechanism) or as a mechanism involving homolytic o r heterolytic dissociation of one of the sulfur-to-ligand bonds (dissociati ve mechanism). The inversion mechanism can be assigned by the analyses of k inetic data determined by various experimental methods: the inversion mecha nism is reflected in the entropy of activation, solvent effect and volume o f activation. The general features in kinetic data for each mechanism are s ummarized to facilitate elucidation of the mechanism. The scope and limitat ions of the guidelines are reviewed through typical studies on the sulfur i nversion in sulfonium salts, sulfoxides and metal-sulfide complexes.