A G2 study of SH⁺ exchange reactions involving lone-pair donors and unsaturated hydrocarbons

The ligand exchange reactions between mono-adducts of the sulfenium ion ([SH-X]⁺) and either unsaturated hydrocarbons or lone-pair donors have been investigated computationally at the G2 level. The mono-adducts react with acetylene or ethylene to form a thiiranium or a thiirenium ion, in most cases without an overall barrier. In the reactions involving lone-pair donors, the original lone-pair donor is expelled from the [SH-X]⁺ mono-adduct with the formation of a new mono-adduct. The reaction proceeds in this case via an intermediate di-adduct. Both the hydrocarbon and the lone-pair donor attack the mono-adduct with the relevant orbitals aligned in a near-collinear fashion, as was also the case for previously investigated reactions involving PH₂⁺ and Cl⁺. The reaction energies and the binding energies of the intermediate complexes in the exchange reactions are primarily determined by the electronegativities of the lone-pair donors. The thermochemical data can be rationalized within the framework of qualitative molecular orbital theory, and the results are compared with our previous findings for the corresponding reactions involving PH₂⁺ and Cl⁺.