Homoatomic Sulfur Cations

Abstract

This chapter gives an account on the recent achievements in an understanding of the synthesis, solution behavior, structure and bonding of the homopolyatomic sulfur cations. A focus is put on the developments aimed to understand experimental observations, i.e., quantum chemical calculations of these electronically delicate species. The synthesis of the highly electrophilic sulfur cations requires the use of very weakly basic conditions throughout, that is weakly coordinating counterions such as AsF₆ ⁻, SbF₆ ⁻, Sb₂F₁₁ ⁻ as well as weakly basic solvents such as SO₂, HF, HSO₃F. Structure and bonding of the currently structurally characterized S₄ ²⁺, S₈ ²⁺, and S₁₉ ²⁺ cations are governed by positive charge delocalization brought about by 3p_π-3p_π, π^*-π^* and 3p²→3σ^* bonding interactions. The solution behavior of S₈ ²⁺ salts, which give rise to several sulfur radical cations such as S₅ ^·+, was analyzed in detail based on (calculated) thermodynamic as well as spectroscopic considerations and hitherto unknown D _3d symmetric S₆ ²⁺ and D _4h symmetric S₄ ^·+ are likely players in SO₂ solutions of S₈(AsF₆)₂.