Theory as a viable partner for experiment—The quest for trivalent silylium ions in solution

Abstract

This chapter presents the debate in connection with the nature of silylium ions in condensed phases and the role that quantum chemical investigations played and still play in this debate. It is not so difficult to design a silylium cation that by strong internal complexation should be unable to interact with solvent molecules. The question on the existence of a free silylium cation in solution is approached in the chapter by clarifying the properties that a free silylium cation in solution should have and the ways in which these properties can bedetermined by either experimental or theoretical means. After these basic considerations, the chapter discusses and evaluates the various attempts of preparing free silylium cations in solution. There is a large overlap between the silylium cation problem and the problem of investigating ion solvation in general. Chemists understand the various steps in a solvation process, the accompanying changes in the properties of the solute, and its consequences for chemical reactivity. Solvent–solute interactions span the broad spectrum from Van der Waals type interactions to bonding between the interacting molecules.