Halogen Bonds of Halogen(I) Ions―Where Are We and Where to Go?

Halenium ions, X+, are particularly strong halogen-bond donors that interact with two Lewis bases simultaneously to form linear [D<middle dot><middle dot><middle dot>X<middle dot><middle dot><middle dot>D](+)-type halonium complexes. Their three-center, four-electron halogen bond is both fundamentally interesting and technologically valuable as it tames the reactivity of halogen(I) ions, opening up new horizons in a variety of fields including synthetic organic and supramolecular chemistry. Understanding this bonding situation enables the development of improved halogen(I) transfer reactions and of advanced functional materials. Following a decade of investigations of basic principles, the range of applications is now rapidly widening. In this Perspective, we assess the status of the field and identify its key advances and the main bottlenecks. Clearing common misunderstandings that may hinder future progress, we aim to inspire and direct future research efforts.