When HCN ligands are attached to the halonium cation, where would they be bonded? Does the σ-hole steer the trajectory of interaction? The cases of single and double σ-hole halonium cations are considered.

This study explores the fundamental nature of interactions between halonium cations—modeled as fragments of real crystal structures—and Lewis bases, with hydrogen cyanide (HCN) serving as a representative ligand. Two types of halonium cation monomers, namely the iodonium ion of adamantylideneadamantane and ethynyl(phenyl)-λ³-iodane, along with their chlorine and bromine analogues, are examined. Complexes formed with HCN molecules are investigated using quantum chemical calculations and topological analyses of electron density. The findings indicate that the σ-hole plays a decisive role in directing both the geometry and strength of these noncovalent interactions. Systems featuring a single σ-hole exhibit reduced stabilization upon additional ligand coordination, often weakening the primary halogen bond. In contrast, cations with two σ-holes can accommodate up to three or four HCN molecules without compromising the integrity of the original halogen bonds. Nevertheless, the most favorable configuration remains the directional binding of two ligands to two distinct σ-holes.