The halogen bond is a powerful tool for the molecular design and pushing the limits of its strength is of major interest. Bearing the most potent halogen‐bond donor atom, astatine monoiodide (AtI) was recently successfully probed [Nat. Chem. 2018, 10, 428–434]. In this work, we continue the exploration of adducts between AtI and Lewis bases with the tributylphosphine oxide (Bu3PO) ligand, revealing the unexpected experimental occurrence of two distinct chemical species with 1:1 and 2:1 stoichiometries. The 1:1 Bu3PO⋅⋅⋅AtI complex is found to exhibit the strongest astatine‐mediated halogen bond so far (with a formation constant of 10(4.24±0.35)). Quantum chemical calculations unveil the intriguing nature of the 2:1 2Bu3PO⋅⋅⋅AtI adduct, involving a halogen bond between AtI and one Bu3PO molecular unit plus CH⋅⋅⋅O hydrogen bonds chelating the second Bu3PO unit.

As a non-covalent interaction, halogen bonding is now acknowledged to be useful in all fields where the control of intermolecular recognition plays a pivotal role. Halogen-bond basicity scales allow to...