Revisiting the covalent nature of halogen bonding: a polarized three-center four-electron bond

As an important intermolecular interaction, halogen bonding has been studied extensively, but its nature still suffers from controversy without one uniform essence. Electrostatics, charge transfer, polarization and dispersion are emphasized, but the covalent nature is usually overlooked except for the strong halogen bonding species I₃ ^-, which is widely accepted as a result of a three-center four-electron (3c-4e) interaction. In our study, the potential energy surface of I₃ ^- has been evaluated to explore the dissociation from I₃ ^- to I₂⋯I^-. We found that different from an equivalent 3c-4e bond in I₃ ^-, I₂⋯I^- can be rationalized by a polarized one. In addition, when the orbitals are polarized, it is exactly what traditional charge transfer or the popular σ-hole picture describes. I₃ ^- can be described by the Lewis theory model with the middle I⁺ cation serving as the Lewis acid and two terminal I^- anions acting as Lewis base. Therefore, we further extended this model to a series of I-containing species with chemical composition of L-I⁺-L, F^--I⁺-L and H₃P-I⁺-L (L = OH^-, F^-, Cl^-, Br^-, I^-, PH₃, NH₃, H₂S, HI, H₂O, HBr and HCl) to explore the nature of halogen bonding. When the forces of two bases around I⁺ are the same, it corresponds to an equivalent 3c-4e bond, such as I₃ ^-. Otherwise, it is a polarized multicenter bond, such as I₂⋯I^-. This work gives a new insight into the nature of halogen bonding compounds: besides the well-known I₃ ^-, the nature of the other species is also a multicenter bond, existing as equivalent and polarized 3c-4e bonds, respectively.