There has been an upsurge of interest in the halogen bond during the last decade. This non-covalent interaction is less familiar than the hydrogen bond, but is similar to it in several respects. In this article, we first discuss the nature of the halogen bond in the gas phase, as established by systematic investigations of the rotational spectra of complexes B⋯XY, where B is a simple Lewis base and XY is a dihalogen molecule. The geometry of a given B⋯XY is found to be isomorphic with that of the corresponding hydrogen-bonded system B⋯HX, an observation that leads an interim definition of the halogen bond similar to that recently proposed for the hydrogen bond. Selected novel applications of the halogen bond made in the last decade in various areas of chemistry/materials (namely crystal engineering, liquid crystals, nano-materials, polymer chemistry and inorganic chemistry) are then reviewed. These applications generally involve molecules of the type XR (where R is an electron-withdrawing group) acting as the electron donor, rather than dihalogens XY.

Tetrel, pnictogen and chalcogen-bonded complexes: old bonds but new names.

Many complexes of the type B⋯MX, (where B is a Lewis base such as H₂, N₂, ethyne, ethene, cyclopropane, H₂O, H₂S, PH₃, or NH₃, M is a coinage-metal atom Cu, Ag or Au, and X is a halogen atom) have now been characterised in the gas phase through their rotational spectra. It is pointed out that, for a given B, such complexes have angular geometries that are isomorphous with those of their hydrogen- and halogen-bonded counterparts B⋯HX and B⋯XY, respectively. Since the MX are, like the B, HX and XY referred to, closed-shell molecules, the complexes B⋯MX also involve a non-covalent bond. Therefore, the name ‘coinage-metal’ bond is suggested for the non-covalent interaction in B⋯MX, by analogy with hydrogen and halogen bonds. A generalised definition that covers all non-covalent bonds is also presented.

Geometries, equilibrium dissociation energies (D_e), and intermolecular stretching, quadratic force constants (k_σ) are presented for the complexes B⋯CO₂, B⋯N₂O, and B⋯CS₂, where B is one of the following Lewis bases: CO, HCCH, H₂S, HCN, H₂O, PH₃, and NH₃. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of D_e employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B⋯CO₂ complexes involves the interaction of the electrophilic region around the C atom of CO₂ (as revealed by the molecular electrostatic surface potential (MESP) of CO₂) with non-bonding or π-bonding electron pairs of B. The conclusions for the B⋯N₂O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B⋯CS₂ series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C_∞ axis of CS₂ (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B⋯CO₂, B⋯N₂O, and B⋯CS₂, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B⋯HX and B⋯XY. It is also shown that the dissociation energy D_e is directly proportional to the force constant k_σ, with a constant of proportionality identical within experimental error to that found previously for many B⋯HX and B⋯XY complexes.

The dissociation energy D_σ is directly proportional to the intermolecular stretching force constant k_σ for many hydrogen- and halogen-bonded complexes.