Adducts of S/Se Donors with Dihalogens as a Source of Information for Categorizing the Halogen Bonding

The great variety of products ensuing from the reactions between chalcogen donors and dihalogens and their assemblies in the crystal lattice provides a wide experimental basis useful to disembroil the ongoing debate on how to define the halogen bonding. In this paper we present a critical analysis/study of the structural features retrieved from the Cambridge Structural Database (CSD) for linear three-body systems involving either halogens, X-X-X (X = Br, I), or halogen(s) and chalcogen(s) atoms, E-X-Y, X-E-Y, and E-X-E (E = S, Se; X = Y = Cl, Br, I; X = I, Y = Cl, Br, I). The relative elongations (delta) of the two bonds in the examined three-body systems with respect to the sum of the relevant atomic radii can be fitted by a common nonlinear least-squares equation derived from the bond-valence model. The similarities observed in the structural features suggest a common nature of the chemical bond in all systems considered and indicate that the charge transfer and the 3c-4e models can be successfully applied to all the cases considered to explain the nature of the chemical bonding.