On the σ, π and δ hole interactions: a molecular orbital overview

By means of molecular orbital theory and the analysis of charge transfer and electrostatic forces, we discuss the features of non-covalent hole interactions of the halogen, chalcogen and pnictogen bond families. The use of MOs allows us to explain and predict the location of holes, and to design novel interactions such as systems with sigma and pi holes on the same or opposite sides. In view of the orbital origin of the hole interactions, we suggest that many chalcogen and pnictogen bonds are largely based on pi holes and not on the commonly accepted sigma holes. In addition, a new type of hole interaction based on delta holes is found on the sextuply bonded dimolybdenum.