The noncovalent interaction between Lewis bases and Lewis acidic chalcogen atoms in organic or inorganic compounds is defined as chalcogen bonding (ChB).^1–7 

This interaction is closely related to hydrogen bonding (HB)⁸  and halogen bonding (XB).^9,10  Hydrogen bonding in particular has been strongly investigated in recent decades,^11–13  followed by an increased research interest in halogen bonding.^10,14,15  However, chalcogen bonding has been mostly neglected although it has been known from nature for decades.^16–18 

One illustrative example shows the ability of chalcogens to act as Lewis bases (as already commonly known), and as Lewis acids: the complex of iodine with diphenyl diselenide.¹⁹  This complex with a square geometry demonstrates the interrelated interactions of halogen bonding alongside chalcogen bonding (Figure 19.1). First of all, the complex contains two strong halogen bonds with intermolecular distances of 2.99 Å (3.88 Å is the sum of van der Waals radii²⁰ ), in which the chalcogens act as Lewis bases. But also the second face of chalcogens is observed, in which the selenium centers act as Lewis acids and form noncovalent bonds with the Lewis basic region of iodine, with a distance of 3.59 Å (which is again shorter than the sum of the van der Waals radii). Therefore, the complex nicely showcases the amphiphilic behavior of both halogens and chalcogens.