According to the IUPAC definition,¹  a chalcogen bond (ChB) is a net attractive interaction R–Ch⋯A occurring between an electron-rich donor site A and an electron-poor chalcogen atom (Ch) included in a R–Ch molecule or ion (R = organic framework). Although chalcogen atoms can also behave as electron-rich species, the term chalcogen bond is reserved to the former case. The definition does not imply any model for the ChB, and the nature of the interaction is a current matter of debate.

In a molecular orbital (MO) view, the interaction between a ChB donor R–Ch and a ChB acceptor A to give a RCh⋯A assembly can be interpreted as a charge transfer from a LP of the Lewis base A to a virtual σ*-MO localized on a R–Ch bond. In the case of double-bonded sp²-hybridized and hypercoordinated chalcogen species, a slightly different CT scheme occurs. In such cases, charge is transferred from the LP on A to a π*-MO, as reflected in the formation of remarkably bent ChB adducts.²