with the three dots representing the bond. R–X is the XB donor, and X is a halogen atom covalently bound to the R group and having an electrophilic region, or a potentially electrophilic region, on its electrostatic potential surface. It may happen that X is covalently bound to more than one group. In such cases the halogen may also form more than one halogen bond ( Figure Figure6 6 ). Y is the XB acceptor (donor of electron density) and can be an anion or a neutral species possessing at least one nucleophilic region, ., a lone-pair-possessing atom or π-system. This IUPAC definition has been framed as simply and comprehensively as possible to account for all the cases wherein there is evidence of bond formation involving a nucleophile and a positive region on a halogen atom X from a molecule or molecular fragment R–X.