Anion - pi bonding.
The interaction between the centroid of electron poor aromatic rings (e.g. triazine) and anions (fluoride, chloride, azide) was first described by us. A high-level theoretical study demonstrated that a bonding interaction existed which was similar in magnitude to the intensively studied aromatic cation-pi bond. This seminal work has led to numerous studies of related systems and to a series of papers in which the interaction was demonstrated experimentally.
Lead references:
"Anion-Aromatic Bonding: A Case for Anion Recognition by p-Acidic Rings." M. Mascal, A. Armstrong, M. D. Bartberger, J. Am. Chem. Soc. 2002, 124, 6274.
“Precedent and Theory Unite in the Hypothesis of a Highly Selective Fluoride Ion Receptor.” M. Mascal, Angew. Chem. Int. Ed. 2006, 45, 2890.