Noncovalent bonding between anions and the pi-cloud of electron-deficient aromatic rings has recently attracted considerable attention. Complexes of anions with the electron deficient s-tetrazine aromatic ring and other binding units (i.e., urea and squaramide) have been studied and compared using both high level MP2/6-311 + G** ab initio and molecular interaction potential with and without polarization (MIPp and MIP, respectively) and molecular electrostatic potential (MEP) calculations, in order to explore the physical nature of the interactions. Our findings reveal that tetrazine is an excellent candidate for constructing anion receptors. (C) 2003 Elsevier Science B.V. All rights reserved.