The nature of the cation−π interaction has been examined by carrying out high level ab initio calculations of both metallic (Li+,Na+,K+, and Ag+) and organic (NH4+, C(NH2)3+, and N(CH3)4+) cations with different classes of π systems, viz. alkenes (ethene), arenes (benzene), and heteroarenes (pyrrole). The calculations, which include a rigorous decomposition of the interaction energies, indicate that the interaction of these π systems with the metal cations is characterized by contributions from both electrostatic and induction energies, with the contribution of the latter being dominant. Though the contributions of dispersion energies are negligible in the cation−π complexes involving Li+ and Na+, they assume significant proportions in the complexes involving K+ and Ag+. In the π complexes of the organic cations, the repulsive exchange contributions are much larger than the attractive electrostatic contributions in the π complexes of organic cations, and thus, the contributions of both induction and dispe...