MP2(FULL)/6-311++G** calculations are performed on the cation–π complexes of Li⁺ and Mg²⁺ with the π-face of linear (ethylene, butadiene, hexatriene, and octatetraene) and cyclic (benzene, naphthalene, anthracene, phenanthrene and naphthacene) unsaturated hydrocarbons. The interaction energy is found to increase systematically as the size of the π-system increases. The higher interaction energy is in good correlation with the extent of charge transfer. The increase in the interaction energy is more dramatic in the case of acyclic systems. The computations reveal that larger π-systems tend to have higher complexation energy with the metal ions, which will have important implications in our understanding of the structural and functional aspects of metal binding.