The calculations with 'three-state model' on the complexation between alkaline earth metal atoms and 1,3,5,7-cyclooctatetraene (COT) show that the metal atoms transfer their electrons to COT via concerted adiabatic two-electron transfer mechanism, to form cation-pi complexes. The MP2/6-311++G** calculated interaction strength and distance are in scale with chemical bonding. Decomposition on the binding energy showed that electrostatic is the main component. Thus, it could be concluded that the interaction between the alkaline earth metal atoms and COT is a chemical bond, namely cation-pi bond, which is ionic in nature. (c) 2006 Elsevier B.V. All rights reserved.