The equilibrium properties of the pi-bonded Al-ethylene complex in its ground state are calculated by coupled-cluster theory. Significant changes in the geometry of the ethylene molecule upon complexation (elongation of the CC bond, pyramidalization of the CH2 groups) are consistent with the formation of a chemical bond between fragments. The overall interaction is rather weak because bonding is derived from the overlap between: (i) a singly occupied p orbital of Al and the antibonding pi* orbital of ethylene and (ii) a vacant Al sp hybrid and pi of C2H4. Electronically excited states are studied by the equation-of-motion coupled-cluster method. The covalent nature of the interaction between fragments is reflected in the excited-state delocalization over both fragments (as opposed to the corresponding van der Waals complex). In the examined energy range (0-5.18 eV) both valence and Rydberg excited states are found. Bonding in the valence states is explained in terms of a simple molecular orbital picture. Two very intense transitions at 3.50 eV and at 3.79 eV can be used as a probe in experimental studies. (C) 2003 American Institute of Physics.