Electronic structure of the π-bonded Al–C2H4 complex: Characterization of the ground and low-lying excited states

The equilibrium properties of the π-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 π* orbital of ethylene and (ii) a vacant Al sp hybrid and π 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.