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

The equilibrium properties of the p-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 p* orbital of ethylene and ~ii! a vacant Al sp hybrid and p 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. © 2003 American Institute of Physics. @DOI: 10.1063/1.1576212#