A set of first principles calculations has been performed to characterize the interaction of a pi bonded organic molecule with a simple s-p metal surface. The structures of the Al(C2H4) and [AI(C2H4)](+) complex molecules, and of the adsorption of an isolated ethylene molecule onto Al(100) were calculated. We find a C(2)H(4)pi-->Al3p interaction to be the most significant in Al(C2H4) and [Al(C2H4)](+) with additional Al 3p --> C(2)H(4)pi* back-donation in Al(C2H4). In complete contrast, no chemical binding of ethylene to Al(100) is observed, but physisorption is predicted. These calculations show how the binding between a pi bond and the pi orbitals of a simple metal atom cannot be extended to describing the binding with a p band of a solid. Our conclusions may be extended to other systems where this interaction could be significant.