The density functional theory (DFT) has been used to reexamine the Al-C2H4 complex because of discrepancies between the results of post-Hartree-Fock methods concerning the binding energy and the ordering of the metal-ligand stretching frequencies. In this study, equilibrium geometry, binding energy, and harmonic frequencies have been calculated using the 6-311G(2d,2p) basis set, It is shown that the Al-C2H4 complex has a C-2v symmetry equilibrium structure and a B-2(2) ground electronic state, which is strongly bound by -13.3 kcal/mol after BSSE correction (to be compared to the -16 kcal/mol experimental value). The bonding in the Al-C2H4 complex has been investigated by the electron localization function (ELF). The aluminum-ethylene bonding is found to be mostly electrostatic. The degree of weakening of the C=C double bond and the ordering of the two metal-carbon stretching modes have been discussed using a harmonic vibrational and force constant analysis and compared to the experimental results. Furthermore, a comparison of the shifting between the two wagging modes for complexed and free ethylene has allowed us, on the basis of isotopic substitutions, to reassign the symmetry of the only observed wagging mode (B-2 instead of A(1)). We have also suggested the reassignment of the experimental band reported at 781 cm(-1) from the B-1 rocking mode to the A(1) symmetric wagging one.