Quantum chemical calculations on the Li-C2H4 complex have been performed with coupled-cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be B-2(2), With a C-2 upsilon symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including C-13/C-12, Li-7/Li-6, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li-C stretching mode. These calculations also allow to propose an assignment for the observed C-H/C-D stretching modes. The observed blue-shift of the symmetric CH2 bending mode as well as the red-shift of the antisymmetric CH2 bending, CD2 bending, and C-C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na ... C2H4 complex has been found to be unstable in its B-2(2) electronic state. The study of the (2)A(1) electronic state for both Na ... C2H4 and K ... C2H4 complexes show that they are at most very weak van der Waals complexes, This result confirms the conclusions of matrix isolation experiments.