The geometries and vibrational frequencies are reported for the ground and low-lying states of MgC2H2+ and MgC2H4+. The bonding in the X2A1 ground state and in the 2B1 and (2)2A1 excited states is mainly electrostatic. For both ligands, the 2B1 state is more strongly bound (computed relative to its own asymptote) than the ground state, because the Mg out-of-plane 3p orbital has a smaller overlap and hence a smaller repulsion with the ligand than does the Mg 3s orbital. In the 2B2 excited state, Mg+ inserts into the ligand π bond, which leads to a much larger binding energy than that of the ground state or the 2B1 state. © 1994.