Optimized geometries and vibrational frequencies were calculated for Mg+-X and [X-Mg-Y](+) complexes (X, Y = H2O, N2O, CO2, O-2, and O), required for understanding the chemistry of magnesium in the upper atmosphere. B3LYP optimizations were performed employing 6-311+G(2d,p) basis sets. In several cases a number of different orientations were investigated in order to determine the geometries of lowest energy, and in cases involving O and O-2, different spin states also had to be considered. In contrast to the corresponding Ca+-containing complexes, the diligated Mg+ ions have the two ligands approaching from the same side. In order to establish accurate energetics, up to RCCSD(T) single-point energy calculations were also employed, using quadruple-basis sets. Accurate dissociation energies for the Mg+-X and [X-Mg-Y](+) species were derived and discussed. Comparison with available experimental results was made where possible.