Abstract The structure and binding energies are determined for Mg(CO 2) + n and Sr(CO 2) + n for n = 1 and 2. We also consider Mg + 2 and Mg 2CO + 2 to compare the binding of CO 2 to a single metal ion with the binding to a diatomic ion. The vertical excitation energies are computed for all species. The potential energy curves for the low-lying states of Mg + 2 are reported. The MgCO + 2 results are in good agreement with the experimental results of Duncan and co-workers.