Ab initio molecular orbital calculations on the structure and stability of the nitrate and sulfate and sulfate complexes of uranyl (UO22+) and plutonyl (PuO22+) using effective core potentials are reported. It is found that the binding energy of sulfate is greater than that of nitrate to both uranyl and plutonyl, with a slight preference for plutonyl. A method of decomposing the binding energy into electrostatic, Pauli repulsion, polarization and charge-transfer components is described which predicts that electrostatic forces are dominant. A simple molecular mechanics potential is developed by using this finding, which is successful in reproducing the ab initio results.