We report a series of ab initio QM calculations on uranyl and Sr2+ complexes of O=PR3 ligands (R = H Me Ph) to assess the role of substituents R and of NO3- counterions on the intrinsic cation-ligand interaction energy. When there are no counterions, the binding sequence of UO22+ and of Sr2+ complexes follows the order R = H < Me ( Ph, due to polarization and charge-transfer effects. However, in the presence of NO3- counterions, the OPMe3 and OPPh3 complexes become of similar stability, due to the ligand-anion repulsive interactions. Complexes of OPR3 with the spherical Sr2+ cation are found to be less stable than those with the linear UO22+ cation. In the second part of the paper we report molecular dynamics simulations in water on 1:1 and 2:1 complexes of OPR3 with UO2(NO3)(2). The changes in free energies of solvation upon electronic reorganization of the ligand and UO2(NO3)(2) induced by complexation are investigated using statistical perturbation FEP techniques and found to be nearly independent of R, The importance of these results in the context of designing efficient ionophores for uranyl cations is discussed.