The structure, chemical bonding, and thermodynamics of alkali ions in M[12-crown-4](+), M[15-crown-5](+), and M[18-crown-6](+), M[UO2(O-2)(OH2)(2)](4,5)(+), and M[UO2(O-2)(OH)(OH2)]n(1)n (n = 4, 5) complexes have been explored by using quantum chemical (QC) calculations at the ab initio level. The chemical bonding has been studied in the gas phase in order to eliminate solvent effects. QTAIM analysis demonstrates features that are very similar in all complexes and typical for electrostatic M-O bonds, but with the M-O bonds in the uranyl peroxide systems about 20 kJ mol(-1) stronger than in the corresponding crown ether complexes. The regular decrease in bond strength with increasing M-O bond distance is consistent with predominantly electrostatic contributions. Energy decomposition of the reaction energies in the gas phase and solvent demonstrates that the predominant component of the total attractive (Delta E-elec + Delta E-orb) energy contribution is the electrostatic component. There are no steric constraints for coordination of large cations to small rings, because the M+ ions are located outside the ring plane, [O-n], formed by the oxygen donors in the ligands; coordination of ions smaller than the ligand cavity results in longer than normal MO distances or in a change in the number of bonds, both resulting in weaker complexes. The Gibbs energies, enthalpies, and entropies of reaction calculated using the conductor-like screening model, COSMO, to account for solvent effects deviate significantly from experimental values in water, while those in acetonitrile are in much better agreement. Factors that might affect the selectivity are discussed, but our conclusion is that present QC methods are not accurate enough to describe the rather small differences in selectivity, which only amount to 510 kJ mol(-1). We can, however, conclude on the basis of QC and experimental data that M[crown ether](+) complexes in the strongly coordinating water solvent are of outer-sphere type, [M(OH2)n(+)][crown ether], while those in weakly coordinating acetonitrile are of inner-sphere type, [M-crown ether](+). The observation that the M[UO2(O-2)(OH)(OH2)]n(+)n complexes are more stable in solution than those of M[crown ether](+) is an effect of the different charges of the rings.