The results of a study on the ground states of tricarbonato complexes of dioxouranate using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to uranium in the formal oxidation states VI and V, [UO₂(CO₃)₃]^4- and [UO₂(CO₃)₃],^5- have been fully optimized in D_3h symmetry at second-order perturbation theory (MBPT2) level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. The uranyl fragment has also been optimized at CASSCF/CASPT2, to obtain an estimate of the MBPT2 error. Finally, the effect of distorting the D_3h symmetry to C₃ has been investigated. This study shows that only minor geometrical rearrangements occur in the one-electron reduction of [UO₂(CO₃)₃]^4- to [UO₂(CO₃)₃],^5- confirming the reversibility of this reduction.