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 D3h 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 D3h 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.