AbstractThe crystal and molecular structures of the rare earth carbonates with the general formulae [C(NH2)]3[Ln(CO3)4(H2O)]·2H2O (where Ln = Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+) and [C(NH2)]3[Ln(CO3)4]·2H2O (where Ln = Y3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) were determined. The crystals consist of monomeric [Ln(CO3)4(H2O)]5–or [Ln(CO3)4]5–complex anions in which the carbonate ligands coordinate to the Ln3+ion in a bidentate manner. The spectroscopic (UV/Vis/NIR and IR) properties of the crystalline lanthanide carbonates, as well as their aqueous solutions, were determined. Correlation between the spectroscopic and the structural data enabled us to conclude that the [Ln(CO3)4(OH)]6–and [Ln(CO3)4]5–species predominate in the light and heavy lanthanide solutions, respectively. The nature of the Ln–O interaction was also discussed. The experimental data, as well as the theoretical calculations, indicated that the Ln–O(CO32–) bond is more covalent than the Ln–O(OH2) bond. Moreover, the covalency degree is larger for the heavy lanthanide ions. Inspection of the NBO results revealed that the oxygen hybrids, with the approximate composition sp4, form strongly polarized bonds with the 6s6p5d4hybrids of lutetium.