Carbonate is generally a strongly complexing ligand for f-element ions, including trivalent lanthanides. The few lanthanide(III) carbonates previously characterized have extended multidimensional solid stare structures. Spectroscopic and solubility studies indicate that the solution species are monomeric; however, structural details remain unknown. We report the first single-crystal structure of a discrete mononuclear Nd(III) carbonate, the guanidinium salt of Nd(CO3)(4)(H2O)(5-), which corresponds directly to a species formed in highly concentrated carbonate solutions.