Three isostructural hexanuclear lanthanide(III) clusters are reported (Ln(III) = Gd, Tb, and Dy). The metallic core of each complex displays an unusual arrangement of ions, which is stabilized by a mu(6)-carbonate ligand. Magnetic studies show that the Ln(III) ions in each compound are weakly exchange coupled, with the Tb and Dy analogues displaying single-molecule-magnet behavior.