The molecular structure of the complex (see Figure ), as established by X-ray diffraction studies, confirmed the presence of a bridging carbonate ligand with a µ-k(O):k 2 (Oʹ,Oʹʹ) coordination mode between the two Tm III (Cp ttt )2 fragments, as previously observed for the Sm(III) analogue [Sm(Cp ttt )2(CO3)]. A more detailed description of the structure can be found in the Supporting Information.