The solid-state structures of 1-Ln were determined by single crystal XRD (1-Nd is depicted in Figure , see Supporting Information Figures ). The structural data of 1-La and 1-Nd are reported here for the first time, whilst 1-Ce and 1-Sm have been structurally authenticated previously. As expected, the structures of 1-Ln are trigonal planar with respect to the η 5 -Cp tt centroids, with the three C2 atoms (Figure ) in the plane defined by the Ln 3+ ion and the three Cp tt centroids. The three Cp tt ligands adopt the same orientation to form a "picket-fence" motif with three t Bu groups above and three below the trigonal plane. Complexes 1-Ln do not show high symmetry in the solid state, consistent with the solid-state structures of [M(Cp tt )3] (M = Th, U, which has an analogous electronic configuration (Nd 3+ : [Xe]4f 3 ; U 3+ : [Rn]5f 3 ). Conversely, in 1-Ce, the mean M … Cpcentroid distances are longer than those seen in [Th(Cp tt )3] [2.566 Å], which has a different valence electronic configuration (Ce 3+ : [Xe]4f 1 ; Th 3+ : [Rn]6d 1 ) due to the near-degeneracy of the 5f and 6d orbitals for Th 2 and the stabilization of the 6dz2 orbital in trigonal ligand environments.