In this divalent lanthanide sandwich complex, the Tm(II) metal center is surrounded by two h 5coordinated Cp ttt ligands with Tm-C bonds ranging from 2.652(4) to 2.717(4) Å (in average 2.68 ± 0.02 Å) and Tm-Cp(ctr) (ctr = ring centroid) distances of ca. 2.39 Å (see also Table ). In each crystallographically independent molecule, the sum of Tm-Cp(ctr) distances is ca. 4.78 Å, a value only slightly larger than that in the trivalent complex [Tm(Cp ttt )2I] (4.74 Å, see Table ). Although the metal center in 1 is only surrounded by two Cp ttt ligands, a slightly bent arrangement is observed in the solid state, with angles between the mean C5 planes in the range 11.0-13.9° and Cp(ctr)-Tm-Cp(ctr) angles of 164.3-167.7°. Similar bent arrangements were observed for the base-free complexes [Ln(Cp ttt )2] (Ln = Sm, Eu), 22 although 1 presents a Cp(ctr)-Ln-Cp(ctr) arrangement closer to linearity, which may be the result of the smaller ionic radius of Tm(II) compared to Sm(II) (1.03 and 1.17 Å, respectively, in a six-coordinate environment). Consistently, the Ln-Cp(ctr) distance is ca. 0.13 Å shorter in 1 in comparison to that in [Sm(Cp ttt )2] (see Table ).