Structural, Spectroscopic, and Theoretical Comparison of Traditional vs Recently Discovered Ln²⁺ Ions in the [K(2.2.2-cryptand)][(C₅H₄SiMe₃)₃Ln] Complexes: The Variable Nature of Dy²⁺ and Nd²⁺

The Ln3+ and Ln2+ complexes, Cp′3Ln, 1, (Cp′ = C5H4SiMe3) and [K­(2.2.2-cryptand)]­[Cp′3Ln], 2, respectively, have been synthesized for the six lanthanides traditionally known in +2 oxidation states, i.e., Ln = Eu, Yb, Sm, Tm, Dy, and Nd, to allow direct structural and spectroscopic comparison with the recently discovered Ln2+ ions of Ln = Pr, Gd, Tb, Ho, Y, Er, and Lu in 2. 2-La and 2-Ce were also prepared to allow the first comparison of all the lanthanides in the same coordination environment in both +2 and +3 oxidation states. 2-La and 2-Ce show the same unusual structural feature of the recently discovered +2 complexes, that the Ln–(Cp′ ring centroid) distances are only about 0.03 Å longer than in the +3 analogs, 1. The Eu, Yb, Sm, Tm, Dy, and Nd complexes were expected to show much larger differences, but this was observed for only four of these traditional six lanthanides. 2-Dy and 2-Nd are like the new nine ions in this tris­(cyclopentadienyl) coordination geometry. A DFT-based model explains the results and shows that a 4f n5d1 electron configuration is appropriate not only for the nine recently discovered Ln2+ ions in 2 but also for Dy2+ and Nd2+, which traditionally have 4f n+1 electron configurations like Eu2+, Yb2+, Sm2+, and Tm2+. These results indicate that the ground state of a lanthanide ion in a molecule can be changed by the ligand set, a previously unknown option with these metals due to the limited radial extension of the 4f orbitals.