electron localization · lanthanides · lanthanum · samarium · superbulky ligands have been known since the early decades of the 20th century. EuCl 2 , SmCl 2 , and YbCl 2 were the first to be reported. [1,2] For the elements europium, samarium, and ytterbium all twelve MX 2 halides are known. This is not the case for the elements thulium, dysprosium, and neodymium for which only the halides of the triad chlorine, bromine, and iodine have been synthesized and crystallographically characterized. They structurally bear close resemblance to the respective alkaline-earth metal halides. [3, 4] The electronic configurations of the M 2+ ions of these six elements are 6s 0 5d 0 4f n with n = 4 (Nd), 6 (Sm), 7 (Eu), 10 (Dy), 13 (Tm), and 14 (Yb).These halides are produced as solids either by comproportionation reactions (2 MX 3 + M) [4] or by Wöhlers metallothermic reduction from the trihalides with alkali metals.[4c]The reduction potentials for the reactions M 3+ + e À ! M 2+ range between À0.35 V (M = Eu) and À2.6 V (Nd), [5] the highest values being similar to that of the half cell K/K + (À2.92 V).[6] With the proper choice of ligand, it should be possible to produce these six lanthanides in solution in the oxidation state + 2 by alkali metal (potassium) reduction from trivalent precursors.There were two major discoveries in the outgoing 20th century that boosted the solution chemistry of divalent lanthanides: First, the synthesis of [Sm(C 5 Me 5 ) 2 ] [7] and the discovery that it reduces molecular nitrogen to form the dimeric [Sm 2 (C 5 Me 5 ) 4 N 2 ].[8] Second, the discovery that THF or DME would not be reduced by divalent thulium. [9] [TmI 2 (dme) 3 ][10] followed by [DyI 2 (dme) 3 ] [11] and [NdI 2 -(thf) 5 ] [12] were the first molecular complexes of divalent thulium, dysprosium, and neodymium that could be handled in solution under argon (!) and crystallized. Although these latter three complexes were not organometallic compounds, their existence has stimulated vigorous research with organic ligands and, meanwhile, there are organometallic examples for all of these six lanthanides in the oxidation state + 2. [13] Two strategies have proved successful: 1) The ligands should preferably be (super)bulky organic ligands. 2) The formation of an anionic complex in combination with a bulky cation enhances the stability by its gain in lattice energy. Two examples for the use of superbulky ligands alone were first reported in lectures at a conference on rare-earth metals ("Tage der Seltenen Erden 2007") in Bonn.