Small cause – Great effect: What the 4fn+15d0 → 4fn5d1 configuration crossover does to the chemistry of divalent rare-earth halides and coordination compounds.

Abstract The rare-earth elements in the divalent state, i.e. with oxidation number + 2, may either have the electron configuration 4fn+15d° (symbolized as R 2+) or 4fn5d1 (R 3+e-). As R 2+ (R = Eu, Yb, Sm, Tm, Dy, Nd) they can either be contained in extended solids as in the insulating diiodides R I 2 , or in coordination complexes such as samarocene, Kagan's reagent, or TmI 2 (DME) 3. In the case of R 3+e-, the "excess" d electron can either be delocalized and cause (semi)metallic behavior in extended solids, e.g. in LaI 2 , or localized with the R 3+e- = 4fn5d1 ion trapped in a coordination complex with (super)bulky ligands such as in [K(2.2.2-crypt)][LaCp" 3 ]. Thus, the seemingly small cause of a one-electron configuration crossover, 4fn+15d° ↔ 4fn5d1 has a large effect on the chemical behavior and physical properties of the respective compounds where atomic properties and ligand effects play important roles. Graphical abstract The seemingly small cause of the electronic configuration crossover 4fn+15d0 → 4fn5d1, symbolized as R 2+ → R 3+e-, has a great effect on the chemistry and physics of divalent rare-earth compounds in salt-like/metallic extended solids as well as in coordination complexes in solution and in the crystalline solids. fx1 Highlights • Rare-Earth elements form compounds with oxidation number + 2, may be divalent. • The third valence electron may be in a 4f state in solids and coordination compounds. • The third valence electron may be in a delocalized 5d state in solids. • The third valence electron may be trapped in a localized 5d state. • Temperature, pressure and the ligands influence the nature of the third valence electron. [ABSTRACT FROM AUTHOR]

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