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

The rare-earth elements in the divalent state, i.e. with oxidation number + 2, may either have the electron configuration 4f(n+1)5d degrees(symbolized as R2+) or 4f(n)5d(1) (R(3+)e(-)). As R2+ (R = Eu, Yb, Sm, Tm, Dy, Nd) they can either be contained in extended solids as in the insulating diiodides RI2, or in coordination complexes such as samarocene, Kagan's reagent, or TmI2(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 LaI2, or localized with the R(3+)e(-) = 4f(n)5d(1) 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, 4f(n+1)5d degrees <--> Hf(n)5d(1) has a large effect on the chemical behavior and physical properties of the respective compounds where atomic properties and ligand effects play important roles.