Electronic Configuration of Sm II

Samarium, like all lanthanide elements, preferentially exists in the + 3 oxidation state. The loss of the three outermost electrons, namely the 5d 6s2 electrons, results in enhanced thermodynamic stability in which a closed-shell 

Xe-like electronic configuration is adopted. The + 2 oxidation state is most relevant for samarium (f6, near half-filled), europium (f7, half-filled), thulium (f13, nearly filled) and ytterbium (f14, filled). In order to attain the more stable + 3 oxidation state, Sml2 readily gives up its final outer-shell electron, in a thermodynamically driven process, making it a very powerful and synthetically useful single-electron transfer reagent. 

---