Other oxidation states

As per to the ionic model the comparative stability of Ln²⁺ and Ln³⁺ compounds is determined through a balance among the third ionization energy (I₃) of the lanthanide, and the variation of lattice (or solvation) energies related with the two ions. The I₃ value for lanthanides is sufficiently small that most Ln²⁺ compounds are unstable regarding to disproportionation to Ln and Ln³⁺. The exceptions are of two types. For Sm, Eu and Yb, I₃ is sufficiently large to stabilize some compounds like SmO, EuF₂ and YbCl₂. The aqueous Ln²⁺ ions are strongly reducing, particularly so for Sm and Yb. Alternatively, compounds with large anions contain small lattice energies and so disproportionation is less favorable. so LnS and LnI₂ are identified for all Ln. Several of these compounds are metallic in look and highly conducting that implies a not usual electron configuration like 4f orbitals on one atom cannot overlap adequately with orbitals on other atoms to create bands. A formulation like  (Ln³⁺)(S^2-)(e^-) is occasionally given, implying a (4f)ⁿ configuration suitable to Ln³⁺ with one electron delocalized with in a band (formed probably from overlapping 5d orbitals). This peculiarity disappears for compounds of Sm, Eu and Yb, and, for instance, EuS and YbI₂ are not metallic but contain 'normal' Ln²⁺ ions.

Ln⁴⁺ compounds are known just for elements that have the lowest I₄ values. Ce⁴⁺ is known in aqueous solution and forms several compounds like CeO₂. Pr⁴⁺ and Tb⁴⁺ are very strongly oxidizing, proiding fluorides LnF₄, and being exists together with Ln³⁺ in mixed-valency oxides like Pr₆O₁₁ (that is actually nonstoichiometric).