Oxidation States of Lanthanoids

The single most common, dominant oxidation state shown by every one of the fourteen lanthanoids, without exception, is +3 -- this universal +3 tendency is, as already noted in section 4.5.1, one of the key reasons the lanthanoids are treated as a closely related family rather than as fourteen chemically distinct elements. In addition to this universal +3 state, however, a number of individual lanthanoids also show either a +2 or a +4 oxidation state under suitable conditions, and these deviations from the +3 norm are not random -- they can be explained systematically in terms of the same extra-stability argument (favouring half-filled or fully-filled f sub-shells) already used repeatedly throughout this chapter.

Gadolinium's Gd³⁺ ion and lutetium's Lu³⁺ ion both carry particular extra stability, precisely because their electronic configurations at the +3 oxidation state happen to correspond exactly to the especially stable half-filled and fully-filled 4f configurations respectively: Gd³⁺ has the configuration [Xe]4f⁷ (exactly half-filled), while Lu³⁺ has the configuration [Xe]4f¹⁴ (exactly fully filled). Because these two ions are already, by pure coincidence of atomic number, sitting exactly at a maximally stable f-sub-shell configuration when in their common +3 state, neither shows any strong additional tendency to gain or lose a further electron beyond +3.

Similarly, but now involving the +4 oxidation state rather than +3, cerium and terbium each attain the particularly stable 4f⁰ and 4f⁷ configurations respectively when pushed to the +4 state -- cerium's Ce⁴⁺ reaches the fully-EMPTY, and hence maximally symmetric/stable, 4f⁰ configuration, while terbium's Tb⁴⁺ reaches the half-filled 4f⁷ configuration. This extra stability at +4 is precisely why cerium in particular is well known and widely used as a strong, reliable +4 oxidant in analytical/volumetric chemistry (Ce⁴⁺ solutions being reduced back to the extra-stable Ce³⁺⁻adjacent... more precisely, cerimetric titrations exploit exactly this thermodynamic driving force toward the stable 4f⁰/Ce³⁺-favouring couple). …