Oxidation States

The Dominant State: $+3$

Across the lanthanoid series, compounds of lanthanum(III) and of $\text{Ln(III)}$ in general are by far the predominant species — the $+3$ state is the one every member of the series shows reliably. Occasionally, however, $+2$ and $+4$ ions also turn up, either in solution or in solid compounds, as exceptions to this main pattern.

Why the Exceptions Appear

Just as irregular jumps show up in the ionisation enthalpies of these elements, the occasional $+2$ or $+4$ ion is a sign of the same underlying cause: certain $4f$ occupancies are unusually stable. An empty, a half-filled, or a completely filled $f$ subshell all confer extra stability on the ion that has one, and an element will favour a $+2$ or $+4$ state over the "default" $+3$ state whenever reaching that state lands it on one of these especially stable configurations.

Worked Examples From the Series

• Cerium, $\text{Ce}^{4+}$: favoured because it has a $4f^{0}$ configuration — the noble-gas-like empty-shell arrangement. Even so, $\text{Ce}^{4+}$ is a fairly strong oxidant and tends to revert to the common $+3$ state; its standard potential $E^{\ominus}(\text{Ce}^{4+}/\text{Ce}^{3+}) = +1.74\ \text{V}$ is large enough to oxidise water in principle, but the reaction is kinetically very slow, which is exactly what makes $\text{Ce(IV)}$ a useful, well-behaved analytical reagent rather than a hazard.
• Pr, Nd, Tb and Dy also show a $+4$ state, but only within oxides of formula $\text{MO}_2$ — not as a general solution-phase ion.
• Europium, $\text{Eu}^{2+}$: formed by losing only the two $s$ electrons, which leaves behind a stable $f^{7}$ (half-filled) configuration. Because that configuration is so favourable, $\text{Eu}^{2+}$ is nonetheless a strong reducing agent and readily reverts to the common $+3$ state.
• Ytterbium, $\text{Yb}^{2+}$: analogous case, with a stable $f^{14}$ (completely filled) configuration; also acts as a reductant.
• Terbium, $\text{Tb}^{4+}$: has a half-filled $f$-orbital set and, in line with the pattern, behaves as an oxidant. …