Why `Ln^(2+)` ions are reductants and `Ln^(4+)` are oxidants ?

Explanation

Lanthanides are stable in +3 oxidation state only than in other oxidation states. Thus `Ln^(2+)` ion are easily oxidised to `Ln^(2+)` ions and acts as reductants. `Ln^(4+)` ions are easily reduced to `Ln^(3+)` ion and act as oxidants.