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+)` ions are easily oxidised to `Ln^(2+)` ions and act as reductants. `Ln^(3+)` ions are easily reduced to `Ln^(3+)` ions and act as oxidants.