Ionisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?

For Th, Pa and U, 5f electrons start filling and they have lower penetration than 4f electrons for Ce, Pr and Nd. Removal of weakly bound 5f electrons is easier than removing 4f electrons, so ionization enthalpy for Th, Pa and U is lower than Ce, Pr and Nd.

The 5f electrons will therefore, be more effectively shielded from the nuclear charge than 4f electrons of the corresponding lanthanoids. Therefore, outer electrons are less firmly held and they are available for bonding in the actinoids.