Lanthanoid ion with no unpaired electron is diamagnetic in nature.
Ce58Ce2+=[Xe]4f25d06s2=[Xe]4f2 (two unpaired electrons)
Sm62Sm2+Eu63Eu2+Yb70Yb2+=[Xe]4f65d06s2=[Xe]4f6(six unpaired electrons) =[Xe]4f75d06s2=[Xe]4f7 (seven unpaired electrons) =[Xe]4f145d06s2=[Xe]4f14 (no unpaired electrons)
Because of the absence of unpaired electrons,
Yb2+ is diamagnetic.