Always remember that the difference in ionization enthalpy of two successive states is approx or equal to 10 to 15 eV or less. The lower oxidation state is not stable; conversely if the difference is more than 15 eV, the lower oxidation state is more stable. Here the 3rd and 2nd ionization enthalpy difference is more than 15 eV; that's why second ionization enthalpy is more stable