The doubly or more highly charged ions have d
n
configurations with no 4s electrons A general trend of increasing values
of second ionisation enthalpy is expected as the effective nuclear charge
increases because one d electron does not shield another electron from
the influence of nuclear charge because d-orbitals differ in direction However, the trend of steady increase in second and third ionisation
enthalpy breaks for the formation of Mn2+ and Fe3+ respectively In both
the cases, ions have d5 configuration