The trend in the third ionisation enthalpies is not complicated by
the 4s orbital factor and shows the greater difficulty of removing an
electron from the d
5 (Mn
2+) and d
10 (Zn
2+) ions In general, the third
ionisation enthalpies are quite high Also the high values for third
ionisation enthalpies of copper, nickel and zinc indicate why it is
difficult to obtain oxidation state greater than two for these elements Although ionisation enthalpies give some guidance concerning the
relative stabilities of oxidation states, this problem is very complex and
not amenable to ready generalisation