How do the high third ionization enthalpies of copper, nickel, and zinc explain the difficulty in obtaining oxidation states greater than two for these elements?

The high third ionization enthalpies of copper (Cu), nickel (Ni), and zinc (Zn) indicate the difficulty in obtaining oxidation states greater than two for these elements. The third ionization involves removing an electron from the d⁸ configuration of Cu²⁺, d⁸ configuration of Ni²⁺, and d¹⁰ configuration of Zn²⁺, making it energetically unfavorable. The stable electronic configurations in these divalent ions discourage further electron removal. The substantial energy required for the third ionization contributes to the challenge in achieving higher oxidation states, underscoring the reluctance of these elements to adopt oxidation states beyond two in their compounds.