The stepwise stability constants in aqueous solution at 25° C for the formation of the i...

The stepwise stability constants in aqueous solution at 25° C for the formation of the ions [M(en)(H₂O)₄]²⁺, [M(en)₂(H₂O)₂]²⁺, and [M(en)₃]²⁺ for copper and nickel are given in the table. Why is there such a difference in the third values? (Hint: Consider the special nature of d⁹ complexes.)