Q5(c). Successive ethylenediamine formation constants

The equilibrium constants for the successive reactions of 1,2-diaminoethane with Co²⁺, Ni²⁺ and Cu²⁺ are as follows: [M(OH₂)₆]²⁺ + en ⇌ [M(en)(OH₂)₄]²⁺ + 2H₂O (K₁); [M(en)(OH₂)₄]²⁺ + en ⇌ [M(en)₂(OH₂)₂]²⁺ + 2H₂O (K₂); [M(en)₂(OH₂)₂]²⁺ + en ⇌ [M(en)₃]²⁺ + 2H₂O (K₃). Values of log K₁, log K₂, log K₃: Co²⁺ 5.89, 4.83, 3.10; Ni²⁺ 7.52, 6.28, 4.26; Cu²⁺ 10.72, 9.31, −1.0. (i) What does the above data inform about successive formation constants? (ii) How do you account for very low value of K₃ for Cu²⁺?