Question

Though [CuCl4]^2- exists, but [CuI4]^2- doesn't - Why?

Answer 1

Cu⁺² when reduced to Cu⁺ and Cu, it has standard electrode potential 0.16 V
and 0.34 V respectively.

Cl₂ has an E0 of 1.36 V and I2 has an E⁰ of 0.53 V which shows that iodine is less
electronegative so the iodide anions would react with the cupric cations (Cu²⁺) yielding
iodine(I₂) and either metallic copper(Cu) or cuprous cations (Cu⁺).
Thus, the [CuI₄]^2- does not exists while [CuCl₄]^2- exists.