The equilibrium $2{\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{Cu}}_2^{2+}$ moves to the right in the presence of

.Cl⁻, S₂O^-2_3,SCN⁻ and CN⁻ have the tendency to lose electrons and hence, reduce Cu²⁺ to Cu⁺ thus, shifting the reaction towards left.