Question

Which of the following is formed in the solution when $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ is treated with $KCN$ till the colour of the complex is discharged?

• A. $Cu(CN{)}_2$
• B. $\left[Cu\right(CN{)}_4{]}^{2-}$
• C. $\left[Cu\right(CN{)}_4{]}^{3-}$
• D. $\left[Cu\right(CN{)}_6{]}^{4-}$

Answer 1

Answer: (C)

$\left[Cu\right(CN{)}_4{]}^{3-}$

Addition of $KCN$ to $CuS{O}_4$ solution first causes reduction and then precipitates $CuCN$ (cuprous cyanide). This reacts with excess $C{N}^{-}$, forming a soluble four co-ordinate complex $\left[Cu\right(CN{)}_4^{-}]$ which is tetrahedral in shape.
$2C{u}^{2+}+4C{N}^{-}\to 2CuCN+(CN{)}_2$
Cyanogen
$CuCN+3C{N}^{-}\to \left[Cu\right(CN{)}_4{]}^{3-}$
Colourless complex