Question

$CuS{O}_4$ when reacts with $KCN$ forms $CuCN$ which is insoluble in water. It is soluble in excess of $KCN$, due to formation of the following complex:

• A. $K_2\left[Cu\right(CN{)}_4]$
• B. $K_3\left[Cu\right(CN{)}_4]$
• C. $CuC{N}_2$
• D. Cu$\left[KCu\right(CN{)}_4]$

Answer 1

Answer: (B)

$K_3\left[Cu\right(CN{)}_4]$

Here, $CuCN$ is insoluble in water. This means $Cu$ in $CuCN$ is in $+1$ state. Because $Cu$ is unstable in water only when it is in $+1$ state.

Co-ordination sphere exactly contains $4C{N}^{-}$ ions. So, its structure would be $\left[Cu\right(CN{)}_4{]}^x$

Calculating $x$, $Cu$ is $+1$ state

$1+4(-1)=x$

$1-4=x$

$x=-3$

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

It combines with $3K^{+}$ ions to form $K_3\left[Cu\right(CN{)}_4]$