If $C d^{2 +}$ and $C u^{2 +}$ both are present, both are complexed by $C N^{-}$ and $H_{2} S$ gas is passed, which one of the following pairs of complexes and their relative stability enables the separation of $C u^{2 +}$ and $C d^{2 +}$ ?

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$K_{3} [C u {(C N)}_{4}]$ : more stable and $K_{2} [C d {(C N)}_{4}]$ : less stable

$K_{2} [C u {(C N)}_{4}]$ : less stable and $K_{2} [C d {(C N)}_{4}]$ : more stable

$K_{2} [C u {(C N)}_{4}]$ : more stable and $K_{2} [C d {(C N)}_{4}]$ : more stable

$K_{3} [C u {(C N)}_{4}]$ : less stable and $K_{2} [C d {(C N)}_{4}]$ : less stable

answer is A.

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Detailed Solution

Both $C u^{2 +}$ and $C d^{2 +}$ ions reacts with KCN to from complex
The complex of copper is stable but the complex but the complex of cadmium is unstable
When $H_{2} S$ gas is passed unstable complex of $C d^{2 +}$ gives yellow precipitate.
No such precipitate is obtained with complex of copper