The equilibrium \[ 2\mathrm{Cu}^{1} \rightleftharpoons \mathrm{Cu}^{0}+\mathrm{Cu}^{11} \] in aqueous medium at 25 °C shifts towards the left in the presence of

(B) $\mathrm{Cl}^{-}$; (C) $\mathrm{SCN}^{-}$; (D) $\mathrm{CN}^{-}$

JEE Advanced · 2011 · 10 Apr 2011 · Paper 2 · Coordination Compounds

The equilibrium

2 Cu^{1} ⇌ Cu^{0} + Cu^{11}

in aqueous medium at 25 °C shifts towards the left in the presence of

Answer: (B), (C), (D)

The equilibrium represents the disproportionation of

Cu^{+}

into

Cu^{0}

and

Cu^{2 +}

.

For the equilibrium to shift left (favoring

Cu^{+}

), we need ligands that stabilize

Cu^{+}

more than

Cu^{2 +}

$Cl^{-}$ : Forms stable $CuCl$ (insoluble) and complexes like $[CuCl_{2}]^{-}$ , stabilizing $Cu^{+}$
$SCN^{-}$ : Forms stable complexes with $Cu^{+}$
$CN^{-}$ : Forms very stable $[Cu (CN)_{4}]^{3 -}$ complex with $Cu^{+}$
$NO_{3}^{-}$ : Does not form strong complexes with $Cu^{+}$

Therefore,

Cl^{-}

SCN^{-}

, and

CN^{-}

shift the equilibrium to the left.

JEE Advanced · 2011 · 10 Apr 2011 · Paper 2 · One or more correct options