• # question_answer Direction: The dissociation of complex may be expressed as ${{[M{{L}_{x}}]}^{4+}}\,+xL$ and equilibrium constant of this is known as instability constant which is a measure of stability. The stability of complex depends on EAN of central atom, charge on metalion, basic nature of ligand and chelation. Which complex is most stable where ${{K}_{d}}$ is un stability constant? A)  ${{[Cu\,{{(CN)}_{2}}]}^{-}}\,{{K}_{d}}=1\times {{10}^{-16}}$ B)  ${{[Fe{{(CN)}_{6}}]}^{4-}}{{K}_{d}}=1\times {{10}^{-37}}$ C)  ${{[Fe{{(CN)}_{6}}]}^{3-}}{{K}_{d}}=1\times {{10}^{-44}}$ D)  $[Ag{{(CN)}_{2}}^{-}{{K}_{d}}=1\times {{10}^{-20}}$

Stability constant, ${{K}_{s}}=\frac{1}{{{K}_{d}}}=1\times {{10}^{44}}$ Thus, ${{[Fe\,{{(CN)}_{6}}]}^{3-}}$ is the most stable among the given.