KCET 2026 Chemistry Solved Paper

Concept:The stability of a complex ion depends on the ligand field strength and the strength of metal-ligand bonding.Explanation:The cyanide ion $(\mathrm{CN}^-)$ is a strong field ligand.It forms strong coordinate bonds with the metal ion.Consider $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$.Let the oxidation state of iron be $x$.$x + 6(-1) = -3$So, $x = +3$.Thus, $\mathrm{Fe}^{3+}$ has a high positive charge and small size.This increases the electrostatic attraction with $\mathrm{CN}^-$.Also, $\mathrm{CN}^-$ is a $\pi$-acceptor ligand, which stabilises the complex through back bonding.Hence, $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$ is very stable.Among the other options:$\left[\mathrm{Fe}(\mathrm{CO})_5\right]$ is a neutral molecule, not an ion.$\left[\mathrm{Fe}\left(\mathrm{C}_2\mathrm{O}_4\right)_3\right]^{3-}$ is stabilised by chelation, but oxalate is weaker than cyanide.$\left[\mathrm{Fe}\left(\mathrm{H}_2\mathrm{O}\right)_6\right]^{3+}$ has water, a weak field ligand, so it is less stable.Therefore, $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$ is the most stable complex ion.Answer:Option B: $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$