Which of the following is correct statement?

Which of the following is correct statement?
  1. Complex $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{2+}$ is more stable than $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{3+}$
  2. All complexes of $\mathrm{Ni(II)}$(II) are bound to be outer d-orbital complexes.
  3. Stability constant for $\left[\mathrm{Ni}(\mathrm{en})_3ight]^{2+}$ is greater than stability constant for $\left[\mathrm{Ni}\left(\mathrm{NH}_3ight)_6ight]^{2+}$
  4. $\Delta_{\text {oct }}$ for $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{3+}$ is greater than $\Delta_{\text {oct }}$ for $\left[\mathrm{Cr}\left(\mathrm{NH}_3ight)_6ight]^{3+}$

Solution

(A) In $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{2+}$, the $\mathrm{Co}^{2+}\left(\mathrm{d}^7ight)$ ion undergoes $\mathrm{sp}^3 \mathrm{~d}^2$ hybridization to form outer orbital complex. Thus the electronic arrangement of the metal ion will be $\mathrm{t}_{2 \mathrm{~g}}^6 \mathrm{e}_{\mathrm{g}}^1$.

In $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{3+}$, the $\mathrm{Co}^{3+}\left(\mathrm{d}^6ight)$ ion undergoes $\mathrm{d}^2 \mathrm{sp}^3$ hybridization to form inner orbital complex. Thus the electronic arrangement of the metal ion will be $t_{2 \mathrm{~g}}^6 \mathrm{e}_{\mathrm{g}}^0$.

$\mathrm{t}_{2 \mathrm{~g}}^6 \mathrm{e}_{\mathrm{g}}^0$ is more stable than $\mathrm{t}_{2 \mathrm{~g}}^6 \mathrm{e}_{\mathrm{g}}^1$.
Thus complex $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{2+}$ is less stable than the complex $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{3+}$.

Thus option $\mathrm{A}$ is incorrect.

(B) $\left[\mathrm{Ni}(\mathrm{CN})_4ight]^{2-}$ involves $\mathrm{dsp}^2$ hybridization and is inner d-orbital complex.

Thus, all complexes of $\mathrm{Ni}$ (II) are not bound to be outer d-orbital complexes. Thus option (B) is incorrect.

(C) Ethylene diamine is chelating ligand and forms stable metal chelates whereas ammonia is unidentate ligand and does not form chelates.

Hence, Stability constant for $\left[\mathrm{Ni}(\mathrm{en})_3ight]^{2+}$ is greater than stability constant for $\left[\mathrm{Ni}\left(\mathrm{NH}_3ight)_6ight]^{2+}$.

Thus option $\mathrm{C}$ is correct.

(D) Water is weaker ligand when compared to ammonia.

$\Delta_{\text {oct }}$ for $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{3+}$ is smaller than $\Delta_{\mathrm{oct}}$ for $\left[\mathrm{Cr}\left(\mathrm{NH}_3ight)_6ight]^{3+}$.

Thus option D is incorrect.

Asked in: JEE-TOPICTESTS-CHEMISTRY

Practice more COORDINATION COMPOUNDS questions on Aicharya