MHT CET · Chemistry · Coordination Compounds

Which among the following is a correct decreasing order of thermodynamic stability of the complexes?

A \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\gt\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}\gt\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\)
B \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\gt\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\gt\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}\)
C \(\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}\gt\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\gt\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\)
D \(\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\gt\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{--}\gt\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\).

Verified Solution

Answer & Solution

Correct Answer

(B) \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\gt\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\gt\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}\)

Step-by-step Solution

Detailed explanation

Higher the charge on the central metal ion, higher is the stability of the complex. The order of oxidation state of metal ions in the given complex is \(\mathrm{Co}^{3+}\gt\mathrm{Cu}^{2+}\gt\mathrm{Ag}^{+}\).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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