The stability constants of the complexes formed by a metal ion (M²⁺) with NH₃, CN^–, H₂O and ‘en’ are of the order of 10¹¹, 10²⁷, 10¹⁵ and 10⁸ respectively. Then (en = ethylene diamine)

CN^– is the strongest ligand

The correct answer is (3) CN^– is the strongest ligand.

To compare the strength of ligands based on their stability constants, we need to understand that ligands with higher stability constants form more stable complexes with the metal ion. Therefore, the larger the stability constant, the stronger the ligand.

Given stability constants:

NH₃: \(10^{11}\)

CN^-: \(10^{27}\)

H₂O: \(10^{15}\)

en (ethylene diamine): \(10^8\)

Based on the given stability constants, the ligands can be ranked in order of increasing strength as follows:

1. en (ethylene diamine) with a stability constant of \(10^8\) is the weakest ligand among the four.

2. H₂O (water) with a stability constant of \(10^{15}\) is stronger than en but weaker than NH₃ and CN^-.

3. NH₃ (ammonia) with a stability constant of \(10^{11}\) is stronger than en and water but weaker than CN^-.

4. CN^- (cyanide) with a stability constant of \(10^{27}\) is the strongest ligand among the four.

Therefore, the correct answer is: (c) CN^- is the strongest ligand.