Why is [Ni(en)₃]²⁺ more stable than [Ni(NH₃)₆]²⁺?

Due to the chelate effect:

• Ethylenediamine (en) is bidentate each en forms a stable 5-membered chelate ring with Ni
• Replacing 6 NH₃ with 3 en molecules releases a net gain of 3 free particles → entropy increases (ΔS > 0)
• More positive ΔS makes ΔG more negative → greater thermodynamic stability
• Stability constant (log K) for [Ni(en)₃]²⁺ is significantly higher than for [Ni(NH₃)₆]²⁺