Which one of the following metal complexes is most stable?

Solution

Chemistry ma’am · 3 steps

All four are Co²⁺ with six N donors, so count the chelate rings: the most rings wins.

1. Same metal, same donors. In every option cobalt is Co²⁺ (two Cl⁻ outside the bracket) and is bonded to six nitrogen atoms. The only difference is how many of those nitrogens come from ethylenediamine (en).
2. Chelate effect. Each en is bidentate. It binds through both of its N atoms and closes a five-membered ring with the metal. Each ring makes the complex more stable, because one en replaces two NH₃ and frees an extra molecule into solution, which raises the entropy.

   Complex	en ligands	NH₃ ligands	Chelate rings
   (a) [Co(en)(NH₃)₄]Cl₂	1	4	1
   (b) [Co(en)₃]Cl₂	3	0	3
   (c) [Co(en)₂(NH₃)₂]Cl₂	2	2	2
   (d) [Co(NH₃)₆]Cl₂	0	6	0

3. So. Stability follows the number of chelate rings: (b) > (c) > (a) > (d). The most stable complex is [Co(en)₃]Cl₂, option (b).

The idea to keep. When the metal and the donor atoms are the same, more chelate rings means a more stable complex. Polydentate ligands win over the same number of monodentate ones because of entropy.

Answer from the NTA final answer key. Solution by PrepWiser.