$\left[ \mathrm{Co} \left( \mathrm{en} \right)_3 \right]^{3+}$ is more stable than that of the $\left[ \mathrm{Co} \left( \mathrm{NH}_3 \right)_6 \right]^{3+}$. This is because the chelating ligands form a more stable complex than the non-chelating ligands. Since, ethylenediamine is a bidentate ligand and forms a stable chelate than that of the ammonia ligand.