Which one of the following metal complexes is most stable?
\( [\mathrm{Co}(\mathrm{en})_3]\mathrm{Cl}_2 \)
\( [\mathrm{Co}(\mathrm{en})_2(\mathrm{NH}_3)_2]\mathrm{Cl}_2 \)
\( [\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_2 \)
\( [\mathrm{Co}(\mathrm{en})(\mathrm{NH}_3)_4]\mathrm{Cl}_2 \)