Which complex compound is most stable?
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{3}\right)_{3}\right]$
$\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right] \mathrm{NO}_{3}$
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]_{2}\left(\mathrm{SO}_{4}\right)_{3}$
$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Br}\right]\left(\mathrm{NO}_{3}\right)_{2}$
