How many stereoisomers (geometrical plus optical) are possible for the octahedral complex ion $[\mathrm{Co}(\mathrm{en})(\mathrm{NH}{}_3){}_2\mathrm{Cl}{}_2]{}^{+}$, where en is a symmetrical bidentate ligand?

Treat en as a cis-spanning $AA$ unit, giving an $M(AA)b_2{c}_2$ octahedron. Enumerating the arrangements gives three geometrical isomers (the two $\mathrm{NH}{}_3$ trans; the two $\mathrm{Cl}{}^{-}$ trans; neither pair trans). The last has no mirror plane and exists as an enantiomeric pair, so the total is $3+1=4$ stereoisomers.