Draw the structures of optical isomers of:
$(i)$ $[Cr(C_2O_4)_3]^{3-}$
$(ii)$ $[PtCl_2(en)_2]^{2+}$
$(iii)$ $[Cr(NH_3)_2Cl_2(en)]^{+}$

(N/A) Optical isomers are non-superimposable mirror images of each other.
$(i)$ $[Cr(C_2O_4)_3]^{3-}$: This complex shows optical isomerism due to the presence of three bidentate oxalate ligands. It exists as a pair of enantiomers (dextro and laevo forms).
$(ii)$ $[PtCl_2(en)_2]^{2+}$: The cis-isomer of this complex is optically active and exists as a pair of non-superimposable mirror images. The trans-isomer is optically inactive due to the presence of a plane of symmetry.
$(iii)$ $[Cr(NH_3)_2Cl_2(en)]^{+}$: This complex exhibits optical isomerism in its cis-form,where the two $Cl^-$ ligands are adjacent to each other,allowing for chiral arrangements.