Explanation:
Optical isomerism is exhibited by only those complexes in which elements of symmetry are absent. Octahedral complexes of the types \(\left[M(a a)_{3}\right]\) \(\left[M(a a) \,x_{2},\,y_{2}\right]\) and \(\left[M(a a)_{2}\, x_{2}\right]\) have absence of elements of symmetry, thus exhibit optical isomerism. Here, aa represents bidentate ligand, \(x\) or \(y\) represents monodentate ligand and \(M\) represents central metal ion.
Hence, \(\left[C o\left(N H_{3}\right)_{3} \,C l_{3}\right]^{0}\) due to presence of symmetry elements does not exhibit optical isomerism, or Octahedral complexes of \(\left[M(A A)_{2}\, B_{2}\right]\) type, e.g. \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+},\left[\mathrm{M}(\mathrm{A} A) \,\mathrm{B}_{2} \mathrm{C}_{2}\right]\) type, e.g. \(\left[\mathrm{Co}(\mathrm{en}) \mathrm{Cl}_{2}\left(\mathrm{NH}_{3}\right)_{2}\right] \operatorname{and}\left[M(\mathrm{AA})_{3}\right]\) type, e.g. \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) show optical isomensim, whereas complexes of \(\left[M A_{3} B_{3}\right]\) type, e.g. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}_{2}\right]^{0}\) do not show optical isomerism.
