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Q.
Which of the following does not show optical isomerism? (en = ethylenediamine)
AIPMTAIPMT 2009Coordination Compounds
Solution:
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, $\left[ Co \left( NH _{3}\right)_{3} Cl _{3}\right]^{0}$ 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.
$\left[ Co ( en )_{2} Cl _{2}\right]^{+}, \left[M( AA ) B _{2} C _{2}\right]$ type, e.g.
$\left[ Co ( en ) Cl _{2}\left( NH _{3}\right)_{2}\right]$ and $\left[M( AA )_{3}\right]$ type, e.g.
$\left.[\text { Co(en })_{3}\right]^{3+}$ show optical isomensim, whereas complexes of $\left[ MA _{3} B _{3}\right]$ type, e.g. $\left[ Co \left( NH _{3}\right) Cl _{2}\right]^{0}$ do not show optical isomerism.