Question

Indicate the type of isomerism exhibited by the following complex: $[Co(en)_3]Cl_3$

Hint:

Any octahedral complex of the type $[M(AA)_3]^{n\pm}$ (where AA is a symmetrical bidentate ligand) is always chiral and shows optical isomerism.

Answer 1

Step 1: Structure of the complex.
In $[Co(en)_3]Cl_3$, the $Co^{3+}$ ion is bonded to three bidentate ethylenediamine (en) ligands, giving a coordination number of 6 and an octahedral geometry.
Step 2: Symmetry analysis.
The three chelate rings wrap around the metal ion in a propeller-like arrangement. The resulting structure has no plane of symmetry and no centre of inversion, making it chiral.
Step 3: Type of isomerism.
Because the complex is non-superimposable on its mirror image, the two mirror-image forms are enantiomers. This is optical isomerism. The two forms are designated $\Delta$ (right-handed) and $\Lambda$ (left-handed).
Step 4: Consequence.
A pure sample of one enantiomer rotates plane-polarised light. An equimolar mixture of both forms gives an optically inactive racemic mixture. The complex $[Co(en)_3]Cl_3$ exhibits optical isomerism.