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: Concept
Stereoisomerism in octahedral coordination complexes containing bidentate ligands.

Step 2: Meaning
A complex exhibits optical isomerism if it lacks a plane of symmetry and forms non-superimposable mirror images (enantiomers).

Step 3: Analysis
The complex \[ [Co(en)_3]^{3+} \] contains a central cobalt ion octahedrally surrounded by three bidentate ethylenediamine ligands. The three chelate rings wrap around the metal ion in a propeller-like arrangement, making the complex chiral.

• In this complex, cobalt is coordinated by three ethylenediamine (en) ligands.

• Each ethylenediamine molecule is a bidentate ligand and donates two nitrogen atoms to the metal ion.

• Consequently, the coordination number of cobalt is: \[ 3\times2=6. \]

• A coordination number of six gives the complex an octahedral geometry.

• The three bidentate ligands wrap around the metal ion to form three chelate rings.

• These chelate rings can twist in two different ways, producing right-handed and left-handed arrangements.

• As a result, the complex possesses neither a plane of symmetry nor a centre of inversion.

• Therefore, the complex exists as two non-superimposable mirror images known as optical isomers or enantiomers.

• These are commonly designated as: \[ \Delta \quad\text{and}\quad \Lambda \] forms.

• Hence, \[ [Co(en)_3]^{3+} \] is an optically active coordination compound.

Step 4: Conclusion
Molecules lacking a plane of symmetry are chiral and consequently exhibit optical isomerism, existing as dextro (d) and laevo (l) forms.

Final Answer: Optical Isomerism.