Find the optical isomerism of coordinate complex

Question: Which of the following complex species do not show optical isomerism?

a)     [Co(en)₃]³⁺

b)    [Co(en)₂Cl₂]⁺

c)     [Co(NH₃)₃Cl₃]

 

Solution: Lets us recall the basic of Optical Isomerism

a)      [Co(en)₃]³⁺

In this complex Co is central metal while en (ethylene diammine) is bidentate chelating ligand which is coordinate to the central metal atom Co. [Co(en)₃]³⁺ shows d²sp³ hybridisation.

Hybridisation of [Co(en)₃]³⁺:

 

This means that [Co(en)₃]³⁺  is an octahedral complex.

Such complex does not possesses any element of symmetry thus, it is optically active. It has been resolved into d- and l- forms.

 

b)    [Co(en)₂Cl₂]⁺

In this complex, two symmetrical bidentate chelating ligands ‘en’ (ethylene diammine) and two monodentate ligands ‘chlorine’ are coordinate to central metal atom Co.

[Co(en)₂Cl₂]⁺ shows d²sp³ hybridisation. This means that it is an octahedral complex.

Hybridisation of [Co(en)₂Cl₂]⁺ :

 

 [Co(en)₂Cl₂]⁺ exhibit the phenomenon of optical isomerism and can be resolved into their optical isomers. Its cis form is unsymmetrical, while the trans form is symmetrical because it contain plane of symmetry.

 Hence, optical isomerism is shown by cis form only. The cis form is resolved into d- and l- forms.

 

 c)     [Co(NH₃)₃Cl₃]

In this complex, Co is attached to three monodentate ligand ammonia and three (other) monodentate ligands chlorine.

 [Co(NH₃)₃Cl₃] shows d²sp³ hybridisation. This means that it is an octahedral complex.

Hybridisation of [Co(NH₃)₃Cl₃]:

In such octahedral complex, metal is placed at the centre and six ligands occupy their position at the vertices. Due to different spatial arrangement of the groups around the central metal, these complex show geometrical isomerism. This gives rise to two types of isomers called cis and trans isomers.

The cis and trans forms of [Co(NH₃)₃Cl₃] are given below.

 

 

 

In the cis-isomer, the three Cl- are one triangular face (position 1, 2, and 3) and the three NH₃ molecules are placed on the opposite triangular face (position 4, 5 and 6). Since three identical ligands occupy one face of an octahedron, the isomer is said to be facial or fac. 

In the trans isomer, Cl- are placed on the position 1, 2 and 3 while NH2 molecules are present on the positions 3, 4 and 5. It is also called meridional isomer (in which each set of three identical ligands occupies a plane passing through the metal atom).

[Co(NH₃)₃Cl₃] show facial as well as meridional isomerism. But both contain plane of symmetry, hence it is optically inactive.