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.

 

 

 

Optical Isomerism in coordinate complex

What is optical isomerism?

It is a type of stereoisomerism in which the isomer molecules have the tendency to rotate plan of polarized light and also show chiral behaviour. These compounds are called as Optically active compounds.

Condition required for optical isomerism:

• The molecule should be disymmetrical means they should not contain; a plane of symmetry which divides the molecule in two equal halves.

• Presence of a chiral molecule which makes a non superimposable mirror images of the compound. (A Chiral molecule is that in which a carbon is attached with four different groups.)

• The isomers should have non superimposable mirror images, and these images are called the enentiomers.

Coordinate complex that show optical isomerism:

1) Four Coordination Compound:

• Optical isomers are possible for tetrahedral complex but not for square planar complex.

• Optical isomerism is shown by tedrahedral complexes of type [Mabcd]  where central metal (M) is attached to four different monodentate ligands (a, b, c and d).

• The tetrahedral complexes of Be(II), Zn(II)and B(III) which contain unsymmetrical bidendate ligands also show optical isomerism.

  

2) Six Coordination Compound:

• The octahedral complexes of the type [Ma₂b₂c₂], [Ma₂b₂cd], [Ma₂bcde] and [Mabcdef] are optically active where a,b,c,d,e and f are monodendate ligands.

• The octahedral complexes of the type [M(A-A) ₃]^+_n show optical isomerism where (AA) is a symmetrical bidendate ligand.

• Complexes of the type [M(A-A) ₂X₂] also show optical isomerism where(AA) is symmetrical bidentate ligand whereas X is monodentate ligand. The cis form of this type of complex is optically active whereas trans form is optically inactive.

• Similarly the cis form of the complex [M(A-A) ₂BC]^+-n is optically active.

• Complexes of the type [M(A-A)B₂C₂] also show optical isomerism.

• Complexes containing optically active ligands show optical isomerism. Example:[Co(en)₂(pn) ₂ (NO₂)₂]⁺ and the complexes containing polydendate ligands also show optical isomerism. Example:[Co(EDTA)]^-