Isomerism In Coordination Compounds-2 - Coordination Chemistry | Inorganic Chemistry PDF Download

• Facial and Meridional Isomers: In facial isomers three identical donor atoms lie on the corners of a triangular face and in meridional isomer three identical donor atoms lie on the corners of the plane bisecting the complex.

There are four types of complexes which exist in fac- or mer-isomeric forms.

(i) [Ma3b3]^n±: These types of complexes exist as fac- and mer- isomers.
For examples: [Co(NH₃)₃Cl₃]

In fac- isomer, the three Cl—Co—Cl bond angles are 90˚ while in mer- isomer, two Cl-Co-Cl bond angles are 90˚ and one is 180˚.

(ii) [M(AB)₃]^n±: In this type of complexes AB is unsymmetric bidentate ligand.
For example: [Co(gly)₃] and [Cr(gly)₃]. The facial and meridional isomers of [Co(gly)₃] are shown below:

(iii) [M(AAA)b₃]^n±: The complex [Co(dien)(NO₂)₃] (where dien = NH₂-CH₂-CH₂-NH-CH₂-CH₂-NH₂)
also exists in facial and meridional isomeric forms.

(iv) [M(AAA)₂] The complex [Co(dien)₂] also exist as facial and meridional isomers

• [Ma₂b₂c₂]^n±: Five geometrical isomers are possible for these type of complexes.
  For example: [Pt(NH₃)₂ (Py)₂Cl₂ ]²⁺

• [Mabcdef]^n±: Fifteen isomers are possible for this type of complex.

For example: Only one example, [Pt(Py)(NH₃)(NO₂)(Cl)(Br)(I)]

• [M(AA)ab]^n±: Two geometrical isomers (cis and trans).

For examples: [Co(en)₂ (NH₃)(Cl)]²⁺ , [Co(en)₂ (Py)(Cl)]²⁺ [Cr(OX)₂ (NO₂ )(Cl)]^3-

• [M(AA)a₂b₂]^n±: Three geometrical isomers are possible for this type of complexes.

For example: [Co(en)(NH₃)₂Cl₂]⁺ , [Co(en)(Py)₂Cl₂ ]⁺

• [M(AB)₂a₂]^n±: Five geometrical isomers are possible.
  For example: [Co(gly)₂Cl₂ ]^-

• [M(AB)₂ab]^n±: Six geometrical isomers are possible for this type of complexes.
  For example: [Co(gly)₂ (NH₃)(Py)]⁺ Ion

• Octahedral Complexes Containing Optically Active Bidentate Ligand: An example of this type of complex is [Co(en)(pn)(NO₂)₂]⁺ ion.

Three geometrical isomers are possible for this complex.

Optical Isomerism:
Optical isomers or enantiomers are pairs of molecules or ions which are non-superimposable mirror images of each other. Non-superimposable means that if one structure is laid over the other of the same molecule, the positions of all the atoms will not match. Isomers that are non-superimposable of each other are called chiral or optically active because they rotate the plane of polarised light.
The essential condition for a substance to be chiral (optically active) is that the substance has no plane of symmetry. The substances having no plane of symmetry are always non-superimposable on their mirror images.

(a) Square Planar Complexes
Square planar complexes rarely show optical isomerism because all the four ligands and the metal cation lie in the same plane and hence have a plane of symmetry.
Meanwhile, there are some exceptions. For example, (isobutylenediamine) (meso-diphenylethylenediamine) palladium (II) or platinum (II) complexes are square planar complexes and are optically active.

EDTA forms a square planar complex ion with palladium (II) in which EDTA4- acts as tetradentate
ligand. This complex ion also exists as optical isomers.

Tetrahedral Complexes
Only [Mabcd]^n± type tetrahedral complexes show optical isomerism. For example,
[As³⁺(CH₃)(C₂H₅)(S)(C₆H₅COO)]₂₊ ion show optical isomers.

Tetrahedral complexes of Be (II), B (III) and Zn (II) with unsymmetric chelating ligand also exist as
optical isomers.

Octahedral Complexes
Octahedral complexes of [Ma₆]^n±, [Ma₅b]^n± are optically inactive and do not show optical isomerism
because of the presence of plane of symmetry.

For octahedral complexes which exist as cis and trans isomers, see the optical activity in both the
isomers individually.

•  [Ma₄b₂]^n±: These types of complexes exist as cis and trans isomeric forms. Both these forms have plane of symmetry and therefore, are optically inactive. Thus this type of complexes do not show optical isomerism.

For example: [Co(NH₃)₄Cl₂ ]⁺