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
For example: Only one example, [Pt(Py)(NH3)(NO2)(Cl)(Br)(I)]
For examples: [Co(en)2 (NH3)(Cl)]2+ , [Co(en)2 (Py)(Cl)]2+ [Cr(OX)2 (NO2 )(Cl)]3-
For example: [Co(en)(NH3)2Cl2]+ , [Co(en)(Py)2Cl2 ]+
Three geometrical isomers are possible for this complex.
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.
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
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
For example: [Co(NH3)4Cl2 ]+