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  Stereoisomerism

Stereoisomerism (space isomerism):

Consider two compounds having similar ligands attached to the same central metal ion, however the arrangement of ligands in space around the central metal ion are distinct, then these two compounds are termed to be stereoisomers and this phenomenon is termed as stereoisomerism. There are two different kinds of stereoisomerism.

a) Geometrical or cis-trans isomerism:

Geometric isomers are possible for both octahedral complexes and square planar, however not tetrahedral. In cis-isomer two identical or similar groups are adjacent to one other while in a trans-isomer they are diametrically contrary to each other.

Square planar complexes of the type [Ma2b2]n+, here a and b are monodentate ligands, subsist as cis and trans-isomers as shown below. Illustration of this kind of complexes is [Pt (NH3)2Cl2] and [Pd(NH3)2 (NO2)2]. The cis-trans isomers of such compounds are symbolized as:

430_geometrical isomerism.jpg

898_geometrical isomerism2.jpg


 
In octahedral complex, the different coordination positions are numbered as shown:

1486_geometrical isomerism3.jpg

All along the twelve edges of the octahedron, there are twelve cis places. They are (1,2) (1,3) (1,4) (1,5) (2,6) (3,6) (4,6) (5,6) (3,4) (4,5) (2,5) & (2,3). In order to evade confusion, usually it is supposed that the 1, 2 positions are cis-positions. There are 3 trans positions; they are (1,6) (2,4) and (3,5).Usually it is taken that 1,6 positions are trans-positions in order to evade confusion. An octahedral complex of the type [Ma4b2], here a and b are monodentate ligands, exists as two geometrical Isomers:

1361_geometrical isomerism4.jpg

A particular illustration for such Isomerism is [Co(NH3)4 Cl2]+ that exists as two geometrical isomers.

The octahedral complex are of the type [M (AA)2a2]n±, here (AA) is a symmetrical bidentate ligand like ethylenediamine H2N-CH2-CH2-NH2 and ‘a’ is a monodentate ligand. A particular illustration for this is [Co(H2N-CH2-CH2-NH2)2 Cl2]+

2028_geometrical isomerism5.jpg

The octahedral complex of the type, [Ma3b3]n±, here a and b are monodentate ligands too exist as geometrical isomers, For illustration, [Rh(py)3Cl3] exist as cis-(1,2,3 trichlorocomplex) and trans-(1,2,6-trichloro complex) isomers as symbolized below:

2469_geometrical isomerism6.jpg

b) Optical Isomerism:

This is a phenomenon in which some organic or inorganic compounds contain the property of rotating plane polarised light. The compounds that exhibit this property are termed as optical isomers. The optical isomers of a compound have similar physical and chemical properties. The only differentiating property is that the isomers rotate the plane of polarised light either to the left or right. In coordination compound of type [PtCl2(en)2]12+, two geometrical isomers are possible, namely cis and trans. Among these two isomers, cis isomer exhibits optical activity since the entire molecule is asymmetric.
 

206_optical isomerism.jpg

                                                     Figure: Optical isomers of cis (PtCl2(en)2]2+

 

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