Stereo-isomerism

ISOMERISM IN COORDINATION COMPOUNDS

·      The compound which have the same molecular formula but different arrangement of their atoms are called isomers and the phenomenon is called isomerism 

·      Isomers have different physical and chemical properties because of different arrangement of their atoms.

·      Isomerism in coordination compounds may be divided into two

             (i)Stereo-isomerism

             (ii)Structural Isomerism

i). Stereo-isomerism-

·     
stereoisomers have the same chemical formula and chemical bonds but they have different special arrangement of atoms or ligands.

                          Or

When two compounds contain the same ligands coordinated to the central ion, but the arrangement of ligands in space is different. The two compounds are said to be stereoisomers and the type of isomerism is called stereoisomerism.

i)                   Geometrical or cis-trans Isomerism

ii)                 Optical or mirror- image Isomerism

           i). Geometrical isomerism –

·      Isomers having same molecular formula but different arrangement of ligands around central atom/ion, are called geometrical isomers and phenomena is called geometrical isomerism

This isomerism is due to ligands occupying different positions around the central metal atom or ion.

·      The ligands occupy positions either adjacent or opposite to one another. This type of isomerism is also known as cis- and trans- isomerism.

·      Geometrical isomers in which two similar ligands are arranged adjacent to each other are called cis-isomers

·      Geometrical isomers in which two similar ligands are arranged opposite to each other are called Trans-isomers

·      Square planar complexes (coordination number four) exhibit geometrical isomerism

·      complexes of the type Ma₄b₂ and Ma₃b₃ exhibit geometrical isomerism(coordination no-6)

 

                                                                                                      

 Facial and Meridional Isomers

·      A set of three, ligands (similar) may be arranged on an octahedron in all cis-fashion, giving facial or 'fac' isomer. In other words, three similar ligands on the same face of octahedron give rise to the facial isomer.

·      A set of three similar' ligands may be arranged on an octahedron with one pair trans (meridian of octahedron), giving rise to meridional or 'mer' isomer.

 Note:

 1. Geometrical isomerism is not observed in complexes of coordination number 2 and 3.

 2. Geometrical isomerism is not observed in complexes of coordination number 4 of tetrahedral geometry.

(ii). Optical isomerism:

·      A coordination compound which can rotate the plane of polarised light is said to be optically active.

·      When the coordination compounds have same formula but differ in their abilities to rotate directions of the plane of polarised light are said to exhibit optical isomerism and the molecules are optical isomers.

·      The isomer which rotates the plane of polarised light to right direction is termed dextro (d-form) while the isomer which rotates the plane of polarised light to left direction is termed laevo (l-form).

The two optically active isomers are collectively called enantiomers.

·      Optical isomers which are mirror image that can not superimposed on one another are called enantiomers 

If Mirror image are superimposable,are called non-enantiomers

·      The essential requirement for a substance to be optically active is that the substance should not have a plane of symmetry in its structure.

·      Optical isomerism is expected in tetrahedral complexes of the type [Mabcd] but No mirror image isomerism is possible with tetrahedral and square planner complex of the type – [Ma₄], [Ma₃b], [Mab₃] because all possible arrangement of ligands round the central metal ion, M are exactly equivalent.

Example-