Chemistry > Coordination Compounds > 10.0 Isomerism in Co-ordination compounds
Coordination Compounds
1.0 Basics
2.0 Addition Salt
3.0 Nomenclature of Co-ordination Compounds
4.0 Werner's Co-ordination Theory
5.0 Valence bond theory
6.0 Crystal field splitting theory (CFST)
7.0 Effective atomic number
8.0 Magnetic Moment
9.0 Application of Crystal Field Splitting Theory (CFST)
10.0 Isomerism in Co-ordination compounds
11.0 Organo-metallic compounds
12.0 Stability of Co-ordination compounds
10.2 Stereo Isomers
Stereo isomerism are divided into two types:
1. Geometrical isomerism
2. Optical isomerism
- here $a$ and $b$ represent monodentate ligands and $AA$ is a bidentate ligand.
Stereoisomers have same molecular formula and chemical bonds but have spatial arrangment. Structural isomers have different bonds. A detalied account of these isomers are given below:
- Geometric isomerism: These types of isomerism arises in heteroleptic complexes due to different possible geometric arrangement of the ligand. Important example of the this behaviour are found with coordination number of "$4$ or $6$". In n square planar complex of formula.
Geometrical isomerism with Co-ordination number Four:
1. The tetrahedral complexes does not exhibits geometrical isomerism because all the ligands are equidistant from one another.
2. Square planar complexes exhibits geometrical isomerism.
- The square planar complex with general formula ${M_4},M{a_3}b,M{(aa)_2}$ does not exhibit geometrical isomerism.
- The square planar complexes with general complex $M{a_2}{b_2},{\text{ }}Mabcd,{\text{ }}M{a_2}bc,{\text{ }}M{(ab)_2},{\text{ }}M(ab)cd$
$aa$ - symmetric bidantate ligand
$ab$ - unsymmetric bidantate ligand
Geometrical isomerism with Co-ordination number six:
- General formula that does not exhibit geometrical isomerism $M{a_6},M{a_5}b,M{(aa)_3}$
- General formula that exhibit geometrical isomerism $M{a_4}{b_2},M{a_2}{b_2}cd,M{a_3}bcde,M{a_2}{b_2}{c_2},Mabcdef,M{(aa)_2}{b_2},M{(aa)_2}bc,M{(aa)_2}{b_2}{c_2},M{(aa)_3},M{a_4}bc,M{a_3}{b_3}$
- The octahedral complex of formula $M{a_3}{b_3}$ if three donor atom of same ligand occupy adjacent position at the corner of an octahedral face. It is known as facial (fac) isomer. When the position are around the meridian of octahedral is known as meridonal (mer).
| S.No | Geometry | Number of Geometrical Isomers |
| 1. | $M{a_4}{b_2}$ | $2$ |
| 2. | $M{a_4}bc$ | $2$ |
| 3. | $M{a_3}{b_3}$ | $2$ |
| 4. | $M{a_3}{b_2}c$ | $3$ |
| 5. | $M{a_3}bcd$ | $4$ |
| 6. | $M{a_2}{b_2}{c_2}$ | $5$ |
| 7. | $M{a_2}{b_2}cd$ | $6$ |
| 8. | $M{a_2}bcde$ | $9$ |
| 9. | $Mabcdef$ | $15$ |
Optical Isomerism
Optical isomers are mirror images that cannot be superimposed an one another. These are called enantiomers. The molecule or ions that cannot be superimposed are called as Chiral . The two forms are called dextro($d$) and laevo($l$) depending upon the direction they rotate the plane of polarised light in a polarimeter ($d$ rotate to the right, $l$ to the left). Optical isomerism is common in octahedral complexes involving didentate ligands.
Co-ordination number four ($4$):
In tetrahedral complexes with four different types of ligand of ($Mabcd$) can exhibit optical isomerism but this type of complexes are very unstable hence the isolation of such types of complexes is different but the tetrahedral complexes with unsymmetric bidentate ligands are stable and exhibit geometrical isomerism.
- Tetrahedral complexes which contains unsymmentrical bidentate ligand $M{(ab)_2}$
Example: ${\left[ {Cr{{(en)}_2}C{l_2}} \right]^ + }$ show optical activity.
- The square planar complexes does not exhibit optical isomerism due to their structure. But some of the square planar complexes with bidentate ligand can exhibit optical isomerism.
Optical isomerism in octahedral Complex
- In octahedral complexes with general formula $M{a_6}$, $M{a_5}b$, $M{a_4}{b_2}$, $M{a_4}bc$, $M{a_3}{b_3}$, $M{a_3}{b_2}c$, does not exhibit optical isomerism due to the presence of plane of symmetry.
- The cis form of $M{a_2}{b_2}{c_2}$, can exhibit optical isomerism.
- $M{a_2}{b_2}cd$, $M{a_2}bcde$, $Mabcdef$ these complexes of the general formula can exhibit optical isomerism.
- $[Mabcdef]$ is expected to give 15 geometrical isomers. In the case of $[PtBrClI(N{O_2})(N{H_3})(pyr)]$, several of these were isolated and characterized by Anna Gel'man and reported in 1956. Optical isomers are possible for each of these 15 forms, making a total of 30 isomers.
- The cis form of $M{(aa)_2}bc$, $M{(aa)_2}{b_2}$ exhibit optical isomers but the Trans form compounds do not exhibit.
- The compounds with general form of formula $M{(aa)_3}$ exhibit optical isomerism but do not exhibit geometrical isomerism.
- $M{(ab)_3}$ exhibit optical isomerism.
- Some of the EDTA complexes exhibits optical isomerism.
- Octahedral complexes of the type $M{(aa)_3}$, $M(aa){b_2}{c_2}$, $M{(aa)_2}bc,$, $M{a_2}{b_2}{c_2}$, $M{a_2}bcde$, $M{a_2}{b_2}cd$, $Mabcdef$.
