Stereoisomerism

Stereoisomerism of Bs level.

1. STERIOISOMERISM BY SAMEEN FATIMA

2. Isomerism in Coordination compounds . Two or more chemical compounds that have the same molecular formula but different structures are called isomers and the phenomenon is termed isomerism.   TYPES OF ISOMERISM: Structural isomerism. Stereoisomerism.

3. STEROISOMERISM Two compounds that contain the same ligands coordinated to the same central metal ions, but differ only in the arrangement of ligands in space are called stereoisomers and the  type of isomerism is called stereoisomerism or space isomerism.

4. Types of Stereoisomerism . It is of two types. Geometrical Isomerism or Positional isomerism. Optical isomerism. Stereoisomerism can exist only in complexes with coordination number four or greater. Most common coordination numbers among coordination complexes are four and six, they will be used to illustrate stereoisomerism.

5. 1. Geometrical isomerism or Positional isomerism: I n geometrical isomers the ligands occupy different positions round the central metal ion. Cis-trans is one kind of geometrical isomerism. Cis means "adjacent to" and trans means "on the opposite side of". In cis isomer two identical ligands occupy the positions adjacent to each other while in a trans-isomer the two identical ligands occupy the position diagonal to each other.   the following figure briefly describe the phenomena of isomerism.

6. COORDINATIONS Coordination no. 4: Complexes showing the coordination number 4 can have two common spatial geometries:  1.tetrahedral 2.square planar Geometrical isomerism is not exhibited by tetrahedral complexes, since all the four ligands in tetrahedral complexes are adjacent to each other. Geometrical isomerism can occur in four-coordinate complex with square planar geometry.  Square planar complexes of types MA2B2  show two stereoisomers: the cis-trans isomers. For example the complex [Pt(NH3)2Cl2] exist as two geometric isomers. Its cis form is in pale yellow color and trans is in dark yellow.  In cis complex, the chloro groups are closer to each other (on the same side of the square), while they are at opposite corners in the trans complex. The ammine groups are also closer together in the cis complex. Square planar complexes of Ni(II), Cu(II) and Co(II) also exist in cis and trans forms.

7. MA2BC type of complexes also exist in cis-trans form e.g. [Pt(NH3)2CINO2] can exist in the following two geometrical forms. In cis form the two ammine groups are on the same side of Pt whereas in trans form the two ammine groups are on the opposite side of Pt. Complexes of type MABCD exhibit three isomers, e.g., [Pt(Py)(NH3) BrCl ] can exhibit as These can be written by fixing any one ligand (sayNH3) and then placing other ligands trans to it.

8. Geometrical isomers are also found in square planar complexes which have unsymmetrical bidentate ligands of the type [MA2B2], e.g., the Pt(II)-glycinate complex exists in cis-and-trans forms as shown below: Geometrical isomers is shown by bridged binuclear planar complexes of the type M2A2X2, e.g. [Pt Cl2 PEt3]2  where PEt3 is triethlenephosphine . Geometrical isomerism is possible only when no more than two positions of the square are occupied by identical ligands. If there are three identical ligands, as in the complex [Pt(NH3)3Cl+], all the arrangements are equivalent and geometrical isomerism is impossible.

9. Coordination no . 6: The geometry of coordination no. 6 is almost always based on octahedron. Octahedral coordination is the most common of all coordinations and permits both cis-trans and optical isomerism. Octahedral complexes of the type namely MA6 and MA5B give only single isomer. Complexes of the type MA4B2 is: 

10. Type MA3B3 two isomers are again possible; these are the facial (abbreviated fac), and meridional ( mer ) forms, only a few fac- mer isomers have been isolated, among them those of [Co(NH3)3(NO2)3], [Cr(NH3)3Cl3] etc. Complexes of the type MA2B2C2 can exhibit several isomeric forms. Consider as an example [Cr(OH)2(NH3)2Br2]+. Each of the ligands may be trans to each other or cis to each other.

11. For complex type MABCDEF, no less than 13 geometrical isomers would be possible. Unsymmetrical bidentate ligands in octahedral complexes also exhibit geometrical isomerism (similar to square planar complexes). For example chromium- triglycinato complex is found to exist in cis and trans forms.

12. 2. Opticalisomerism: Stereoisomers that are non –superimposable mirror images of each other, like left and right hands, are called optical isomers or enantiomers. Optical isomers have identical physical and chemical properties except that they interact with polarized light in different ways. Separate equimolar solutions of the two will rotate a plane of polarized light by equal amounts but in opposite directions. One solution is dextrorotatory, And other is levorotatory. The isomers are called dexo and levo isomers. And the phenomena is called optical activity. We can measure it by polarimeter. A single solution. solution containing equal amounts of two isomers is called a racemic mixture, does not rotate a plane of polarized light because the equal and opposite effects of the two isomers exactly cancel. We shall consider optical isomerism only in 4- and 6- coordination complex compounds.

13. Coordoination no. Involve in optical isomerism: Coordination no. 4 Coordination no. 6

14. Coordination no. 4 In a square planar complex, since all the ligands and the metal ion liein the same plane, the complex has a plane or a center of symmetry and consequently it canot exhibit optical isomerism, even if all the ligands may be different. The square complexes are seldom optically active and hence rarely show optical isomerism.  Chirality: the optical isomerism is seldom observed in simple observed in simple tetrahedral complexes. Two optical isomers differ only in a property called chilarity. The central atom in the complex MABCD tetrahedral which display optical isomerism is called chiral.

15. Complex type MA2BC: it is one in which two of the four ligands are the same, while the rest are different cannot exhibit optical isomerism. the model of this complex is at the left and its mirror image can be superimposed on the structure at left, showing that the two are really identical. The example of it is shown below. It is a non superimposable and chiral pair.

16. Coordination no. 6 The complex compounds with coordination number 6 are octahedral in shape.  The octahedral complexes containing onlythe nondentate ligands such as MABCDEF, MA2B2CD and MA2B2C2 show optical isomerism.  For example: complex of the type MABCDEF, [Pt(Py)(NH3)(NO2)(Cl)(Br)(I)] exist in d-form andl-forms. 

17. Optical isomerism occur in octahedral complexes [M(AA)3] type in which AA represents a symmetrical group like ethylenediammine or oxalate ion, e.g.,   Compounds of [M(AA)B2C2] type also show optical isomerism. For example, the complex ion 8 also exist in d-isomer ans l- isomer.