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:
In 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. a)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.