Transcript 6.Coordination Compounds

Isomerism
Coordination Compounds
1.
2.
3.
4.
5.
6.
Ionization isomerism
Hydrate isomerism
Coordination isomerism
Coordination Position isomerism
Linkage isomerism
Polymerisation
Isomers have the same molecular formula, but their atoms are arranged either in a
different order (structural isomers) or spatial arrangement (stereoisomers).
2
Structural Isomerism - 1. Ionization Isomerism
In this type of isomerism the difference arises from the positions of groups within or
outside the coordination sphere. Therefore, these isomers give different ions in solution,
hence the name ionisation isomerism. For example, there are two distinct compounds of
the formula Co(NH3)5 BrSO4. Position of groups differ within and outside coordination
sphere.
e.g. – [CoBr(NH3) 5]SO4
and
[Co SO4 (NH3) 5] Br
1. [CoBr(NH3) 5]SO4  [CoBr(NH3) 5 ]2+ + SO4 2Red – Violet
Gives precipitate with Barium chloride
2. [Co SO4 (NH3) 5] Br  [Co SO4 2- (NH3) 5 ]+ + BrRed
Gives precipitate with silver nitrate
ii. [Co Cl2 (NH3) 4] NO 2 and [Co Cl NO 2 (NH3) 4] Cl
Structural Isomerism - 2. Hydration Isomerism
•This type of isomerism arises from replacement of a coordinated group by water of hydration.
example - CrCl3. 6H2O:
Three isomers are [Cr(H2O)6]Cl3, [Cr Cl(H2O)5]Cl2-H2O and [CrCl2(H2O)4]Cl.2H2O
They differ largely from one another in their physical and chemical properties as illustrated
below.
[Cr(H2O)6]Cl3
Violet
[CrCl(H2O)5]Cl2.H2O
Green
[CrCl2(H2O)4]Cl. 2H2O
Green
Example - 2
- It does not lose water when treated with conc.H2SO4
- three chloride ions are precipitated with AgNO3
- It loses one water molecule when treated with conc.H2SO4
- two Cl—ions are precipitated with AgNO3
- It loses one water molecules on treatment with conc. H2SO4
- one Cl—ion is precipitated with AgNO3
[CoCl(en)2(H2O)]Cl2
and [CoCl2 (en)2] Cl.H2O
Structural Isomerism - 3. Coordination Isomerism
Structural Isomerism - 4. Coordination Position Isomerism
Structural Isomerism - 5. Polymerisation Isomerism
Structural Isomerism - 6. Linkage Isomerism
Same complex ion structure but point of attachment of at least one of the ligands differs.
I [Co(NH3)4(NO2)Cl]Cl
and
II [Co(NH3)4(ONO)Cl]Cl
I Nitro group - (NO2) linkage through N , II Nitrito group -(ONO) link thru O
Stereoisomerism - Geometrical Isomerism
Geometrical isomerism is due to ligands occupying different positions around the central
ion. The ligands occupy positions either adjacent to one another or opposite to one
another. These are referred to as cis form and trans form, respectively. This type of
isomerism is, therefore, also referred to as cis-trans isomerism .
Coordination no. 4
1. Tetrahedral complexes - Cis-trans isomerism is not possible in tetrahedral complexes
because all the four lignads are adjacent to one another.
2. Square Planar Complexes - cis-trans isomerism is very common amongst square planar
complexes of the following types.
1. Complexes of the type Ma2b2 can exist in cis and trans forms. Amongst the best known
examples are the complexes of platinum (II) and palladium (II), such as [PtCl2 (NH3)2],
[Pd(NO2 )2(NH3)2 ] and [PtCl2(py)2].
[PtCl2 (NH3)2],
Geometrical Isomerism
2. Complex of the type Ma2bc - there are cis and trans isomers depending upon whether
the two ‘a’ groups are adjacent or opposite to each other. Ex. Pt(py)2 NH3Cl]
Py
Cl
Py
Pt
Cl
Pt
Py
NH3
NH3
Py
3. Complexes of the type Mabcd produce three isomers. The structures of these isomers can
be written by fixing the position of one ligand (say ,a ) and placing the other ligands b, c and d
trans to it
NH2OH
NO2
NO2
NH3
NH3
Py
NH2OH
NH2OH
Pt
Pt
Pt
Py
NH3
Py
NO2
Geometrical Isomerism
Coordination no. - 6 Octahedral complexes
The complexes having coordination number 6 adopt octahedral geometry. The
octahedral complexes also exhibit geometrical isomerism. The complexes of the
type Ma6 and Ma5b would not show geometrical isomerism.
1. Complexes of the type Ma4b2 or Ma2b4 or Ma4bc - A large number of other
octahedral complexes of these types where M is Co(III) Cr(III), Rh(III), Pt(IV), have
been prepared.
Geometrical Isomerism
2. Complexes of the type Ma3b3 exist in two isomeric forms. Example - [RhCl3(py)3]
It is seen in the cis form, like groups occupy the corners of one of the triangular faces of the
octahedron while in the trans form., they do not.
Cl
Cl
Cl
Py
Py
Rh
Cl
Rh
Py
Py
Cis -
Py
Cl
Py
Cl
Trans -
Geometrical Isomerism
3. Complexes of the type [Mabcdef], where all the ligands are different, there is a
possibility for the existence of 15 different geometrical forms. However, the only
compound of this type that has been prepared so far is [PtBrCII(NO2) (C5H5N)(NH3) ]
It has been possible to isolate only three different forms of this compound.
4. Complexes of the type (M(aa)2b2] or [M(aa)2bc] containing bidentate ligands (aa) show
geometrical isomerism. The other two ligands may be the same (b2) or different (bc). The
common examples are [M(en)2bc] where M is Co (III), Cr(III), Ir(III), Rh(III), Pt(III), Pt(IV) and
b and c are ligands such as NO2, I , Br, Cl , NH3, py, etc. For example , the complex
[Co(en)2Cl2]+ where en is H2NCH2CH2NH2, exists in cis and trans forms.
Geometrical Isomerism
en
en
Cl
Cl
(M(aa)2b2]
Co
Co
Cl
Cl
en
en
Cis -
Trans -
en
en
Cl
Cl
Co
Co
Py
Py
en
Cis -
en
[M(aa)2bc]
Trans -
Stereoisomerism - Optical Isomerism
• Optical isomers have non-super imposable mirror images.
•A molecule which has no plane of symmetry is described as chiral. The carbon atom with
the four different groups attached which causes this lack of symmetry is described as a
chiral centre or as an asymmetric carbon atom. The molecule with a plane of symmetry
is described as achiral. Only chiral molecules have optical isomers.
Simple substances which show optical isomerism exist as two isomers known as enantiomers.
•A solution of one enantiomer rotates the plane of polarisation in a clockwise (left) direction.
This enantiomer is known as the (+) form or Laevoritatory (l-)
The solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise
direction. This enantiomer is known as the (-) form or Dextrorotatory (d-)
•If the solutions are equally concentrated the amount of rotation caused by the two isomers is
exactly the same - but in opposite directions, mixed in equal amounts they form Racemic
mixture.
if you rotate molecule B.
Optical activity


If one enantiomer of a chiral compound is placed in a polarimeter and polarized
light is shone through it, the plane of polarization of the light will rotate.
If one enantiomer rotates the light 32° to the right, the other will rotate it 32° to
the left.
Optical Isomerism
Metal complexes not having any elements of symmetry - Centre, Plane of symmetry
etc can show optical isomerism. Theoritically all such complexes must show optical
activity but not many have been resolved into d- and l- isomers.
Complexes with coordination number 4
1. Tetrahedral complexes - Not many tetrahedral complexes show optical activity.
Complexes of Be (II), B(III) and Zn(II), containing two symmetrical bidentate ligands
have been resolved into optically active forms. For example the complex
bis(benzoylacetonato)beryllium(II) has been shown to exist in two optical forms as
shown below.
Optical Isomerism
2. Square planar complexes are seldom optically active. This is because all the ligands
and the metal atom lie in the same plane. The molecule has a plane of symmetry and
hence cannot exhibit optical isomerism. One such platinum compound which has ,
however been resolved into two forms is as shown below.
H
Ph
Me
H
Me
Ph
Optical Isomerism
Complexes with coordination number 6
1. Complexes of the Type [M (aa)3 ]where aa= bidentate ligand. The complexes such as
[Co(en)3]Cl3, [Cr(ox)3]3- etc., exist as optical isomers because they form non- superimposable
mirror images,
en
en
Co
Co
en
en
en
[Cr(ox)3]3-
en
Optical Isomerism
Complexes with coordination number 6
2. Complexes of the type [M(aa)2 b2] The cis isomer is asymmetric and can be resolved into
d- and I- isomers. For example , the complex ion dichlorobis (ethylenediamine)rhodium (III)
[RhCl2 (en)2]+ exists in the form of three isomers (trans optically inactive and cis shows d- and
en
l- isomers.
Cl
Rh
Trans -
Cis-
Cl
d-
en
[CoCl2 (en)2]+
en
Cl
Co +2
Cl
en
Optical Isomerism
Complexes with coordination number 6
3. Complexes of the type [M(aa) 2bc] The complexes containing two different unidentate
ligands and two symmetric bidentate ligand also show optical isomerism. For example, the
complex ion [CoCl (en) 2(NH3)] +2 exists in d- and I -forms.
en
en
NH3
NH3
Co +2
Co +2
Cl
Cl
en
en
Optical Isomerism
Complexes with coordination number 6
4. Complexes of the type [M(aa)b2 c2] The complexes containing only one symmetric bidentate
ligand also show optical isomerism. For example, the complex ion [CrCl2(en)(NH3)2]+ exists in dand I -forms.
NH3
NH3
en
NH3
Cl
Cl
NH3
Cr +
Cr +
Cl
Cl
en
en
Optical Isomerism
Complexes with coordination number 6
5. Complexes containing hexadentate ligands - The complexes containing hexadentate ligands
such as ethylenediaminetetraacetato (EDTA) also show optical activity. Fox example, the
complex [Co(EDTA) exists in two forms d- and I- .
6. Complexes containing unidentate ligands - The complexes of the type [Mabcdef] are also
expected to show optical isomerism. A complex of this type can theoretically have 15
geometrical forms, each of which should be optically active.