Chem i str y | 28.

11

5.1 Structural Isomerism

Structural isomerism occurs due to the difference in chemical linkages and distribution of ligands within and
outside the coordination sphere. In Structural isomerism, isomers possess dissimilar bonding pattern. Different
types of isomers are discussed below:
(a) Ionization Isomerism: Ionization isomerism is the result of the exchange of groups or ions between the
coordinating sphere and the ionization sphere.
This isomerism occurs only in compounds where counter ions act as potential ligands. Ionization isomers
exhibit different physical as well as chemical properties.

Ionisation
[CoBr(NH3 )5 ]SO 4  →[CoBr(NH3 )5 ]2 + + SO2–
4
( A ) Red violet
Pentaamminebromocobalt (III) sulphate

Ionisation
[Co(SO 4 )(NH3 )5 ]Br  →[Co(SO 4 )(NH3 )5 ]+ + Br –
(B) Red
Pentaamminesulphatocobalt (III) bromide

Here (A) and (B) are ionization isomers. (A) forms white precipitate (BaSO4) with BaCl2 whereas (B) does not
react with BaCl2. Similarly (B) gives yellowish white precipitate (AgBr) with AgNO3 while (A) does not react with
AgNO3. Other examples of ionization isomers are:
(i) [PtCl2 (NH3)4] SO4 and [Pt (SO4) (NH3)4] Cl2
(ii) [CoCl2 (NH3)4] NO2 and [CoCl (NO2) (NH3)4] Cl
(iii) [Pt (OH)2· (NH3)4] SO4 and [Pt (SO4) (NH3)4] (OH)2

(b) Hydrate Isomerism (Solvate Isomerism): In a complex compound, water molecules behave in two ways:
(i) W
 ater molecules which behave as ligands are coordinated with the metal atom and are part of the complex
ion, e.g. [M (H2O)x].
(ii) W
 ater molecules act as water of crystallization and these appear outside the coordination sphere, e.g.
[MLx].nH2O.
Isomerism which occurs due to dissimilar number of water molecules as ligands (inside the sphere) and as water
of crystallization (outside the sphere), is known as hydrate isomerism. This isomerism is analogous to ionization
isomerism, in which water molecules inside and outside the sphere are exchanged.
For example,
Cr (H2O)6Cl3 has three possible structures:
•• [Cr (H2O)6] Cl3 (violet)
•• [Cr (H2O)5Cl] Cl2H2O (green)
•• [Cr (H2O)4Cl2] Cl.2H2O (dark green)
These complex compounds differ from one another with respect to the number of water molecules acting as
ligands.
Other hydrate isomers are:
•• [Co (NH3)4 (H2O)Cl]Cl2.
•• [Co (NH3)4Cl2] Cl H2O
2 8. 12 | Co-ordination Compounds

(c) Linkage or Salt Isomerism:

(i) Linkage isomerism occurs in complex compounds having ambidentate ligands like
—CN, —NC, —NO2, —ONO, —CNO, —NCO, —CNS, —NCS, —SCN, etc.
(ii) In this isomerism, an ambidentate ligand coordinates with different atoms.
(iii) These isomers can be differentiated by IR spectroscopy.

For example,
[Co (NO2) (NH3)5] Cl2 and [Co (ONO) (NH3)5] Cl2
(A) (B)
Pentaamminenitrocobalt (III) chloride Pentaammine nitritocobalt (II) chloride
(Yellow-red) (Red)

(A) is not decomposed by the action of acids whereas (B) liberates HNO3 by the action of acid. Other examples of
linkage isomers are:
(i) [Cr (SCN) (H2O)5]2+ and [Cr (NCS) (H2O)5]2+
(ii) [Co (NO2) (py)2 (NH3)2] NO3 and [Co (ONO) (py)2 (NH3)2] NO3

(d) Polymerization Isomerism: When two compounds possess stoichiometric composition but different
molecular formulas, they are known as polymerization isomers of each other. Molecular formula of one isomer
will be the integral multiple of the other one.
Example: [PtCl2 (NH3)2] and [Pt (NH3)4] [PtCl4]

(e) Coordination Isomerism:

(i) This isomerism occurs only in those complexes in which both cation and anion are complex.
(ii) It occurs as a result of the exchange of ligands between the cation and anion.
(iii) It may occur in those complexes also in which both cation and anion have the same metal atoms.

Example:
(i) [Cr (NH3)6] [Cr (SCN)6] and [Cr (SCN)2 (NH3)4] [Cr (SCN)4 (NH3)2]
(ii) [Co (NH3)6] [Cr (C2O4)3] and [Cr (NH3)6] [Co (C2O4)3]

(f) Coordination Position Isomerism: It occurs in complexes containing bridge ligands and is the result of
OH
dissimilar arrangement of metal atoms forming bridge, e.g. (NH3)4Co Co (NH3)2Cl2 SO4 and
Cl
OH
Cl (NH3)4Co Co (NH3)3Cl SO4
Cl

5.2 Stereoisomerism
Stereoisomerism occurs as a result of the different arrangements of ligands around the central metal atom. It may
be of two types: (1) Geometrical isomerism and (2) Optical isomerism.
 Chem i str y | 28.13

5.2.1 Geometrical Isomerism

Isomerism which occurs due to different relative arrangements of ligands around the central metal atom is known
as geometrical isomerism. Geometrical isomers are of two types:
(a) Cis-isomer: In a disubstituted complex molecule/ion, when two similar ligands are at right angle (90º), the
geometrical isomer is known as Cis-isomer.

(b) T
 rans-isomer: When two ligands are positioned in opposite directions, i.e. at 180º to each other, the isomer
formed is trans-isomer.
Cis- and Trans- positions are indicated in figures:
5
4 1 4 1

M M

3 2 3 2
6
Square planar Octahedral

Cis- positions: (1, 2), (2, 3), (3, 4), (1, 4) (1, 2), (2, 3), (3, 4), (1, 4), (1, 5),
(4, 5), (3, 5), (2, 5), (1, 6), (2, 6),
(3, 6) and (4, 6)
Trans- positions: (1, 3) and (2, 4)

Geometrical Isomerism and Coordination Numbers

Geometrical Isomerism with Coordination Number 4:
Tetrahedral complexes do not show geometrical isomerism as all the four valences are identical.

Square–planar complexes:
(a) C
 omplexes of type MA4, MA3B and MAB3 do not show geometrical isomerism, where A and B are monodentate
ligands.
(b) C
 omplexes of formula MA2B2 and MA2BC types have two geometrical isomers, where A and B are monodentate
ligands.

Example:
(i) [PtCl2 (NH3)2] resembles MA2B2 in formula and exists in two isomeric forms:
Cl NH3 Cl NH3

Pt Pt

Cl NH3 NH3 Cl
Cis-isomer Trans-isomer
(light yellow) (Dark yellow)

(ii) [PtCl (NH3) (py)2] resembles MA2BC and exists in two isomeric forms:
Py NH3 NH3 Py

Pt Pt

Py Cl Py Cl
Cis Trans
2 8. 14 | Co-ordination Compounds

(c) Complexes of formula MABCD exist in three isomeric forms:

A B A C A C A B

M M M M

D C D B B D C D
(I) (II) (III) (IV)

(III) and (IV) are similar.

e.g. [Pt (NO2) (NH2OH) (NH3) (py)] + exists in 3 isomeric forms.
A = NO2, B = NH2OH, C = NH3, D = py

(d) If A is an unsymmetrical bidentate ligand, then compounds having formula MA2 tend to exhibit geometrical
isomerism, e.g.

[Pt (gly)2]  —CH —COO–)

gly = glycinate ( NH 2 2

CH2—NH2 NH2—CH2 OC O NH2—CH2

Pt Pt
and
OC O O CO CH2—NH2 O CO
Cis Trans

(e) Bridged dinuclear complexes of formula M2A2B4 also exhibit geometrical isomerism, e.g. PtCl2 P ( C6H5 )3 
 2 ( )
(C6H5)3P Cl Cl Cl Cl Cl
Pt Pt Pt Pt
Cl Cl P(C6H5)3 and (C6H5)3P Cl P(C6H5)3
Trans Cis

Geometrical Isomerism with Coordination Number 6:

(a) Complexes of type MA6 and MA5B type do not show geometrical isomerism.
(b) Complexes of type MA4B2 or MA4BC exist in two isomeric forms, e.g. [CoCl2· (NH3)4]+
Cl Cl
NH3 Cl NH3 NH3

+ +
Co Co

NH3 NH3 NH3 NH3

NH3 Cl
Cis Trans

(c)  omplexes of type MA3B3 exist in two geometrical forms which are named as facial (fac–) and meridonial
C
(mer–) isomers. When three ligands of the same type are arranged in one triangular face, then isomer is facial.
fac- and mer- isomers of complex MA3B3 are as follows:
A B
B A A A
M M
B A B A
B B
fac-isomer mer-isomer
 Chem i str y | 28.15

E.g. [Co (NO2)3) (NH3)3] can be represented in fac- and mer- isomeric forms as follows:

NO2 NH3
NH3 NO2 NO2 NO2
Co Co
NH3 NO2 NH3 NO2
NH3 NH3
fac-isomer mer-isomer

Similarly, [RhCl3 (py)3] also exists in fac- and mer- forms.

(d) C
 omplex compound of formula MABCDEF may exist in 15 isomeric forms and only one compound of this type
is identified so far [Pt (Br) (Cl) (I) (NO2) (NH3) (py)].
(e) C
 omplexes of formula M(AA)2B2 and M(AA)2BC also exhibit geometrical isomerism, where A is the symmetrical
bidentate ligand, e.g. ethylenediamine (en), oxalate (ox), etc. [CoCl2 (en)2]+
Cl Cl
Cl

en CO+ en CO
+ en

en
Cl
Cis-isomer Trans-isomer

(f) Complex of type M(AA')3 also exists in Cis- and Trans- forms. Where AA’ is unsymmetrical bidentate ligand, e.g.
[Cr (gly)3], gly: glycinate (NH2CH2COO–)
O O
CH2 C O CH2 C O
NH2 NH2 NH2 NH2
CH2 CH2
+ +
CO CO
C= O C= O
O O NH2 O
O=C NH2 CH2 C O
CH2
=

O
Cis-isomer Trans-isomer

Illustration 8: Draw the structure of geometrical isomers of [Pt (gly)2] where gly is NH2CH2COO–. (JEE ADVANCED)

Sol: CH2 NH2 NH2 CH2 and CH2 CO

NH2 O

Pt Pt
OC O O CO OC O H2N CH2
Cis-isomer trans-isomer

5.2.2 Optical Isomerism

Optical activity: Compounds which rotate on the plane of polarized light are optically active. If the plane rotates
clockwise, then the isomer is said to be dextro rotator (d or +) and if the plane rotates anticlockwise then the isomer
is said to be laevo rotator (l or -). Equimolar mixture of d– and isomer is optically inactive and is called racemic
mixture. Optical isomers differ in optical properties.
2 8. 16 | Co-ordination Compounds

(a) Optical isomerism in complexes with coordination number 4:

(i) T
 etrahedral complexes: Like carbon compounds, complex MABCD must be optically active but due to
their labile nature, such complex cannot be resolved in d or l form. However, tetrahedral complexes with
unsymmetrical bidentate ligand are optically active. In optically active tetrahedral compounds, the ligand
must be unsymmetrical. It is not necessary whether it is chiral (asymmetric) or not, e.g. bis (benzoyl
acetonato) beryllium (II)

H5C6 C6H5 H5C6 C6H5

C= O O=C C= O O=C
CH Be CH CH Be CH
and
C O O C C O O C
CH3 CH3 CH3 CH3
Dextro
Leavo

Another example of this type is [Ni (CH2NH2COO)2]—bis (glycinato) nickel (II)

O O O O
Ni Ni
N N N N

NO2
Illustration 9: Draw all the optical isomers for [(en)2Co Co(en)2]4+ (JEE MAIN)
NO2

Sol: Complex compound shows optical isomerism and exists in d l and meso forms.

4+ en en 4+
en en en 4+
NO2 NO2 NO2

(1) CO CO 2) CO CO (3) en
CO CO

NO2 NO2 NO2

en en en en en en

I and II d and l form (mirror image of each other), III meso-form

(ii) S
 quare planar complexes: Generally square planar complexes are not optically active as they have all the
ligands and metal atoms in one plane. That is why there is a plane of symmetry.

Note: However some optically active square planar complexes are identified, e.g. isobutylenediaminemesostilben-
ediaminoplatinum (II) ion.
2+
C6H5 CH NH2 NH2 CH NH2
Pt
C6H5 CH NH2 NH2 CH C6H5

(a) Optical isomerism in compounds of coordination number 6 – Octahedral complexes:

(i) C
 omplexes of type MA4R2 exist in cis- and trans- forms and both forms are optically inactive due to plane
of symmetry.
(ii) Complexes of type MA3B2 exist in facial and meridonial forms but both are optically inactive.
(iii) C
 omplexes of type MA2B2C2 are optically active, e.g. five geometrical isomers of [PtCl2 (NH3)2 (py) 2]2+ are
possible. Out of these five possible isomers, three have been prepared. Their cis- form is optically active
while trans- forms are optically inactive due to symmetry.
 Chem i str y | 28.17

py 2+ py 2+
Cl NH3 py Cl

Pt Pt

Cl NH3 NH3 Cl
Py NH3
Cis-isomer Trans-isomer

(iv) C
 omplex MABCDEF has 15 geometrical isomers and each isomer exists as pair of enantiomers and hence
total 30 optical isomers will be possible. Only one such compound has been identified so far – [Pt (Br)
(Cl) (I) (NO2) NH2) (py)].

MASTERJEE CONCEPTS

Number of Possible Isomers for Specific Complexes

Formula Number of Stereoisomers Pairs of Enantiomers

MA6 1 0

MA5B 1 0

MA4B2 2 0

MA3B3 2 0

MA4BC 2 0

MA3BCD 5 1

MA2BCDE 15 6

MABCDEF 30 15

MA2B2C2 6 1

MA3B2CD 8 2

MA3B2C 3 0

M (AA) BCDE 10 5

M (AB)2 CD 11 5

Saurabh Gupta (JEE 2010, AIR 443)