ISOMERISM IN

Isomerism in Coordination Complexes

UNIT
135

COORDINATION
COMPLEXES 3
Isomerism in Coordination Complexes
Isomers are compounds having the same number and kinds of atoms arranged differently. The phenomena
of existence of isomers is called isomerism. Since their structural arrangements are different, the
properties of isomers are also different. Isomers can be broadly classified into two types, each type
having further sub-classifications.

Isomerism

Structural Isomerism Stereoisomerism

1.Ionization isomerism 1. Geometrical isomerism

2. Hydrate isomerism 2. Optical isomerism
3. Linkage isomerism
4. Coordination isomerism
5. Coordination position isomerism
6. Ligand isomerism
Structural Isomerism
Structural isomers are those which contain different atom-to-atom bonding sequences. These involve
either more than one coordination sphere or different donor atoms of the same ligand in the same
coordination sphere.
(1) Ionization Isomerism
The isomers which involve exchange of ligands between coordination sphere and ionization sphere
are called ionization isomers. Ionisation isomers show different properties. For example,
(a) [Co(NH3)5Cl]SO4 and
pentamminechlorocobalt(III) sulfate
[Co(NH3)5SO4]Cl
Isomerism in Coordination Complexes 136

pentamminesulfatocobalt(III) chloride
In pentamminechlorocobalt(III) sulfate complex, the chloro ligand is in the coordination
sphere, while the sulfate group is in the ionization sphere. An aqueous solution of this
complex, on treatment with barium chloride, yields white precipitates of barium sulfate.

[Co(NH3 )5 Cl]SO 4  BaCl2 

 [Co(NH 3 )5Cl]Cl2  BaSO 4 
White ppt

In pentamminesulfatocobalt(III) chloride complex, the sulfato ligand is in the coordination

sphere, while the chloride group is in the ionization sphere. With silver nitrate, an aqueous
solution of this complex yields white precipitates of silver chloride.

[Co(NH3 )5SO 4 ]Cl + AgNO3 

 [Co(NH 3 )5SO4 ](NO3 ) + AgCl 
White

Other examples of ionisation isomers are:

(a)  Co  NH 3 5 Br  SO 4  Red violet  and  Co  NH 3 5 SO 4  Br  Red 

(b) [Co(NH3)4Cl2]NO2 and [Co(NH3)4Cl(NO2)]Cl

(c) [Pt(NH3)4Cl2]Br2 and
tetramminedichloroplatinum(IV) bromide
[Pt(NH3)4Br2]Cl2
tetramminedibromoplatinum(IV) chloride
(d) [Co(en)2NO2Cl]SCN
chlorobis(ethylene diamine)nitrocobalt(III) thiocyanate
[Co(en)2NO2SCN]Cl and
bis(ethylene diamine)nitrothiocyanato-Scobalt(III) chloride
[Co(en)2SCNCl]NO2
chlorobis(ethylene diamine)thiocyanato-S cobalt(III) nitrite
(2) Solvate Isomerism or Hydrate Isomerism
When water (solvent) molecules are interchanged between coordination sphere and ionisation
sphere, the resulting isomers are called hydrate (or solvate) isomers.
This is a somewhat special case of the above interchange of ligands involving neutral solvate
molecules. The best known example involves isomers of “chromic chloride hydrates” of which
three are known:
[Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2 . H2O and [Cr(H2O)4Cl2]Cl . 2H2O. These differ in their
reactions:
Isomerism in Coordination Complexes 137

dehydr.over H SO
2 4  [Cr(H O) ]Cl (no change)
[Cr(H 2 O)6 ]Cl3  2 6 3
dehydr.over H SO
2 4  [Cr(H O) Cl]Cl  H O
[Cr(H 2 O)5 Cl]Cl2  H 2 O  2 5 2
dehydr.over H SO
2 4  [Cr(H O) Cl ]Cl+2H O
[Cr(H 2 O)4 Cl2 ]Cl  2H 2 O  2 4 2 2
Ag +
 [Cr(H 2 O)6 ]3+  3AgCl(s)
[Cr(H 2O)6 ]Cl3 
Ag +
 [Cr(H 2 O)5 Cl]2+  2AgCl(s)
[Cr(H 2O)5Cl]Cl2 
Ag +
 [Cr(H 2 O) 4 Cl2 ]+  AgCl(s)
[Cr(H 2 O) 4 Cl2 ]Cl 

(3) Linkage Isomerism

Linkage isomerism is the existence of coordination compounds that have the same composition
differing with the connectivity of the metal to a ligand. Typical ligands that give rise to linkage
isomers. SCN (thiocyanato) and NCS (isothiocyanato)

e.g.  Co  NH 3 5 NO 2  Cl2 and Co  NH 3 5 ONO  Cl2 are linkage isomers.

 yellow colour   Re d colour 

(4) Coordination Isomerism

Salts that contain complex cations and anions may exhibit isomerism through the interchange of
ligands between cation and anion. For example, both hexaamminecobalt(III)
hexacyanochromate(III), [Co(NH3)6][Cr(CN)6] and its coordination isomer, [Cr(NH3)6][Co(CN)6],
are known. Another example is [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4] in which the isomers
differ in color (as a result of the d9 Cu2+ chromophore), being violet and green, respectively.
Coordination isomers are obtained when some or all ligands of both the coordination spheres are
interchanged with each other. In these pairs the central metallic atom in the two coordination
spheres may be the same or different.
3+ 3+ 3+ 3–
(i) [Cr (NH3)6] [Cr (CN)6] and [Cr3+(NH3)4(CN)2]+ [Cr3+(CN)4(NH3)2]–
2+ 2+
(ii) [Pt (NH3)4] [Pt2+Cl4]2– and [Pt2+(NH3)3Cl]+ [Pt2+Cl3(NH3)]–
(iii) [Co3+ (NH3 )6 ]3+ [Cr 3+ (CN)6 ]3– and [Co3+ (CN) 6 ]3– [Cr(NH3 )6 ]3+

(5) Coordination Position Isomerism

This type of isomerism is shown by those complex compounds which contain bridging ligands and
arises when the non-bridging ligands are differently placed round the central metal atom. Thus (I)
and (II) are coordination position isomers to each other, since NH3 molecules and Cl– ions (non-
bridging ligands) are differently placed round the two Co3+ ions.
2 2
 H   H 
 O   O 
e.g. (NH ) Co3+ Co3+ (NH3 )2 Cl2  Cl(NH ) Co3+ Co3+ (NH3 )3Cl 
 3 4   3 3 
 O   O 
 H   H 
Tetrammine cobalt(III) -dihydroxo diammine Chloro triammine cobalt(III) -dihydroxo chloro
dichloro cobalt(III) ion. (I) (unsymmetrical ) triammine cobalt(III) ion. (II) (symmetrical )
Isomerism in Coordination Complexes 138

2 2
 NH 2   NH 2 
e.g. (NH3 )4Co3+ Co3+ (NH3 )2 Cl2  (NH 3 )3ClCo 3+ Co3+ Cl(NH3 )3 
 O2   O2 
   
(a) unsymmetrical (b) symmetrical

(6) Ligand Isomerism

Diamine derivatives of propane exist in two isomeric forms which are called 1, 2-diamino propane
or propylene diamine (pn) and 1, 3-diamino propane or trimethylene diamine (tn). The structures of
propane, pn and tn are given below:
1 2 3 1 2 3
1 2 3 CH 2 — CH 2 — CH 3 CH 2 — CH 2 — CH 2
CH 3 — CH 3 — CH3 | | | |
Propane NH 2 NH 2 NH 2 NH 2
1, 2-diamino propane 1, 3-diamino propane
or propylene diamine (pn ) or trimethylene diamine (tn)

pn and tn both are bidentate (neutral) ligands. When these ligands get coordinated to the metal
atom, two isomers are obtained. These are called ligand isomers and the isomerism is called ligand
isomerism. [Co3+(pn)2Cl2]+ and [Co3+(tn)2Cl2]+ are ligand isomers.

Stereo Isomerism
Stereo isomerism is exhibited by those compounds which have the same position of atoms or groups but
these atoms or groups have different arrangement round the central atom. Stereo isomerism may be of
two types viz. geometrical (or cis-trans) isomerism and optical (or d-l or mirror-image) isomerism.
(1) Geometrical (or cis-trans )Isomerism
 The complex compounds which have the same ligands in the coordination sphere but the relative
position of the ligands round the central metal atom is different are called geometrical isomers and
the phenomenon is called geometrical isomerism.
 The complex compound having two ligands occupying the adjacent positions to each other is called
cis-isomer while that in which the two ligands occupy opposite positions is called trans-isomer.
Thus geometrical isomerism is also called cis-trans isomerism.
 Geometrical isomerism is not found in complex compound with coordination number 2 and 3.
 Geometrical isomerism is most common with the complex compounds having coordination number
4 and 6.
(i) Geometrical Isomerism in 4-coordinated Complex Compounds
Complexes having central atom with 4-coordination number may have either tetrahedral or square
planar geometry.
(ii) Geometrical Isomerism in Tetrahedral complexes
Geometrical isomerism cannot be shown by tetrahedral complexes, since all the four ligands in this
geometry have adjacent position (i.e., cis position) to one another and all the four bond angles are
the same (= 109.5°).
Isomerism in Coordination Complexes 139

(iii) Geometrical Isomerism in Square Planar Complexes

A square planar complex having similar ligands at adjacent positions (90° apart) is called cis-
isomer while a square planar complex having two similar ligands at opposite positions (180° apart)
is called trans-isomer.

(1) [Ma2b2] type complexes : Important examples of square planar complexes of this type are
[Pt(NH3)2Cl2]0, [Pt(py)2Cl2]0, [Pd(NH3)2(NO2)2]0 etc. These complexes exist in cis- and trans-
isomers.

(2) [Ma2bc] type complexes : Square planar complexes of this type also show cis-transisomerism. For
example [Pd2+Cl2BrI]2– ion exists in cis and trans isomers as shown below.

[Pt2+(py)2(NH3)Cl]+ is another example of square planar complex of [Ma2bc] type which exists in
cis- and trans-isomers.
Geometrical Isomerism in 6-coordinated Complexes : Octahedral Complexes
A complex compound having central atom with coordination number equal to 6 is octahedral in shape. In
an octahedral complex, if two similar ligands are placed on any of the twelve edges of the octahedron,
they are said to be in cis position. On the other hand, if two similar ligands are lying on a straight line
which passes through the centre (where the metal ion is placed), they are said to be in trans position.
(1) [Ma4b2] type complexes :
Isomerism in Coordination Complexes 140

(2) [Ma3b3] type complexes : [Co(NH3)3Cl3], [Co(NH3)3(NO2)3], [Cr(H2O)3F3], [Cr(NH3)3Cl3],

[Rh(py)3Cl3], [Ru(H2O)3Cl3], [Ir(H2O)3Cl3], [Pt(NH3)3Br3]+, [Pt(NH3)3I3]+ etc. are important
examples of octahedral complexes of [Ma3 b3] type. Two geometrical isomers are for complex
Ma3b3.
(i) The ligands of one type may form an equilateral triangle on one of the faces (called facial
isomer).
(ii) In other isomer, the ligands of one type occupy the positions such that two are opposite (or
trans) to each other, called meridional isomer.
As an example these isomers for [Co3+(NH3)3Cl3]0 have been shown below.

(4) [Ma2b2c2] type complexes : [Pt4+(NH3)2(py)2Cl2]2+ ion is an important example of octahedral

complex of [Ma2b2c2] type. This ion can exist theoretically in five geometrical isomers that have
been shown below., but it is only three isomers that have been actually isolated. It may be noted
from the figure that (I) form is a cis-form since in this form two identical ligands are occupying
adjacent positions. Form (II) is a trans-form because in it the two identical ligands are placed at
opposite positions.

(9) [M(AA)2ab] type complexes : [Co3+(en)2(NH3)Cl]2+ is an important example of octahedral

complex of [M(AA)2ab] type. This complex ion exists in cis- and trans-isomers. In cis-isomer the
two monodentate ligands viz. NH3 and Cl– occupy the adjacent (i.e., cis) positions while in trans-
isomer these ligands occupy opposite (i.e., trans) positions [Ru3+(C2O4)2(py)(NO2)]2– also exists in
cis- and trans-isomers.
Isomerism in Coordination Complexes 141

To Distinguish between cis- and trans-isomers

The following methods may be used to distinguish between cis and trans-isomers.

(1) Dipole moments measurements : The Pt(II) complexes [Pt2+A2X2] type, where A = substituted
phosphine, arsine or stilbine such as (C2H5)3P, (C2H5)3As or (C2H5)3Sb and X = a halogen, have
their dipole moments () either equal to zero or between 8 and 12 Debye units. In the compounds
with  = 0, the individual moments have cancelled one another and so these are trans-isomers. The
compound with  = 8 – 12 Debye units are cis-isomers.

(2) Infra-red spectroscopic technique : In a trans octahedral complex such as [Co3+(NH3)4Cl2] or in

trans square planar complex like [Pt2+(NH3)2Cl2]0 the Cl-metal-Cl symmetrical stretching vibration
produces no change in the dipole moment of the molecule and thus no band corresponding to this
vibration is observed in the infra-red spectrum. However, in the cis-form of each compound, the
symmetrical stretching vibration as well the unsymmetrical stretching vibration produce appreciable
changes in the dipole moment; hence the infra-red spectrum of the cis-isomer will contain a large
number of bands due to Cl-metal-Cl stretching.
Isomerism in Coordination Complexes 142

Optical (or d-l or mirror-image) Isomerism

When the solution of certain complex compounds are placed in the path of a plane-polarised light (the
waves of the plane-polarised light vibrate only in one direction; vibrations in other directions are cut off),
they rotate its plane through a certain angle which may be either to the left or to the right. This property
of a complex of rotating the plane of polarised light is called its optically activity and the complex
possessing this property is said to be optically active. Optically active complexes are said to exist in the
following forms:
(a) One which rotates the plane of polarised light towards right (i.e., in clockwise direction) is said to
be dextro-rotatory or d-form. d-form is also represented by placing (+) sign before its name or
formula.
(b) One which rotates the plane of polarised light towards left (i.e., in anti-clockwise direction) is called
levo-rotatory or l-form. l-form is also represented by putting (–) sign before its name or formula.
(+), dextro, and (–), levo. The d- and l-forms have the following characteristics: (i) Since d- and l-
forms are capable of rotating the plane of polarised light, these are said to be optically active forms
or optical isomers. These two forms have exactly identical physical and chemical properties. (ii) d-
and l-forms are mirror images to each other. Thus d- and l-forms can not be superimposed on each
other and hence d- and l-forms are also called mirror-image isomers or enantiomer.
(c) The isomer which is not capable of rotating the plane of polarised light is called optically
inactive. Such an isomer is called meso, racemic, dl-or () form. A racemic substance is composed
of 50% d- and 50% l-form.
Conditions for a Molecule to Show Optical Isomerism
A molecule in which the grouping of the atoms is asymmetric is called an asymmetric or dissymmetric
molecule. An asymmetric molecule has the following features:
(i) An asymmetric molecule never has a plane of symmetry (also called mirror-image plane) which is
defined as an imaginary plane dividing the molecule in such a way that the part of it on one side of
the plane is the mirror image of that on the other side of the plane. The molecules possessing such a
plane of symmetry are always optically inactive while those having no plane of symmetry are
optically active and hence show optical isomerism.
(ii) An asymmetric molecule cannot be superimposed on its mirror image.
Thus the most necessary and sufficient condition for a molecule to show optical isomerism (i.e., to
exist in d- and l-forms) is that the molecule should be asymmetric, (i.e., it should have no plane of
symmetry) and should not be superimposable on its mirror image.
Isomerism in Coordination Complexes 143

Optical Isomerism in 6-coordinated Complexes : Octahedral Complexes

(1) [Ma4b2] and [Ma3b3] type complexes : (a) Octahedral complexes of [Ma4b2] type exist in cis- and
trans-isomers. Both these isomers are optically inactive due to the presence of symmetry and hence
donot show optical isomerism.
(b) Octahedral complexes of [Ma3 b3] type exist in cis (or facial) and trans (or meridional)
isomers. Both these isomers are optically inactive and hence donot show optical isomerism.
(2) [Ma2b2c2] type complexes : [Pt4+ (NH3)2(py)2Cl2]2+ ion is an important example of octahedral
complex of [Ma2b2c2] type. Cis-isomer exists in two optical (mirror-image) isomers which are
mirror-image to each other. Trans-form of this complex ion is symmetrical and hence is optically
inactive form. Thus it is a trans-meso form.

(3) [M(AA)3] type complexes : Due to the absence of a plane or centre of symmetry, the octahedral
complexes of this type are resolvable into dextro and levo forms. For example d- and l-forms of
[Co3+(en)3]3+ ion are shown below. Other example [Cr3+(ox)3]3–.

(4) [M(AA)2a2] type complexes : Example of this type is [Co3+(en)2Cl2]+ ion. This ion shows
geometrical isomerism and hence exists as cis- and trans- isomers. The cis-isomer does not have
any plane or centre of symmetry and hence is optically active.
Isomerism in Coordination Complexes 144

(5) [M(AA)a2b2] type complexes : One example of this type is [Co3+(en)(NH3)2Cl2]+ ion. This ion
exists in cis- and trans-isomers. Cis-isomer is optically active and hence gives optically active d-
and l-forms. On the other hand trans-isomer is optically inactive and hence does not give any
optical active isomer.

(6) Octahedral complexes containing polydentate ligands : Complex containing hexadentate ligand
(polydentate ligand) like ethylene diamine tetraacetate ion also exists in two optical isomers namely
d- and l-isomers. Ethylene diamine tetracetate ion is abbreviated as EDTA4– and its structure is
given as follows:
Isomerism in Coordination Complexes 145

SOLVED PROBLEM

1. The existence of two different coloured complexes of Co(NH3)4Cl2 is due to

(a) optical isomerism (b) linkage isomerism

(c) geometrical isomerism (d) coordination isomerism
Soln. The existence of two different coloured complexes of [Co2+(NH3)4Cl2] is due to geometrical
isomerism.

Correct answer is (c)

2. The isomerisms that are possible in the Co(III) complexes [Co(NH3)3(NO2)3] and
[Co(NH3)5NO2]Cl2 respectively are
(a) coordination and position (b) optical and linkage
(c) geometrical and linkage (d) optical and optical
Soln. [Co(NH3)3(NO2)3]  Geometrical isomerism possible
The ligand of one type may form an equilateral triangle on one of the faces (called facial or cis
isomer) and the other isomer, the ligands of one type occupy the positions such that two are
opposite (or trans) to each other, called meridonal or trans isomer.

[Co(NH3)5NO2]Cl2  linkage isomerism possible

Here, NO2 ligands having two potentially ligating atoms that are capable of bonding through one
type of donar atom in the situation but a different donor atom in another complex.
[(NH3)5CO-ONO]Cl2 and [(NH3)5CO-NOO]Cl2
Correct answer is (c)
Isomerism in Coordination Complexes 146

3. The complexes [Co(NH3)4(H2O)Cl]Br2 and [Co(NH3)4Br2]Cl.H2O are examples of

(a) Ionization isomerism (b) linkage isomerism
(c) geometric isomerism (d) optical isomerism
Soln. The isomers which involve exchange of ligands between coordination sphere and ionization
sphere are called ionization isomers.
[Co(NH3)4(H2O)Cl]Br2 and [Co(NH3)4Br2]Cl.H2 O is a ionization isomers, because, Br2 and
H2O.Cl exchange between coordination sphere and ionization sphere.

Correct answer is (a)

4. The total number of isomers of Co(en)2Cl2 is

(a) 4 (b) 3 (c) 6 (d) 5

Soln. Co(en)2Cl2 exists in three isomers; two are cis (optical isomers) and one is trans, en is a ethylene
diamine

Correct answer is (b)

5. The complex that exists as a pair of enantiomers is

(a) trans-[Co(H2NCH2CH2NH2)2Cl2]+ (b) cis-[Co(NH3)4Cl2]4+

(c) [Pt(PPh3)(Cl)(Br)(CH3)]– (d) [Co(H2NCH2CH2NH2)3]3+

Soln. [Co(H2NCH2CH2NH2)3]3+ exists in a pair of enantiomers.

Isomerism in Coordination Complexes 147

Correct answer is (d)

6. The number of possible isomers for the square planer mononuclear complex [(NH3)2M(CN)2] of a
metal M is
(a) 2 (b) 4 (c) 6 (d) 3
Soln. The possible isomers of [(NH3)2M(CN)2] are

Thus, the number of possible isomers is 2

Correct answer is (a)
7. The complexes [Co(H2O)4Cl2]NO2 and [Co(H2O)4Cl(NO2)]Cl are

(a) linkage isomers (b) positional isomers

(c) ionization isomers (d) optical isomers
Soln. These are ionization isomers because, NO2 and Cl exchange between ionization sphere and
coordination sphere respectively.
Correct answer is (c)

Co  NH3 5 Br  SO4 Co  NH3 5 SO4  Br

8. and  constitute an example of [JNU-2004, Biotech]
(a) linkage isomerism (b) ionisation isomerism
(c) coordination isomerism (d) optical isomerism
Soln. Ionisation isomerism arise due to difference in the position of groups being inside or outside the
coordination sphere of the complex.
For example : [Co(NH3)5Br]SO4 (Violet compound with Co  Br bond) and [Co(NH3)5SO4]Br
(Red compound with Co  SO4 bond).

Correct option is (b)