Structure and Isomerism in Coordination

compounds
• Coordination number and geometry of the complexes are related to one
another.

• Eg complexes with coordination number 4 are either have tetrahedral or square

planar geometry.

• The coordination number and geometry of the complexes depends upon the
following factors:

• The size of metal atom or ion.

• Size of the ligands and the steric interaction between the ligands.
• Electronic interaction and the number of d-electrons in metal
atom or ion.
• Whether the ligands form pi- bonds with metal ion or atom.

• In general, the metal atoms or ions of larger size favour the formation of complexes of higher coordination
number because steric repulsion decreases with increase in size of central metal ion.
 Coordination number – 2:

coordination number of
metals in complexes- 2
to 9
coordination number
2, 4 and 6- most
common.

• A few number of complexes are known.

• They are limited to the d10 species i.e. Cu+, Ag+, Au+ and Hg2+
• These complexes have linear geometry.
• Some examples are
• [H3N-Cu-NH3]+, [Cl-Cu-Cl]-, [H3N-Ag-NH3]+, [NC-Hg-CN]

➢ These complexes are typically unstable react with additional ligands to

form complexes of higher coordination number.
[Cu(CN)2]- + 2CN- [Cu(CN)4]3-

[Ag(NH3)2]+ + 2NH3 [Ag(NH3)4]+

Coordination number – 3:
➢ Rare number of complexes are known.
➢ These complexes have trigonal planar and trigonal pyramidal geometry.
➢ Some examples are
K[Cu(CN)2], HgI - and pyramidal SnCl -.
3 3
Cu
-
N
Sn
C
Cl Cl
Cu Cl
C C
N N
Cu Cu

➢ Many compounds appear to be three coordinate as judged from

stoichiometry are found to have higher coordination number.

Eg: CsCuCl3 (Infinite single chain, -Cl-CuCl2-Cl- with C.N = 4)

KCuCl3 (Infinite double chain, -Cl4-(Cu2Cl2)-Cl4- with C.N = 6)

Coordination number – 4:
➢ Second most important coordination number in coordination chemistry.
➢ These complexes have tetrahedral and square planar geometry.
➢ Tetrahedral complexes are favored by larger ligands like Cl-, Br- & I- and
small metal ion or atoms with
(i) d0 and d10 configuration
(ii) dn configuration where square planar or octahedral is not
favored such as Fe2+ (d6), Co2+ (d7), Ni2+ (d8), Cu2+ (d9) ions
which for tetrahedral complexes with Cl-, Br- ions.

➢ Square planar complexes are less favored sterically than tetrahedral

complexes.
➢ Square planar complexes are thus formed by only a few metal ions. The
best known are the d8 species such as Ni2+.
➢ Prerequisite for stability of these square planar complexes is the presence
of non bulky strong field π - acceptor ligand such as CN-.
➢ The metal ions belonging to 4d- and 5d- transition elements such as Rh+,
Pd2+, Pt2+ & Au3+ form invariably square planar complexes
regardless of the π – donor or π – acceptor character of the ligands.

2- 2-
Cl
NC CN
Ni Ni
Cl Cl NC CN
Cl

Tetrahedral Square planar

2- 2-
Cl Cl Cl Cl
Pd Pt
Cl Cl Cl Cl

Square planar
Coordination number – 5:
➢ The complexes of CN – 5 are less common than that of CN – 4 & 6.
➢ These complexes have Square pyramidal or trigonal bipyramidal geometry.
➢ These two geometries can be interconverted by small change in bond
angles because these two geometries differ little in energy from one
another.
[CdCl5]3- [Co(C6H7NO)5]2+ , [Ni(CN) 5]3- [Sb(C 6H 5) 5

Trigonal Square
bipyramidal Intermediate geometries pyramidal

➢ [Ni(CN)5]3- ion exist as both square pyramidal and trigonal bipyramidal in

the same crystal. CN 3-
CN 3-
NC CN CN
Ni NC Ni
NC CN CN
CN
Square
pyramidal Trigonal
bipyramidal
Coordination number – 6:
➢ This is the most common and enormously important coordination number
for transition metal complexes.
➢ The possible geometries corresponding to CN – 6 may be hexagonal
planar, trigonal prismatic, octahedral or tetragonally distorted octahedron.
➢ In a regular octahedral complex all the M-L bond distances are equal and
the complexes have plane as well as centre of symmetry i.e. regular
octahedral complexes are symmetric and have Oh symmetry.
L
[Co(NH3)6]3+

L L [Cr(H2O)6]3+

M [Co(H2O)6]3+
L L
[Fe(CN)6]4-

L [Ni(NH3)6]2+
➢ There are some complexes of CN – 6 which have all the six ligands same
but undergo some sort of distortion due to the electronic effect. This type of
distortion is Jahn-Teller effect.

L
L
L L
L
L L L L

M M
M
L L L L
L
L

Tetragonal compression
L L
L
Trigonal prismatic
Tetragonal elongation
Coordination number – 7:
➢ This coordination number is not common. Few 3d and some 4d & 5d
complexes are known. Because of larger metal ion size it can accommodate
more than six ligands.
➢ The possible geometries corresponding to CN – 7 are
pentagonal bipyramidal, a capped octahedron and a capped trigonal
prism. Examples are

[Os(CN)7]3- [ZrF7]3- [NbF7]2-

pentagonal bipyramidal capped octahedron capped trigonal prism

Coordination number – 8:
➢ This coordination number is also not common. Only few complexes are
known.
➢ The possible geometries corresponding to CN – 8 are distorted cubic
structures i.e. square antiprismatic and trigonal dodecahedral.
➢ Examples [Mo(CN)8]4-

Square antiprismatic trigonal dodecahedral

Coordination number – 9:
➢ This coordination number requires larger transition metals and f-block
elements.
➢ The possible geometry corresponding to CN – 9 is tricapped triogonal
prismatic.
➢ Examples includes [Sc(H2O)9]3+, [La(H2O)9]3+ as well as [TcH9]2- and [ReH9]2-
 Isomerism in Coordination compounds
The compounds having same chemical composition but different properties
due to the structural difference are called isomers and the phenamenon of
existence of isomers is called isomerism. (isos means same; meros means
parts).
Isomerism in Coordination compounds

Structure / Constitutional Spin isomerism Stereoisomerism Conformational

isomerism or polytopal
isomerism

Low Spin High Spin Geometrical Optical

Ionization Hydrate Linkage Coordination Ligand Polymerization

 Ionization isomerism:

Hydrate isomerism

Structural Linkage isomerism

isomerism arises
due to different
bonding between
Coordination isomerism
metal and ligands.

Ligand isomerism

Polymerization isomerism
 Ionization isomerism:
There is exchange of ligands b/w coordination and ionization
sphere and give different ions when dissolved in water.
 Hydrate isomerism
There is exchange of water molecule b/w coordination and ionization
sphere.
 Linkage isomerism:
When ambidentate ligands can coordinate to a
metal through either of the two different donor atoms.
 Coordination isomerism

• Observed in the coordination compounds having both cationic and anionic

complex ions and there may be exchange of ligands.
 Coordination position isomerism
The difference in the position of ligands in a bridge complex causes a
special type of coordination isomerism. This special type of isomerism
is called co-ordination position isomerism. In polynuclear complexes,
an interchange of ligands between the different metal nuclei gives rise
to co-ordination positional isomerism.
Ligand isomerism
If a ligand itself exists in two or more isomeric form, then the
complex containing such ligands also exists in isomeric forms.

[Co(pn)2Cl2]+ [Co(tn)2Cl2]+
pn- propylenediamine tn- trimethylenediamine
Polymerization isomerism:
It occurs between compounds having the same empirical
formula but different molecular weight.
[Pt(NH3)2]Cl2 ; [Pt(NH3)4Cl4] ; [Pt(NH3)3Cl]2 [PtCl4]
 Stereoisomers

The isomers in which the same type and number

of ligands are coordinated to the metal atom or
cation but with different spatial arrangements
are called stereoisomers.

Stereoisomers is classified into two

types
• Geometrical isomerism
• Optical isomerism
 Stereoisomers
Other stereoisomers, called optical isomers or enantiomers, are mirror images of each
other.
Just as a right hand will not fit into a left glove, two enantiomers cannot be
superimposed on each other.
 • A molecule or ion that exists as a pair of
Enantiomers enantiomers is said to be chiral.
 • Physical properties of chiral molecules
are identical (boiling point, freezing
point, density, etc.)
Enantiomers • One exception:
• interaction of a chiral
molecule with plane-
polarized light.
 Enantiomers
A chiral compound will rotate plane polarized light. If one enantiomer rotates the light 32° to
the right, the other will rotate it 32° to the left.
Generally, only when 2 chiral things interact is there a difference in properties.
 Geometrical isomers

Geometrical isomers are the one in which relative position of the ligands round the
metal ion is different.

Geometrical isomers exist only in pairs, one isomer with two ligands adjacent to
each other (cis) and in the other two ligands are opposite to each other (trans).

Geometrical isomerism is most common in complexes having coordination number

of 4 and 6.

The complexes having coordination numbers 2 and 3 do not exhibit geometrical

isomerism.
Geometrical isomerism in complexes having coordination number - 4
ML4

Tetrahedral Square planar

[MA4]n± (or) [MABCD]n± [MA4]n±, [MA3B]n±, [M(A A)2]n±,

Do not exhibit geometrical isomerism
whether all ligands are same or [M(A A)AB]n± and [M(A A)A 2]n±
different because all the ligands in this
Above types square planar complexes
geometry are at adjacent positions
do not exhibit geometrical isomerism
relative to each other.
because all the possible spatial
n arrangement of the ligands in this
A geometry are same.
n
M A A

A M
A A
A A
 n n
A B A B
[MA2B2]n± M M
type complexes
A B B A

Cis Trans

Examples of this type of complexes are [Pt(NH3)2 Cl2], [Pt(py)2 Cl2] etc

H3N Cl Cl
H3N

Pt Pt

H3N Cl NH3
Cl

Cis Trans

Geometrical isomers possible is 2

 n± n±
A B A B

[MA2BC]n± M M
type complexes
A C C A

Cis Trans

Examples of this type of complexes are [Pt(Py)2(NH3) Cl]+, [Pt(NH3)2PyCl]+

etc
+ +
Py NH3 NH3
Py

Pt Pt

Py Cl Py
Cl

Cis Trans

Geometrical isomers possible is 2

 Cis Cis
n± n±
A B A C

M M

D C B D
Trans Trans

[MABCD]n± Cis
n±
type complexes A D

C B
Trans

This type of complexes exists in three isomeric forms. The three isomers
are obtained by fixing one ligand at corner and then placing the other
three ligands one by one trans to the fixed ligand.
 Examples of this type of complexes are [Pt(Py)(NH3) ClBr],
[Pt(NH3)(C2H4)ClBr] etc

H3N Py Cl
H3N

Pt Pt

Br Cl Py
Br

H3N Py

Pt

Cl Br

Geometrical isomers possible is 3

 [M(AB)2]n± type complexes exists in two isomeric forms

AB is unsymmetrical ligand in which A and B are two different donor

atoms.
n±
A A

M Cis

B B

n±
A B

M Trans

B A
Examples of this type of complexes are [Pt(gly)2], [Cu(gly)2] etc

H2 H2
H2C N N CH2

Pt Cis

OC O O CO

H2
H2C N O CO

Pt Trans

OC O N CH2
H2

Geometrical isomers possible is 2

 [M2A2B4]n± Bridged binuclear square planar complex
n±
A B A

M M Cis

B B B

n±
This type complex
A B B exists in three
isomeric forms (Cis,
M M Trans
trans and
unsymmetric).
B B A

n±
A B B

M M Unsymmetric

A B B
Example of this type of complex is [Pt2(PEt3)2 Cl4]

Et3P Cl PEt3

Pt Pt Cis

Cl Cl Cl

Et3P Cl Cl

Pt Pt Trans

Cl Cl PEt3

Et3P Cl Cl

Pt Pt Unsymmetric

Et3P Cl Cl
Square planar complexes with symmetric ligands carrying one or
more substituents can form geometrical isomers.

Example of complex with one substituent [Pt(pn)2]2+

pn = propylenediamine

H2 H2 2+
N N

Pt Cis
CH3
H3C
N N
H H2 H2 H

H2 H2 2+
N N

Pt
Trans
H
H3C
N N
H H2 H2 CH3
 Example of complex with two substituent [Pt(bn)2]2+
bn = butylenediamine

H3C H2 H2 CH3 2+
N N
H H
Pt Cis
CH3
H3C
N N
H H2 H2 H

H3C H2 H2 H 2+
N N
H CH3
Pt Trans
H
H3C
N N
H H2 H2 CH3
Geometrical isomerism in complexes having coordination number - 6

ML6

1 Octahedral complex if the two ligands

L occupy either of the positions
(1,2), (1,3), (1,4), (1,5)
(2,3), (3,4), (4,5), (5,2)
(6,2), (6,3), (6,4), (6,5)
5L L2
it is cis isomer
M

4L L3 Octahedral complex if the two ligands

occupy either of the positions
(1,6), (2,4), (3,5)
L it is trans isomer
6
 n±
n±
A B

ML6 A A A A

M M

A A
Octahedral complex A A

[MA6]n±, [MA5B]n± A A

And
n±
[M(A A)3]n± A

Above types of octahedral

A A
complexes do not exhibit
geometrical isomerism since all the M
corners are equivalent.
A A

A
 n± n±
B B

A B A A
[MA4B2]n± M M
type complexes
A A A A

A B

Examples of this Cis Trans

type of complexes
+
are [Co(NH3)4 Cl2]+, Cl
+
Cl
[Co(NH3)4 (NO2)2]+
etc. H3N Cl H3N NH3
The complex of this
Co Co
type exist in two
isomeric forms. H3N NH3 H3N NH3

NH3 Cl

Cis Trans
 n± n±
A B

A B A A

[MA4BC]n± M M
type complexes A C A A

A C

Cis
Examples of this type Trans

of complexes are
[Co(NH3)4 Cl (H2O)]2+, 2+ 2+
NH3 Cl
[Co(NH3)4 (Py)Cl]2+
etc.
H3N Cl H3N NH3
The complex of this
type exist in two Co Co
isomeric forms. H3N OH2 H3N NH3

NH3 OH2

Cis Trans
 n± n±
A B

A B A A

[MA3B3]n± M M

type complexes A B A B

B B

facial meridional
Examples of this type
of complexes are
[Co(NH3)3 Cl3], NH3 Cl
[Cr(NH3)3 Cl3]
etc. H3N Cl H3N NH3
The complex of this
Co
type exist in two Co

isomeric forms. H3N Cl H3N Cl

Cl Cl

fac mer
 Facial and Meridional isomers
Facial isomer (1,2,3-isomer) three identical donor atoms lie on the
corner of a triangular face and all three bond angles (B-M-B) are 90o .

Meridional isomer (1,2,4-isomer) three identical donor atoms lie on

the corner of a plane bisecting the complex and two bond angles (B-M-
B) are 90o and one (B-M-B) is 180o.
n± 4 n±
A B

3
A B A A

M M

A B A B
2 2
B B
1 1
facial meridional
 n± n±
B B

[M(AB)3]n± A A A A
type complexes M
M
AB-Unsymmetrical ligand B B A B

B
Examples of this type A
of complexes are
[Co(gly)3], facial meridional
[Cr(gly)3]
etc. gly
gly
The complex of this O O
type exist in two
isomeric forms.
N N N N

Co gly Co gly

H2C CO
gly = O O N O
H2N O

N O
gly gly

fac mer
 2+ 2+
Py Py

H 3N Py Cl NH3

Pt Pt

[MA2B2C2]n± H 3N Cl H 3N Cl

type complexes Cl Py

C is tr a n s

Examples of this 2+ 2+
Py
type of complexes
NH3

are H 3N Cl Py Cl
[Pt(NH3)2(Py)2Cl 2]2+ Pt Pt
etc.
H 3N Cl
The complex of this
Py Cl

type exist in five Py NH3

isomeric forms.
2+
Cl

H 3N Py

Pt

H 3N Py

Cl
 [MABCDEF]n± type complexes

There is only one coordination compound of this type and it can exist in fifteen
(15) possible isomeric forms.

[Pt(NH3)(Py)(NO2)(Cl )(Br)(I)]

Py

O2N Cl

Pt

H3N Br

I
 n± n±
A B

A B A A
[M(AA)2B2]n± M M
type complexes A A
A B

A B

Cis Trans
Examples of this
type of complexes
are en + +
[Co(en)2Cl2]+ N Cl
[Co(en)2 (NO2)2]+
[Rh(C2O4)2Cl2]3- N Cl N N
etc. en Co en
Co
The complex of this
N N
type exist in two N Cl
isomeric forms.
N Cl
en

Cis Trans
 n± n±
A B

A B A A
[M(AA)2BC]n±
M M
type complexes
A C A A

A C

Cis Trans
Examples of this
type of complexes
are 2+ 2+
en
[Co(en)2NH3Cl]2+ N NH3
[Co(en)2 (Py)Cl]2+
[Cr(C2O4)2(NO2)Cl]3- N NH 3 N N
etc. en Co en
Co
The complex of this
N N
type exist in two N Cl
isomeric forms.
N Cl
en

Cis Trans
 + +
NH3 NH3

[M(AA)B2 C2]n± N NH3 N Cl

type complexes en Co
en Co
N Cl N Cl

Cl NH3

Examples of this type of +

complexes are Cl
[Co(en)(NH3)2Cl2]+
[Co(en)(Py)2Cl2]+ N NH3
etc.
en Co
The complex of this type
exist in three isomeric N NH3
forms.
Cl
 gly - gly - gly -
O N O

[M(AB)2C2]n± N Cl O Cl N Cl
type complexes Co Co Co
N Cl O Cl O Cl

O N N
gly gly gly

Examples of this
type of complex is
[Co(gly)2Cl2]- - -
Cl Cl
The complex of this
type exist in five N N N O
isomeric forms. gly gly gly Co gly
Co
O O O N

Cl Cl
 gly + gly +
O O

N NH3 N Py
Co Co
[M(AB)2CD]n± O Py O NH3
type complexes
N N
g ly g ly

gly + +
g ly
O N

N NH3 O NH3
Examples of this Co Co
type of complex is N Py O Py
[Co(gly)2(NH3)(Py)]+
O N
g ly gly
The complex of this
type exist in six + +
isomeric forms. NH3 NH3

N N N O

gly gly gly Co g ly

Co
O O O N

Py Py
 To distinguish cis- and trans- isomers

Dipole moment Infrared Chemical

measurement spectroscopy method

Trans isomers has Trans isomers are Grinberg’s

Zero dipole IR inactive method
moment
Cis isomers are
Cis isomers has IR active
some value of Trans isomers
dipole moment form non chelated
complex

Cis isomers form

chelated complex
 Cis isomer

Grinberg’s
method

Trans isomer
 Optical isomerism in complexes having coordination number - 4

ML4

Tetrahedral Square planar

[MA4]n± [MA4]n±, [MA3B]n±, [M(A A)2]n±,

Do not exhibit optical isomerism.
[M(A A)AB]n± and [M(A A)A2]n±
Square planar complexes do not exhibit
optical isomerism because all the four
ligands and metal ion are in the same
plane and hence posses plane of
symmetry.
 [MABCD]n± type of tetrahedral complex exhibits optical isomerism.

For example, [As(CH3)(C2H5)(S)(C6H5COO)]2+ exists as optical isomers

2+ 2+

S S

As As

OOCC6H5 C6H5COO CH3

H3C
C2H5 C2H5
Some complexes of Pd (II) and Pt (II) square planar complexes are optically
active.
Optical isomerism in complexes having coordination number - 6
n±
A

A A
ML6
M
A A
Octahedral complex
A
[MA6]n± and [MA5B]n±

n±
Above types of octahedral B
complexes do not exhibit optical
isomerism because of presence of
plane of symmetry. A A
M
A A
A
 [MA4B2]n±
type complexes
+ +
Cl Cl

H3N Cl H3N NH3

Co Co

H3N NH3 H3N NH3

NH3 Cl

Cis Trans

Above types of octahedral complexes do not exhibit

optical isomerism because of presence of plane of
symmetry.
 [MA4BC]n±
type complexes

2+ 2+
NH3 Cl

H3N Cl H3N NH3

Co Co
H3N OH2 H3N NH3

NH3 OH2

Cis Trans

Above types of octahedral complexes do not exhibit

optical isomerism because of presence of plane of
symmetry.
 [MA3B3]n±
type complexes

NH3 Cl

H3N Cl H3N NH3

Co Co
H3N Cl H3N Cl

Cl Cl

fac mer

Above types of octahedral complexes do not exhibit

optical isomerism because of presence of plane of
symmetry.
 [MA2B2C2]n±
type complexes

2+ 2+
Py
Py

H3N Py
Py NH3
Pt
Pt
H3N Cl
Cl NH3

Cl
Cl

Cis-d-isomer Cis-l-isomer

In the above types of octahedral complex only cis isomer

exhibit optical isomerism and other four isomers are optically
inactive because of presence of plane of symmetry.
 [M(A A)3]n± type complexes

n± n±
A A

A A A A
M M
A A A A

A A

Above types of octahedral complex exhibit optical isomerism

and examples for this type are [Co(en)3]3+, [Cr(Ox)3]3- etc.
 +
en + en
N N

N Cl Cl N

Co Co

N Cl Cl N [M(AA)2B2]n±
type complexes
N N
en en

Cis-d-isomer Cis-l-isomer

+
Cl
Above types of octahedral
N N
complex cis isomer exhibit
optical isomerism
en Co en
and
N N trans isomer is optically
inactive due to presence of
Cl plane of symmetry.

Trans
 2+ 2+
en en
N N

N NH3 H3N N

Co Co [M(AA)2BC]n±
Cl N type complexes
N Cl

N N
en en

Cis-d-isomer Cis-l-isomer

2+
NH3 Above types of octahedral
complex cis isomer exhibit
N N optical isomerism
en Co en and
N N trans isomer is optically
inactive due to presence of
Cl plane of symmetry.

Trans
 + +
NH3 NH3

N NH3 H3N N
[M(AA)B 2 C2]n± en Co en
Co
type complexes
N Cl Cl N

Cl Cl

Cis-d-isomer Cis-l-isomer
Above types of
octahedral
complex cis isomer
exhibit optical + +
NH3 Cl
isomerism
and
N Cl N NH3
trans isomer is
optically inactive en Co en Co
due to presence of N Cl N NH3
plane of symmetry.
NH3 Cl

Trans Trans
 gly
gly
O O

N N N N
Co gly
gly
Co
[M(AB)3]n± O O
type complexes
O O
N N
AB-Unsymmetrical ligand gly gly

fac-d-isomer fac-l-isomer

gly
gly

Above types of O O
octahedral
complex fac and N N N N
mer isomers Co gly gly Co
exhibit optical N O O N
isomerism .
gly O O
gly

mer-d-isomer mer-l-isomer
 Total number of stereoisomers for Coordination number-4

Geometrical Optical Total

Complex Type
isomers isomers stereoisomers
Tetrahedral complex
[MABCD]n± - 2 2
Square planar complex
[MA2B2]n± 2 - 2
[MA2BC]n± 2 - 2
[MABCD]n± 3 - 3
[M(AB)2]n± 2 - 2
[M2A2B4] 3 - 3
 Total number of stereoisomers for Coordination number-6

Geometrical Optical Total

Complex Type
isomers isomers stereoisomers
Octahedral complex
[MA4B2]n± 2 - 2
[MA4BC]n± 2 - 2
[MA3B3]n± 2 - 2
[M(A2B2C2]n± 4 2 (Cis) 6
[MABCDEF] n± 15 15 30
[M(AA)3] n± - 2 2
[M(AA)2B2] n± 1 2 (Cis) 3
[M(AA)2BC] n± 1 2 (Cis) 3
[M(AA)B2C2] n± 2 2 (Cis) 4
[M(AB)3] n± 2 2 4