Coordination Compounds

7. Sodium nitroprusside
 2   
Na2 Fe (CN )5  NO  
  
Sodium pentacyanidonitrosyliumferrate(II)
8. Wilkinson's catalyst
[Rh(PPh3)3Cl]
Chloridotris-(triphenylphosphine)rhodium(I).

9. Switzer's Reagent
[Cu(NH3)4]SO4
Tetraamminecopper(II) sulphate

Classification of isomerism:
Type of isomerism

Structural isomerism Stereo isomerism

Ionization Hydrate Coordination Linkage Ligand Coordination Geometrical Optical

position

Structural isomerism:
It arises due to the difference in the type of chemical linkages and distribution of ligands within and outside
the coordination sphere.

(a) Ionisation isomerism :

The type of isomerism which is due to the exchange of groups or ions between the coordination sphere
and the ionisation sphere.
Example.
(i) Co(NH3 )4 Br2SO4 can be represented as
[Co(NH3)4Br2]SO4 (red violet) and [Co(NH3 )4SO4 ]Br2 (red)
These complexes give sulphate ion and bromide ion respectively
(ii) [Pt(NH3)4 Cl2]Br2 and [Pt(NH3 )4Br2 ]Cl2
(iii) [Co(NH3)4(NO3)2]SO4 and [Co(NH3)4SO4 ](NO3)2

(b) Hydrate isomerism :

This type of isomerism is due to presence of different number of water molecules inside a coordination
sphere.
Example. Cr(H2O)6Cl3 has three possible structures
(i) [Cr(H2O)6]Cl3 violet
(ii) [Cr(H2O)5Cl]Cl2 .H2O green
(iii) [Cr(H2O)4Cl2]Cl.2H2O dark green.
These complexes 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 and [Co(NH3)4Cl2]Cl.H2O

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(c) Linkage isomerism :
(i) This type of isomerism arises due to presence of ambidentate ligands like
NO2Θ , CN⊝ and SCN⊝
(ii) These ligands have two donor atoms but at a time only one atom is directly linked to the central
metal atom of the complex.
(iii) Such type of isomers are distinguished by infra red (I.R.) spectroscopy.
Examples. • [Co(NH3)5NO2]Cl2 and [Co(NH3)5 ONO]Cl2
   • In NO2Θ ligand, the coordinating sites are nitrogen (i.e., NO2Θ Nitro ligand)
or through oxygen (i.e. ONO⊝ Nitrito ligand)
   • The nitro isomer is yellow and is stable to acids whereas nitrito isomer is
red and is decomposed by acids.

(d) Coordination isomerism :

(i) This type of isomerism is exhibited when the complex has two complex ions in it - 'Cationic and
anionic'.
(ii) This type of isomerism is caused by the interchange of ligands between the two complex ions of the
same complex.
Examples. [Co(NH3 )6 ] [Cr(CN)6 ] and [Co(NH3 )6 ] [Cr(C2O4 )3 ]
[Cr(NH3 )6 ] [Co(CN)6 ] [Cr(NH3 )6 ] [Co (C2O4 )3 ]

(e) Ligand isomerism :

(i) Ligands with C3H6(NH2)2 have two different structures i.e. 1, 3-diamino propane and 1,
2-diaminopropane (propylene diamine).
(ii) Those complexes which have same molecular formula, but differ with respect to their ligands are
called as Ligand isomers.
Examples [Fe(H2O)2C3H6(NH2 )2Cl2 ] has two different structures
Fe(H2O)2CH3 CH CH2 Cl2 Fe(H2O)2 CH2 CH2 CH2 Cl2
and
NH2 NH2 NH2 NH2

(f) Co-ordination Position Isomerisation:

It is shown by polynuclear complexes, due interchange of ligands between the different metal nuclei.
Example.
Cl Cl Cl
Pd Pd
Ph3P Cl PPh3

Ph3P Cl Cl
Pd Pd
Ph3P Cl Cl

(g) Polymerization Isomerism :

This is not true isomerism because it occurs between compounds having the same empirical formula,
but different molecular weights.
Example. [Pt(NH3)2Cl2]
[Pt(NH3)4] [PtCl4]

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Coordination Compounds
Stereo isomerism :
(a) They have same molecular formula, same constitution, they differ only with respect to the spatial
orientation of ligands in space around the metal ion.
(b) The two stereo isomers which are possible are - Geometrical and Optical.
Geometrical or cis - trans isomers:
(a) The ligands occupy different positions around the central metal ion.
(b) When two identical ligands are coordinated to the metal ion from same side then it is cis isomer.
(Latin, cis means same).
(c) If the two identical ligands are coordinated to the metal ion from opposite side then it is trans isomer
(in Latin, trans means across).
(d) These geometrical isomers differ in physical as well as in chemical properties.
(e) Geometrical isomerism is most important in compounds with coordination numbers 4 and 6.
(f) 4-coordinated complexes with tetrahedral geometry do not exhibit cis-trans isomerism.
(g) It is exhibited by 4-coordinated complexes with square planar geometry.

1. Geometrical isomers with coordination number = 4 (Square planar complexes):

(i) Complexes with general formula, [Ma2b2] (where both a and b are monodentate) can have cis and trans
isomers.
a a a b
M M
b b b a
cis-isomer trans-isomer
(ii) Complexes with general formula [Ma2bc] can have cis and trans isomers.
a a a b
M M
b c b a
cis trans
Example: [Pt (NH3)2Cl2]
H3N NH3 Cl NH3
Pt Pt
Cl Cl H3N Cl
(cis-platin) anti cancer trans
Example: [Pt(NH3)2ClBr]
Cl NH3 Br NH3
Pt Pt
Br NH3 H3N Cl
cis trans
(iii) Complexes with general formula, [Mabcd] can have three isomers.
a b a d a c

M M M
d c c b b d
(i) (ii) (iii)

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(iv) Complexes with general formula, [M(AB)2] can have two isomers.
Example: Diglycinato platium (iv) complexes
⊝
CH2 NH2 NH2 CH2 CH2 NH2 O CO
Pt Pt
⊝ ⊝ ⊝
CO O O CO CO O NH2 CH2
(cis) (trans)

2. Geometrical and optical isomerism for tetrahedral complexes (Coordination

number=4):

[Ma4]
[Ma3b]
[Ma2b2]
[Ma2bc] • All are optically inactive
[M(AA)2] • geometrical isomers = Not defined
[M(AA)b2]
[M(AA)bc]
[M(AB)c2]

[Mabcd]
• All are optically active
[M(AB)2]
• geometrical isomers = Not defined
[M(AB)cd]

3. Stereo isomerism for Octahedral complexes (for monodentate ligand):

y

M
x
z

Optical isomers:
(a) Optically active complexes are those which are nonsuperimposable over the mirror image structure.
(b) An optically active complex is one which is asymmetric in nature i.e., not divisible into two identical
halves.
(c) The complex which rotates plane polarised light to left hand side is laevo rotatory i.e. 'l' or '–' and if
the complex rotates the plane polarised light to right hand side then it is dextro rotatory 'd' or '+'.
(d) Thus, complexes which have same physical and chemical properties but differ in their action towards
plane polarised light are called as optical isomers.

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Coordination Compounds
(e) The 'd' and '' isomers of a compound are called as Enantiomers or Enantiomorphs.
(f) Only those 6-coordinated complexes in which there are chelating agents i.e. bidentate ligands, exhibit
optical isomerism. This is due to the absence of elements of symmetry in the complex.
(g) Optical isomerism is not found in square planar complexes on account of the presence of axis of
symmetry.

1. For monodentate ligands:

S.No. Complexes Geometrical Stereo Optically Optically Enantiomeric

Isomers Isomers inactive active pairs
1. [Ma6] Not defined Not defined 1 0 0
2. [Ma5b] Not defined Not defined 1 0 0
3. [Ma4b2] 2 2 2 0 0
4. [Ma4bc] 2 2 2 0 0
5. [Ma3b3] 2 2 2 0 0
6. [Ma3b2c] 3 3 3 0 0
7. [Ma3bcd] 4 5 3 2 1
8. [Ma2b2c2] 5 6 4 2 1
9. [Ma2b2cd] 6 8 4 4 2
10. [Ma2bcde] 9 15 3 12 6
11. [Mabcdef] 15 30 0 30 15

Illustration 1:
Draw all the stereoisomers of
1. [CoCl3 (NH3)3]
2. [Pt(py)2(NH3)2Cl2]2
Solution:
1. [CoCl3 (NH3)3] is [Ma3b3] type of complex which shows Facial and Meridional isomerism.
b
b
a
a a
b M
M a
a b
b
a b
Facial (fac) Meridional (Mer)
Optically inactive Optically inactive
2. [Pt(py)2(NH3)2Cl2]2 is [Ma2b2c2] type of complex
2 2
py py
Cl py py Cl

Pt Pt Optically Active
isomers = 2
Cl NH3 Cl
H3N
NH3 NH3
Mirror

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2 2
py NH3
Cl NH3 py Cl

Pt Pt

Cl NH3 py Cl
py NH3
Optically Inactive Optically Inactive

Optically Inactive
2 2
isomers = 4
Cl py
NH3 py NH3 Cl

Pt Pt

NH3 py Cl NH3
Cl py
Optically Inactive Optically Inactive

2. For symmetrical bidentate ligand:

S.No. Complexes Geometrical Stereo Optically Optically Enantiomeric

Isomers Isomers inactive active pairs
1. [M(AA)3] Not defined 2 0 2 1
2. [M(AA)2b2] 2 3 1 2 1
3. [M(AA)2bc] 2 3 1 2 1
4. [M(AA)b4] Not defined Not defined 1 0 0
5. [M(AA)b3c] 2 2 2 0 0
6. [M(AA)b2c2] 3 4 2 2 1
7. [M(AA)b2cd] 4 6 2 4 2
8. [M(AA)bcde] 6 12 0 12 6
Illustration 2:
Draw all the stereoisomers of
1. [Co(en)3]3 2. [Fe(en)2Cl2]
Solution:
1. [Co(en)3]3 is [M(AA)3] type of complex.

3 3
en en

en Optically Active
Co Co en
Isomers = 2

en en

d-form Mirror  -form

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Coordination Compounds
2. [Fe(en)2Cl2] is a type of [M(AA)2b2] complex.

en
en
Cl
Cl
Fe Optically Active
Fe
Isomers = 2
Cl
Cl
en
Mirror en
d-form  -form

Cl

en Fe en Optically Inactive
Isomers = 1

Cl
3. For Unsymmetrical bidentate ligand:

S.No. Complexes Geometrical Stereo Optically Optically Enantiomeric

Isomers Isomers inactive active pairs
1. [M(AB)3] 2 4 0 4 2
2. [M(AB)2c2] 5 8 2 6 3
3. [M(AB)2cd] 6 11 1 10 5
4. [M(AB)c4] Not defined Not defined 1 0 0
5. [M(AB)c3d] 3 4 2 2 1
6. [M(AB)c2d2] 4 6 2 4 2
7. [M(AB)c2de] 7 12 2 10 5
8. [M(AB)cdef] 12 24 0 24 12

Advance level Illustrations on isomerism:

Illustration 3:
Which type of isomerism is shown by [CoሺNH3 ሻ4 Br2 ]Cl ?
(A) Geometrical and Ionisation (B) Optical and Ionisation
(C) Geometrical and Optical (D) Geometrical only
Ans. (A)
Solution:
Ionisation
[CoሺNH3 ሻ4 Br2 ]Cl → [CoሺNH3 ሻ4 BrCl] Br

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NH3 NH3
H3N Br Br NH3
Co Geometrical Co
H3N Br H3N Br
NH3 NH3

Illustration 4:
Statement-1 : The geometrical isomers of the complex [MሺNH3 ሻ4 Cl2 ] are optically inactive.
Statement-2 : Both geometrical isomers of the complex [MሺNH3 ሻ4 Cl2 ] possess axis of symmetry.
(A) Statement-1 is True, Statement-2 is True ; Statement-2 is a correct explanation for Statement-1
(B) Statement-1 is True, Statement-2 is True ; Statement-2 is NOT a correct explanation for Statement-1
(C) Statement-1 is True, Statement-2 is False
(D) Statement-1 is False, Statement-2 is True
Ans. (B)
Solution:
Geometrical isomers of [MሺNH3 ሻ4 Cl2 ] are optically inactive due to the presence of plane of symmetry
Illustration 5:
The ionization isomer of [CrሺH2 Oሻ4 ClሺNO2 ሻ]Cl is –
(A) [CrሺH2 Oሻ4 ሺO2 Nሻ]Cl2
(B) [CrሺH2 Oሻ4 Cl2 ]ሺNO2 ሻ
(C) [CrሺH2 Oሻ4 ClሺONOሻ]Cl
(D) [CrሺH2 Oሻ4 Cl2 ሺNO2 ሻ]. H2 O
Ans. (B)
Solution:
[CrሺH2 Oሻ4 ClሺNO2 ሻ]Cl → Cl⊖ ion
[CrሺH2 Oሻ4 Cl2 ]ሺNO2 ሻ → NO⊖
2 ion
Illustration 6:
Match the complexes in column-I with their stereo properties is column-II
Column I Column II
(A) [CoCl3 ሺNH3 ሻ3 ] (P) Has a facial isomer
(B) [Crሺoxሻ3 ]3⊖ (Q) Cis form is optically active
(C) [CrCl2 ሺoxሻ2 ] (R) Trans form is optically inactive
(D) [RhCl3 ሺPyሻ3 ] (S) Has a meridional form
(T) Two optically active isomer
Ans. A-P,S; B-T; C-Q,R,T; D-P,S
Solution:
(A) [Ma3 b3 ] type complex shows both facial & meridional form of isomer
(B) [MሺAAሻ3 ] type complex due to absence of plane of symmetry are optically inactive

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Coordination Compounds

(C) O Cl
Cl O O O
Cr Cr
Cl O O
O
O Cl
no P.O.S has P.O.S
so optically active so optically inactive
so also has enantiomer
(D) [Ma3 b3 ] type complex shows facial & meridional
a a
a b a b
M ; M
a b b b
b a
fac mer
Illustration 7:
Statement-1: Complexes containing three bidentate groups such as [Crሺoxሻ3 ]3⊖ and [Coሺenሻ3 ]3⊕ do not
show optical activity.
Statement-2: Octahedral complex, [CoሺNH3 ሻ4 Cl2 ] shows geometrical isomerism.
(A) Statement-1 is true, statement-2 is true and statement-2 is correct explanation for statement-1.
(B) Statement-1 is true, statement-2 is true and statement-2 is NOT the correct explanation for
statement-1.
(C) Statement-1 is true, statement-2 is false.
(D) Statement-1 is false, statement-2 is true.
Ans. (D)
Solution:
Statement-1: [MሺAAሻ3 ] type complex shows optical activity.
NH3 NH3
Cl NH3 Cl NH3
Statement-2 : Co ; Co
Cl NH3 H3N Cl
NH3 NH3
cis trans
Illustration 8:
Total number of geometrical isomers for the complex [RhClሺCOሻሺPPh3 ሻሺNH3 ሻ] is.
Ans. (3)
Solution:
[Mabcd] square planar have 3 G.I.
Ph3P CO H3N CO H3N CO
Rh ; Rh ; Rh
H3N Cl Ph3P Cl Cl PPh3

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