This ppt explains double salt and coordination compound, werner's work on coordination compound and Werner's theory of coordination compounds

1. Series of lecture on
Coordination Chemistry
Lecture-1
Werner's Theory
of Coordination Compounds
D. R. Shinde
Associate Professor in
ChemistryDepartment of Chemistry
P.D.E.A’s.
Prof. Ramkrishna More
Arts, Commerce and Science College
Akurdi, Pune-411044

2. The study of coordination compounds always starts with Double salt. Why?
 Doubles salts are the mixture of two salts
2NH4Fe(SO4)2 = (NH4)2SO4 + Fe2(SO4)3 and 3KCN Fe(CN)3 = 3KCN + Fe(CN)3
 Coordination compounds have similarity in formula with double salt but chemical
properties are different from double salt.
 Initially chemists know double salts and they found that some of the double salts
have different properties than expected. This has started the quest for the
coordination compound.
Coordination Compounds and double salts
 Double salt dissociate in aqueous solution. Hence
individual ion such as Fe3+ SO4
2- NH4
+ show their
existence in aqueous solution.

3.  The dissociation reaction can be confirmed by the qualitative test of NH4
+ +Fe3+ + SO4
2- ions.
 2NH4Fe(SO4)2 add in water → 2NH4
+ +Fe3+ + 3SO4
2- in aqueous solution
Few drops of
2N BaNO3
White ppt
Few drops of
KSCN
Blood red Color
SO4
2- ions give white color with BaNO3 due to formation of
BaSO4 solid.
Fe3+ gives red color with KSCN solution due to formation of Fe3+ -
SCN Complex

4. 3KCN∙Fe(CN)3 = in aqueous solution gives
positive test of K+ but not of Fe3+ and CN-.
Thus, in 3KCN∙Fe(CN)3 though dissolve in
water it do not dissociate completely to form
ions.
The solution show existence of K+ but not of
CN-. It means that K+ must not be associated
with CN-.
Few drops of
Sodium cobaltinitrite
Yellow ppt
Few drops of
KSCN
No Change in color
Expected- Red color
Fe3+ - SCN Complex
Few drops of
AgNO3
No Change in color
Expected- White
ppt of AgCN
 After rigorous study, scientists concluded that
3KCN∙Fe(CN)3 is not simple type of double
salt but it is different type of chemical species.

5.  The formula of this species could be written as
K3[Fe(CN)6].
 On dissolution it aqueous solution it will produce 3K+ and
[Fe(CN)6]3- ion. Thus, it can show the presence of K+ ions
but not of Fe3+ and CN- ions. Such double salts are termed
as coordination complexes.
Scientist Alfred Werner was studied such many
complexes and predicted basic theory of Coordination
complexes. For his work on coordination complexes he was
honoured with Noble Prize in 1913.

6. Werner's Work
Werner observed that under different
reaction conditions, Ammonia (NH3)
react with CoCl3 and form series of
compounds which can be identified
from their colours.
He concluded that as colours of these
compounds are different, they must
contain different quantity of NH3. Since
conc. CoCl3 is constant and he used
excess ammonia.
+
CoCl3 Ammonia
violet
Green
Purple
Yellow

7. Sr. No Composition of Compound Colour
1 CoCl3∙3HN3 Violet
2 CoCl3∙4HN3 Green
3 CoCl3∙5HN3 Purple
4 CoCl3∙6HN3 Yellow
Werner Analysed these compounds obtained the composition of these compounds. He
observed that each compounds has different composition. This is in consistent with
difference in their colours. The composition is as given in table.
 All compounds have same number of Co(III) and Cl- ions,
but different numbers of ammonia molecules.
 Werner further analysed these compounds for Chloride
content without decomposition of compound.

8. Werner obtained Cl- ion content in each compound by titration with silver nitrate. He titrated
aqueous solution of these compounds with AgNO3 and obtained moles of AgNO3 consumed
per mole of substance. The experimental results are as follows:
Compound
Moles of AgNO3
required for
complete reaction
Ionisable
Cl- Ions
Dissociation reaction
CoCl3∙3HN3 0 (no reaction) 0 Cl- CoCl3∙3HN3 → (CoCl3∙3HN3)aq + 0Cl-
CoCl3∙4HN3 1 1 Cl- CoCl3∙4HN3 → (CoCl2∙4HN3)aq + Cl-
CoCl3∙5HN3 2 2 Cl- CoCl3∙5HN3 → (CoCl∙5HN3)aq + 2Cl-
CoCl3∙6HN3 3 3 Cl- CoCl3∙6HN3 → (Co∙6HN3)aq + 3Cl-

9. CoCl3(NH3)3
CoCl3(NH3)4
CoCl3(NH3)5
CoCl3(NH3)6
- Chloride
- Ammonia
- Cobalt (III)
ion
+vely charged sphere
of Cobalt (III) ion
Dissociation of different compounds of Co(III) with Cl and NH3

10. The Dissociation reaction in four Co(III), chloride and ammonia complexes is also
confirmed by conductivity experiment.
 Any solution that consists of +ve and –ve ions show conductance.
 From conductance measurement one can find out number ions formed after
dissociation of compound.
Compound
Number of ions by
conductometry per
molecule of complex
Ions possible per molecule
after dissociation
CoCl3∙3HN3 0 No dissociation
CoCl3∙4HN3 2 [Co(Cl)2(HN3)4]+ + Cl-
CoCl3∙5HN3 3 [CoCl(HN3)5]+ + 2Cl-
CoCl3∙6HN3 4 [Co(HN3)6]3+ + 3Cl-

11. Compound
Number of
ionisable Cl- ions
No of non-
ionisable
Cl- ions
Sum of NH3 and
non-ionisable
Cl- Ions
CoCl3∙3HN3 0 3 3 + 3 = 6
CoCl3∙4HN3 1 2 4 + 2 = 6
CoCl3∙5HN3 2 1 5 + 1 = 6
CoCl3∙6HN3 3 0 6 + 0 = 6
i. These compounds consists of ionisable as well as non-ionisable
chloride ions.
ii. Number of ionisable and non-ionisable Cl- ions in compound
depends upon number of ammonia molecules in the compound.
iii. With increasing number of NH3 molecules in the compound,
the ionisable chloride ion decreases.
iv. The sum of ammonia and non-ionisable chlorides remains
constant and it is 6.

12. Explanation / Conclusions for Observations
i. In these compounds, cobalt has +3 oxidation state, hence it requires 3Cl- ions for complete
neutralization of charge on Co(III) ion.
ii. Non-ionisable chlorides are bonded differently to Co(III) ion than ionisable.
iii. All NH3 molecules are bonded similarly to Co(III) ions.
To explain these conclusions, Werner
proposed the concept of –
i. Primary valence
ii. Secondary valence in these compounds.
Nothing but Werner’s theory of coordination
compounds.

13. i. Primary Valency: It is equal to the oxidation state of metal ion.
It is satisfied by -vley charged ion in coordination complex as they are able to
neutralize the +ve charge of the metal ion.
It is not fixed i.e. variable.
ii. Secondary Valency: Secondary valency is the number of non-ionisable chemical species
(negative ion or neutral molecule or positive ions) associated with metal ion in
coordination complex.
iii. Secondary valency is fixed and it is non-ionisable.
For example, in compounds CoCl3∙3HN3,
CoCl3∙4HN3, CoCl3∙5HN3, and CoCl3∙6HN3
secondary valency is 6.

14. 3. Negatively charged ions may satisfy primary as well as secondary valency of metal ion.
ii) In the Complex CoCl3∙6HN3, six NH3 satisfy secondary valency while 3 Cl-
ions satisfy only primary valency. Thus, 6NH3 are non-ionisable while 3Cl-
ions are ionisable.
CoCl3∙6HN3 → [Co(NH3)6] + 3Cl-
Example, i) in the complex CoCl3∙3HN3, three Cl- are satisfying both valencies of Co(III)
ion. In this case, both NH3 and 3Cl- are non-ionizable.
CoCl3∙3NH3 = No dissociation
 Negative ions satisfying only primary valency are ionically
bonded to the metal ion. They dissociated in aqueous
solution and produce ions.
 Ligands / ions satisfying secondary do not dissociate in
aqueous solution. They remain bondeded with metal after
dissolution in water.

15. Thus, on this basis, Werner proposed new scheme of writing formula of these coordination
complexes as follows:
 Metal ion and non-ionisable groups are written in square bracket.
 Ionisable groups are written outside the square bracket
Sr. No
Composition of
Compound
Writing Formula of
coordination complex
1 CoCl3∙3HN3 [Co(NH3)3Cl3]
2 CoCl3∙4HN3 [Co(NH3)4Cl2]Cl
3 CoCl3∙5HN3 [Co(NH3)5Cl]Cl2
4 CoCl3∙6HN3 [Co(NH3)6]Cl3

16.  Metal and ligand together form neutral or -vely charged or +vely charged species is called as
complex or complex ion.
 Example –[CoCl3(NH3)3] represents the neutral complex molecule while [Co(NH3)6]3+ represents
the positively charged complex ions.
 Secondary valency is directional i.e. the ligands are oriented at specific direction in the
space around the central metal ion. This give rise definite structure / geometry to complex
or complex ion has.
CoCl3(NH3)3
+3
-
-
-
Co (NH3)6(Cl)3

17. For example – [Co(NH3)4Cl2]: The six groups are bonded to central Co(III) ion. Thus
following three geometries are possible.
Isomerism and Structure Coordination Compounds
Co
NH3
NH3
Co
NH3
Co
NH3
Hexagonal Planar – three isomers are possible
Co
Cl
Cl
NH3
NH3
H3N
H3N
Co
Cl
Cl
NH3
NH3
H3N
H3N
Co
ClCl
NH3
NH3
H3N NH3
Prismatic – three isomers are possible

18. NH3
Octahedral Geometry – Two Isomers are Possible
 To identify correct structure Werner isolated different isomers of this compound.
He observed that only two isomers can be isolated i.e. this compound form only two
isomers.
 Thus, he concluded that structure of [Co(NH3)4Cl2] must be octahedral.
 Afterword on the basis of X-ray crystallography structure of [Co(NH3)4Cl2] was
studied and is confirmed to be octahedral.

19. Pt(NH3)2Cl2 : This complex may Td or Sq. Pl. structure
 To identify correct structure Werner isolated different isomers of this compound.
He observed that only two isomers can be isolated i.e. this compound form only two
isomers.
 If structure is Td then only one isomer is possible
 If structure is Sq. Pl. then two isomers are possible. Thus, he concluded that
structure of [Pt(NH3)2Cl2] must be Sq. Pl.
Pt
NH3
NH3
Cl
Cl
Pt
NH3
NH3
Cl
Cl
Pt
NH3
NH3
Cl
Cl
Sq. Pl. structure – Two IsomersTd structure – One
isomer is possible