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WERNER’S THEORY OF COORDINATION THEORY:


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(1) Introduction of Werner’s theory:
The systematic study of coordination compounds was started by a very famous Swiss scientist Alfred Werner whose pioneering work opened an entirely new field of investigation in inorganic chemistry. He prepared and characterized a large number of coordination compounds and studied their physical, chemical and isomeric behaviour by simple experimental techniques. On the basis of these studies. Werner, in 1898, propounded his theory of coordination compounds. Which is later termed as Werner’s Theory of Coordinate Compounds. Due to this theory he is awarded by Nobel prize and he is also called the ‘Father of Coordination Chemistry’.

(2)Postulates of Werner’s theory:
In coordination complex central atom metals exert two types of valencies; The important postulates of Werner’s theory are as follows:
(1) The primary or ionizable valencies which are satisfied by negative ions and equal the oxidation state of the central metal atom.
(2) The Secondary or non ionizable valencies which can be satisfied by neutral or negative ions/groups (or sometimes by cationic species -discovered later).
(3) The secondary valencies equal the coordination number of central metal atom/ion. This number is fixed for a metal.
(4) The ions/groups bound by the secondary linkages have characteristic spatial arrangements corresponding to different co-ordination numbers. In the modern terminology, such spatial arrangements are called coordination polyhedra. The various possibilities are
Coordination Number = 2 Linear
Coordination Number = 3 Triangular
Coordination Number = 4 Tetrahedral or sq. planar
Coordination Number = 5 square pyramidal or TBP
Coordination Number = 6 Octahedral
(5) The secondary valencies are generally represented by solid lines while the primary valencies are represented by dashed lines and the ions which satisfy both primary and secondary valencies will be drawn with both solid and dashed lines.
For example the complex [CoCl(NH3)5] Cl2 is represented as
(3) Examples Based on Postulates of Werner’s Theory:


Werner’s study structures of various cobalt ammines and form a series of complex which is called Werner’s series and it is given below:

Cobalt has a primary valency (oxidation state) of three and exhibit secondary valency (coordination number) of 6. The secondary valencies are represented by thick lines and the primary valency is represented by broken lines.

(1) CoCl3.6NH3 ComplexIn this compound, the coordination number of cobalt is and all the 6 secondary valencies are satisfied by NH3 molecules (the black solid lines). The 3 primary valencies are satisfied by chloride ions (represented by dotted line). These are non-directional in character. These chloride ions are instantaneously precipitated on the addition of silver nitrate. The total number of ions in this case is 4, three chloride ions and one complex ion. The central ion and the neutral molecules or ions satisfying secondary valencies are written in a square bracket while writing the formula of the compound. Hence the complex may be written as [Co(NH3)6]Cl3 and as shown as in fig. 


(2) CoCl3.5NH3 complex: In this compound the coordination number of cobalt is also 6 but the number of NH3 molecule is decreased to 5 from 6 and one remaining position is now occupied by chloride ion. This chloride ion exhibits the dual behaviour as it has primary as well as secondary valency. Secondary valency is shown by full line and the primary valency is shown by dotted line in the fig. This structure satisfies the 3 primary and 6 secondary valencies of cobalt. Hence the complex formed may be formulated by writing five ammonia molecules and one chloride ion inside the square brackets and the two chloride ions outside the brackets [CoCl(NH3)5]Cl2.
(3) CoCl3.4NH3 complex: In this compound, two chloride ions exhibit dual behaviour of satisfying both Primary and Secondary Valencies. This compound will give precipitate with silver nitrate corresponding to only one Cl- ion and the total number of ions in this case is 2. Hence it can be formulated as [CoCl2(NH3)4]Cl.
(4) CoCl3.3NH3 complexIn this compound, three chloride ions satisfy primary as well as secondary valency. No Cl- will be precipitated on the addition of silver nitrate at room temperature. Therefore, the complex compound behave as neutral non conducting molecule. It may be formulated as [CoCl3(NH3)3].
Colour Coordination compounds of CoCl3 with NH3: The important aspect of the structure of five different complexes of CoCl3 with ammonia prepared by Werner is shown in table.

SN
Werner Complex
Colour of complexes
Naming of complexes based on colour
1
CoCl3.6NH3
[Co(NH3)6]Cl3
Luteo complex
2
CoCl3.5NH3
[Co(NH3)5Cl]Cl2
Purpureo complex
3
CoCl3.4NH3
[Co(NH3)4Cl2]Cl
Praseo  complex
4
CoCl3.3NH3
[Co(NH3)3Cl3]
Violeo complex

Coordination compounds of CoCl3 with NH3:

SN
Werner Complex
Modern Formula
Number of Cl- Ions precipitate  with AgNO3
Total number ions formed
1
CoCl3.6NH3
[Co(NH3)6]Cl3
3
4
2
CoCl3.5NH3
[Co(NH3)5Cl]Cl2
2
3
3
CoCl3.4NH3
[Co(NH3)4Cl2]Cl
1
2
4
CoCl3.3NH3
[Co(NH3)3Cl3]
0
0 (Non electrolyte)

Conductivity and Colligative Properties:

                   CoCl3.6NH3 > CoCl3.5NH3 > CoCl3.4NH3 > CoCl3.3NH3

Coordination compounds of PtCl4 with NH3:
The important aspect of the structure of five different complexes of PtCl4 with ammonia prepared by Werner is shown in table.

SN
Werner Complex
Modern Formula
Number of Cl- Ions precipitate  with AgNO3
Total number ions formed
1
PtCl4.6NH3
[Pt(NH3)6]Cl4
4
5
2
PtCl4.5NH3
[Pt(NH3)5Cl]Cl3
3
4
3
PtCl4.4NH3
[Pt(NH3)4Cl2]Cl2
2
3
4
PtCl4.3NH3
[Pt(NH3)3Cl3]Cl
1
2
5
PtCl4.2NH3
[Pt(NH3)2Cl4]
0
0 (Non electrolyte)

Conductivity and Colligative Properties:

    PtCl4.6NH3 > PtCl4.5NH3 > PtCl4.4NH3 > PtCl4.3NH3 > PtCl4.2NH3

(4) Werner’s Theory and Isomerism

Werner turned his attention towards the geometrical arrangements of the coordinated groups around the central cation and explained successfully the cause of optical and geometrical isomerism of these compounds. Some examples are given below:
Octahedral complexes:
[CoCl2(NH3)4]Cl: Werner said that theoretically there are three structure possible for this complex. These are planar, trigonal prism, octahedral. The number of possible isomer is for planar, 3 for trigonal prism and 2 for octahedral structure.
Various possible isomers for the planar, trigonal prism and octahedral structures of the complex ion [CoCl3(NH3)4] since only two isomers of the compound could be isolated, werner concluded that geometrical arrangement of the coordinated group around the central atom in this compound was octahedral. In the case of several other complexes in which the coordination number of the central atom was six, werner was able to conclude that in all these cases the six coordinated complex have octahedral geometry.
Square Planar and Tetrahedral:
He studied the geometry of the complexes in which the coordination number of the central metal atom is 4. He proposed that there are two possible structures. 
 [PtCl2(NH3)2] complex: In this complex the coordination number of the metal is 4, werner found that it existed in two isomeric forms, cis and trans. This shows that all the four ligands lie in the same plane. Therefore the structure should be a square planar or tetrahedral.
Cis and trans isomers of [PtCl2(NH3)2] complex

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