Topice Cover :
(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 Complex: In
this compound, the coordination number of cobalt is 6 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 complex: In
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 3 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
