Download Reaction mechanism of Coordination Complexes

Reaction mechanism of Coordination Complexes
Complexes are classified as Inert and Labile ( kinetic stability) depending on
their reactivity.
According to Henry Taube, a Nobel Laureate, the definition is

If the half life period of 0.1 M solution at RT is less than 1 min. ( t½ < 1
min) the complex is called Labile

If the half life period of 0.1 M solution at RT is more than 1 min. ( t½ > 1
min) the complex is called Inert
SUBSTITUTION REACTIONS IN COMPLEXES
Complexes usually undergo substitution reactions by the replacement of
ligands with others.
The reaction is common in Octahedral and Square planar complexes and
follows two different paths SN1 and SN2
I.Dissociation Mechanism (SN1 )
Step (i)
Slow step : dissociation of ligand (Y)
MX5Y
( CN = 6 )
Octahedral
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
MX5
+ Y
( CN = 5 )
Trigonal bipyramidal
1
Step (ii)
Fast step : Addition of new ligand (Z)
MX5

( CN = 5 )
Trigonal bipyramidal
MX5 Z
( CN = 6 )
Octahedral
Mechanism :
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2
II. Association Mechanism (SN2 )
Step (i)
Slow step : Association of ligand (Z) to complex
MX5Y + Z
( CN = 6 )
Octahedral
Step (ii)
MX5 YZ
( CN = 7 )
Pentagonal bipyramidal

Fast step : Leaving the ligand (Y)
MX5 YZ

( CN = 7 )
Pentagonal bipyramidal
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MX5 Z
( CN = 6 )
Octahedral
3
Substitution reactions in Square Planar
complexes
Association mechanism is favorable in square planar complexes
Association Mechanism (SN2 )
Step (i)
Slow step : Association of ligand (Z) to complex
MX3Y + Z
( CN = 4 )
Square planar
Step (ii)

MX3 YZ
( CN = 5 )
Trigonal bipyramidal
Fast step : Leaving the ligand (Y)
MX3 YZ

( CN = 5 )
Trigonal bipyramidal
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MX3 Z
( CN = 4 )
Square planar
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Trigonal bipyramidal
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5
TRANS EFFECT
It is observed that during the substitution reactions of square planar metal
complexes, some ligands preferentially direct the substitution trans to
themselves. i.e., the choice of leaving group is determined by the nature of
ligand trans to it.
The Trans effect can be defined as the effect of a ligand over rate of
substitution of another ligand positioned trans to it in the square planar
complexes.
The ability of a ligand to direct the incoming group to the position trans to it,
is called trans directing effect
The rough order of trans-directing effect of various ligands is:
NO+, CO, CN-, C2H4 > PR3, H- > Me- > Ph- > NO2-, I-, SCN- > Br- > Cl- >
Py, NH3, OH-, H2O
Ex 1:
In the second step, the trans-directing effect of
therefore cis-isomer is formed
Cl- > NH3
In the second step, the trans-directing effect of
therefore trans-isomer is formed
Cl- > NH3
Ex 2:
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Explanation :
Ex 3: synthesis of cis and trans – [Pt (NH3)(NO2)Cl2]
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