Summary Metal ligand equilibria in solution

Unit-I Chapter l.l
METAL -LIGAND EOULIBRIA
IN SOLUTION

Blow up of syllabus: Introduction, Stabilitv of the metal complexes .
Step-wise
formation constant of the complexes, Overall formation constant of the complexes
and their relationship, Factors affecting the stability of Complex lon, Nature of the
metal ion, Nature of the ligand, Chelate effect, Nature of the solvent,
effect and thermodnamic origin. macrocylic
10 hrs


Stability ofmetal complexes are important in understanding the properties of thecomplexes.
Most ofthe complex COmpounds are stable. Some of the complexes are stable in one
may dissociate in another reagent. reagent and
When a complex dissociate in aqueous solution , there exists an equilibrium between central
metal ion and the ligands from which they are derived.
Consider the dissociation of silver ammonia complex ion in solution.
[Ag(NH)J Agt + 2NH,
The equilibrium constant K is given by
[Ag+] [NH}
K
dissoc =6 X 108
[Ag(NH),]
This infers that the degree of dissociation of thecomplex ion is extremely
is highly stable.
small i.e. complex
The equilibrium constant of the above complex ion is called dissociation
complex ion. The smaller the value of dissociation constant , greater will be the constant of the
of
stability the
complex. Reciprocalof the dissociation constant is called Stability constant of the complex.
Stability constant ofthe complex ion [Ag(NH,),]''s the equilibriunm constant ofthe reverse
reaction.
Ag+2NH, [Ag(NH,),]'
K stab
[Ag(NH),J' =1.67 x 107
[Ag+] [NH,}

, Chemistry VI Semester

STABILITY OFTHE COMPLEXES
Stability of the complexes are of twotypes .Thernodynamic stability andKinetic Stability
1. Kinetic Stability : The speed or rate with which transformation leading tothe
of equilibrium willoccur decides the kinetic stability. attachment
In Kinetic sense, conplexes are two types. Labile and inert complexes.
a) Labile complexes : The complexes in which the ligands are readily or rapidly replaced
by others are called"Labile complexes".
Eg : When excessof aqueous ammonia is added to solution of copper() sulphate i
water,there is a rapid colour change to deep blue takes place instantaneously because
of the replacement of water molecules by ammonia molecules.
[Cu(H,0),P +4NH, [ Cu (NH)J+4H,0
Pale blue Deep blue
Therefore, [Cu(H,0),P is labile complex
b) Inert complexes: The complexes in which substitution occurs
slowly are called"
Inert Complexes."
In [Cr(H,O)J if we add ammonia, it may take hours or
days to replace water by
ammonia at room temperature.
Therefore, [Cr(H,0)J is kinetically inert complex.
[Fe(H,0) and [Cr(H,0) ]* have roughly the same bond energy. This means that
these two complexes are of equal stability from the
thermodynamic point of view.
But [Fe(H,0)P is labile and exchanges its ligands with
other ligands rapidly whereas
(Cr(H,0, is inert and hence exchanges its ligands very slowly.
Step wise formation constants of the
complexes
According to Bjerrum, the formation of a complex in solution proceeds by the
replacement of co-ordinated water molecules byy the ligand molecules or ions. Consider the stepwise
of thecomplex MLn in aqueous solution. formation
Where M is metal ion , n -is number of ligands , L- mono dentate
Ligand
Let [M(H,0)] + L [M(H,0), L]+H,o
K,= [M(H,0),,L]
[M(H,0),]L]