Explain the stability factors for co-ordination compounds., Chemistry

The formation of a co-ordination compound involves reaction between a metal ion and ligands. If the force of attraction of the metal ion with ligands is strong a stable complex may result. Generally the complex ions are highly stable. Though, their possibility of dissociation in aqueous solutions cannot be lined out completely though these may be dissociated to a small degree. Thus, a chemical equilibrium may be established between the undissociated complex and the dissociated ions.
(MLn)b+  629_Central ion.png  Ma+ nLx-

Here a+, x- and b+ are the charges on the metal atoms, ligand and complex respectively. Thus, the equilibrium constant of the above reaction is given as 
1833_Coordination compound.png 

The reciprocal of the above constant K, is called stability constant (K8). Greater the value of K8 more is the stability of the complex. 

The magnitude of stability constant helps us to predict that what kind of ligand with a particular metal ion yields a more stable complex. In general, higher is the value of stability constant for a complex ion greater will be its stability. For example a perusal shows that complex of Cu+2 with CN- ligand is more stable than that will ammonia.

Alternatively, 1/K values are sometimes called instability constant.

The stability of complex can also be expressed in terms of its formation constant ( 764_Coordination compound1.png n). Let us understand this idea with the help of an example discussed below.
[M(H2O)n] + nL  629_Central ion.png  [MLn] + nH2O

Follows the following steps:

Step I: 
[M(H2O)n] + L  629_Central ion.png  [ML(H2O)n-1] + H2O

The formation constant for this step,
1244_Coordination compound2.png 

Step II:
[ML(H2O)n-1] + L ? [ML2(H2O)n-2] + H2O

The formation constant of step 2, 
1698_Coordination compound3.png 

Similarly, [ML2(H2O)n-2] + L ? [MLn] + H2O

Thus, for nth step the constant,
2459_Coordination compound4.png 

Now for the overall reaction,
26_Coordination compound1.pngn = K1 × K2 × K3 × ..... × Kn

Here K1, K2, K3, .... etc are stepwise formation constants or stepwise constants while  26_Coordination compound1.pngn is termed as overall formation constant or overall or cumulative stability constant.

Complex ion

Equilibrium reaction



Al3+ 6F-  629_Central ion.png  [AlF6]3-

6.7 × 1019


Cd2+ + 4CN-  629_Central ion.png  [Cd(CN)4]2-

7.1 × 1018


Co3+ + 6NH3  629_Central ion.png  [Co(NH3)6]3+

4.5 × 1033


Cu+ + 3CN-  629_Central ion.png  [Cu(CN3)]2-

2 × 1027


Cu2+ + 4NH3  629_Central ion.png  [Cu(NH3)4]2+

2.1 × 1014


Fe2+ + 6CN-  629_Central ion.png  [Fe(CN)6]4-

1 × 1037


Fe3+ + 6CN-  629_Central ion.png  [Fe(CN)6]3-

1 × 1042


Pb2+ + 3Cl- 629_Central ion.png [PbCl3]-

2.4 × 101


Hg2+ + 4Cl-  629_Central ion.png  [HgCl4]2-

1.2 × 1015


Hg2+ + 4I-  629_Central ion.png  [HgI4]2-

1.9 × 1030


Ni2+ + 4CN-  629_Central ion.png  [Ni(CN)4]2-

1 × 1022


Ag+ + 2NH3  629_Central ion.png  [Ag(NH3)2]+

1.6 × 107


Ag+ + 2 CN-  629_Central ion.png  [Ag(CN)2]-

5.6 × 1018


Ag+ + 2S2O32-  629_Central ion.png  [Ag(S2O3)2]3-

1.7 × 1013


Zn2+ + 4NH3  629_Central ion.png  [Zn(NH3)4]2+

2.9 × 109


Zn2+ + 4CN-  629_Central ion.png  [Zn(CN)4]2-

1 × 1018

While studying the formation of complexes in solution, two kinds of stability of complexes come into question. These are:
Thermodynamic stability: it is a measure of the extent to which the complex will form or will transform into another species when the system has reached equilibrium. This kind of stability deals with metal-ligand bond energy, stability constant etc.

Kinetic stability: this refers to speed with which the transformations occur so as to attain equilibrium. From kinetic view point a complex may be inert or labile rather than stable or unstable. Complexes in which ligands are rapidly replaced by others are called labile or non inert complexes while those in which substitution occurs slowly are called inert complexes.

An example is discussed below to illustrate the above idea.

Cd2+ + NH3 [Cd(NH3)]2+ ...... K1 = 102.65

[Cd(NH3)]2+ + NH3 [Cd(NH3)2]2+ ..... K2 = 102.10

[Cd(NH3)2]2+ + NH3 [Cd(NH3)3]2+ ..... K3 = 101.44

[Cd(NH3)3]2+ + NH3 [Cd(NH3)4]2+ ...... K4 = 100.93

26_Coordination compound1.png4 = K1 × K2 × K3 × K4 = 102.65 × 102.10 × 101.44 × 100.93 = 107.12










Posted Date: 6/25/2012 8:09:52 AM | Location : United States

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