INTRODUCTION:
The co-ordination chemistry of Metal-complex play a vital role in biological system of organism various researchers have studied the mixed ligand complexes of transition metal ions with simple and substituted organic acid 1.
Benzoic acid is taken as primary ligand, it is used as a preservative in food stuff and in soft drinks to inhibits growth of bacteria at low PH values 2.
Mixed-ligand complexes are formed in solutions containing metal ion with two or more different ligands. A number of reviews have appeared on the stability of mixed ligand complexes 3. Study of biologically important ligands with different metals and their ability of complexation have been carried out by many workers 4-5.
The study of ternary complexes of different metal ions with amino acids and bicarboxylic acids have been carried out by many workers 6-7.
Metals form complexes with different ligands. There are a large number of chelating agents, but the donor atom, which co-ordinate with the metal ions is very few. Common donor atoms are Nitrogen, Oxygen and Sulphur 8. Amino acids are well known chelating agents, their importance in biological system and in industry has earned attention of chemists for their metal complexes 9-10.
During past several decades, di-amine and their derivatives are studied for important applications are stable complexes in the fields like biotechnology, environmental science and biochemistry 11.
In the present study, an attempt has been made to determine the stability constants of binary complexes of Mn, Ni and Cr, with benzoic acid and substituted benzoic acid. Various factors influencing the formation and stabilities of binary and ternary complexes.
EXPERIMENTAL:
All the reagents used were of A.R. Grade. The metals nitrate was used. All the solutions for the equilibrium study were prepared in Double Distilled water. Perchloric acid and sodium hydroxide were standardized by acid-base titrations 12.
All the potentiometric titrations were carried out by using carbon dioxide free sodium hydroxide solution. The proton-ligand and metal-ligand stability constant are calculated on the bases of literature methods 13.
RESULT AND DISCUSSION:
The formation constant of the binary complexes formed due to interaction of metal and ligand ions. The proton ligand stability constant is presented in the table-1.
The present study has great importance in the development of co-ordination chemistry. Metal ligand stability constant log k of Cr (II) metal ions with ligand were calculated by point wise and half-integral method as suggest by Irving and Rossotti (Table-2).
Table – 1: Protonation Constants
|
Ligands |
Log k |
Log k |
Cr (II) |
|
Benzoic acid |
3.25 |
-- |
2.61 |
|
p-chloro benzoic acid |
7.98 |
10.96 |
3.79 |
|
p-nitro benzoic acid |
8.62 |
10.88 |
2.68 |
|
p-amino benzoic acid |
7.69 |
11.02 |
3.86 |
The ternary complexes serve as useful models for many biological reactions. A connection between metal-chelation and at least types of cancer was suggest by Furst 14.
In present study we observed increasing trend in the log KMXY values of Metal-ligand chelation.
Table – 2: Stability constant of Mixed ligand complexes
|
Metal ions |
Mixed-ligand system |
log KMXY |
D log k |
|
Cr (II) |
Benzoic acid + P-chloro benzoic acid |
2.92 |
3.06 |
|
Benzoic acid + P-nitro benzoic acid |
5.59 |
3.18 |
|
|
Benzoic acid – P-amino benzoic acid |
5.64 |
4.09 |
The titration curves were carefully analysed for the possibility of protonation of :NH group and COOH. All the chelates formed in simultaneous equilibria by taking M : X : Y in the ration 1 : 5 : 5.
The log KMXY of all the ternary complexes and chelates were determined by Loraas and Thomson method 15 in the PH-region where there was a maximum differences between composite curve and mixed ligand titration curve.
Earlier reported log k values are negative for ternary complexes indicating the primary ligand anions and secondary ligand anions forms mixed-ligand complex 16.
For the present investigation we observed the log k values for mixed-ligand complexes are positive.
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Received on 22.05.2012 Modified on 10.06.2012
Accepted on 05.07.2012 © AJRC All right reserved
Asian J. Research Chem. 5(7): July, 2012; Page 846-847