A Solution Phase Study of Metal - Ligand Interaction of Ni (II), Co (II) and Mg (II) with Glycine and Dicarboxylic Acids: Potentiometric Study

 

Farooque Basheer Ansari*

Rizvi College of Arts, Science and Commerce, Bandra (W), Mumbai- 400050 (M.S.) India.

*Corresponding Author E-mail: farooqueansari2007@rediffmail.com

ABSTRACT:

pH – metry investigation on the complex formation equilibrium of Ni²⁺, Co²⁺ and Mg²⁺ with Glycine (Gly) as a primary ligand and Oxalic acid (OXA) or Malonic acid (MAL) as a secondary ligand shows the formation of ternary systems. For present work different molar ratios of the metal, primary ligands and secondary ligands are used. The ionic strength of 0.1 Molar was kept constant using NaNO₃. The complex formation equilibrium was elucidated with the help of speciation curves. The glycine and OXA or MAL with metal shows an order of

Ni⁺² > Co⁺² > Mg⁺²

 

KEYWORDS: Potentiometric Studies, Ternary Complexes, Glycine, Oxalic acid, Malonic acid, Mixed-Ligand Complexes.

 

 

INTRODUCTION:

The secondary ligand used in the experiment was oxalic acid and malonic acid. Oxalic acid is a dicarboxalic acid having formula HOOC -- COOH (IUPAC name as 1, 2 – Ethan dioic acid) which is found to be 3,000 time stronger acid as compare to acetic acid and its conjugate base is a excellent chelating agent for metal cations at pKa = 1.27 also known as oxalate (C₂O₄ ^-2) where as malonic acid is also a dicarboxylic acid having formula

 

                                                COOH

                               CH₂

                                                 COOH

(IUPAC name 1, 3, - propan dioic acid)

 

Is also chelating agent at pKa = 4.28 thus they form good ternary complexes and there product gives good inside about their stability with various metals.

 

There are various papers appeared in last few decades regarding ternary complexes of transition and non transition metals^1-7. Glycine is a α-amino acid and building block of protein. Its interaction with metals and other molecules will lead to understand various biological processes.

 

Literature survey reveals that very less work has been done on the study of stability constant of ternary complexes (M:L₁:L₂) by using molar ratios 1:1:1, 1:2:2, 1:1:2, 1:2:3. Almost no work has been done in these molar ratios using Glycine as a primary ligand and Oxalic acid and Malonic acid as a secondary ligand therefore in continuation of my work⁸. I decided to undertake present investigation.

 

EXPERIMENTAL:

All chemicals used in this study were of Analar grade obtained from S D. fine chemicals Ltd. Double distilled water was used for preparation of solutions. The NaOH solution of requisite concentration was prepared and standardized by using potassium hydrogen phthalate. The ionic strength in all setup was maintained by using NaNO₃ solutions. Following sets of solutions were prepared.

 

1.     HNO3 (0.1 M) + NaNO3 (1 M)

2.       HNO3 (0.1 M) + NaNO3 (1 M) + Glycine (0.1 M)

3.       HNO3 (0.1 M) + NaNO3 (1 M) + Glycine (0.1 M) +

          Metal (0.1 M)

4.     HNO3 (0.1 M) + NaNO3 (1 M) + OXA or MAL (0.1 M)

5.       HNO3 (0.1 M) + NaNO3 (1 M) + OXA or MAL (0.1 M) +

          Metal (0.1 M)

6.       HNO3 (0.1 M) + NaNO3 (1 M)+Glycine (0.1 M)+ OXA or       MAL (0.1 M)+Metal (0.1 M)

 

Each set of solution was diluted to 50mL in a standard flask. Each solution was titrated separately with standard NaOH solution. A representative titration curves with Ni are shown in fig.1 and fig.2. A Pre-calibrated pH-meter was used to monitor the pH during titration the log K_MX and log K_MXY values were calculated by using computer programme in Excel.

 

RESULTS AND DISCUSSION:

For the present work nitrates of nickel, cobalt and magnesium were taken. The nickel and cobalt are from transition metal series and well-known for Complexation. Magnesium is also equally important from biologically point of view though it belongs to alkaline earth metal.

 

The values of stability constant reveal that-                                        :

1.   A decrease in pH when the secondary ligand (A) is added to the (M: L) solution suggests the release of protons due to ternary complex formation.

2.   The mixed-ligand curve lie below the pure ligand as well as those of binary metal ligand curves indicating the formation of (M:L₁:L₂) complex species.

3.   The pH of hydrolysis in all the mixed-ligand complexes studied was found to be higher than the pH of hydrolysis of the individual complexes.

4.   There was no drift in the pH values due to hydrolysis precipitation up to the pH range studied for

respective system.

5.   Since the mixed-ligand curve did not coincide with either of the individual metal titration curves in the lower pH range, the formation of complex by simultaneous equilibria was inferred.

6.   The formation of mixed complex species in solution was supported by absence of any solid phase during the titration of ternary mixture.

 

 

The log KMXY values for Ternary Complexes of OXA and MAL as secondary ligands (L2) and glycine as a primary ligand (L1) are shown in Table-1.

 

During the present work, we observed that in almost all the cases the stability constant values were in the _{order of-}

_Ni⁺² _{> Co}⁺² _{> Mg}⁺²

This is in accordance with the earlier work⁹   which reported that stability constant of metals with nitrogen protected amino acid shows the following order of stability,

 

Pd⁺² > Hg⁺² > Pb⁺² > Cu⁺² > Ni⁺² > Zn⁺² > Co⁺² > Cd⁺²

This indicates that the metal affinity for Ni, oxygen donor ligands is the major factor determining the stability constant. The order for different metal, ternary complexes reported by Eman Shoukry¹⁰ is-

 

Ba+2 < Sr+2 < Mg+2 < Mn+2 < Cd+2 < Co+2 < Ni+2 < Cu+2 < Zn+2

 

(M1=Nickel, L₁ = Glycine, L₂ = Oxalic Acid, A = HNO₃)

Fig.1: Potentiometric Curves of Ternary Complex

 

(M1=Nickel, L₁ = Glycine, L₃ = Malonic Acid, A = HNO₃)

Fig. 2: Potentiometric Curves of Ternary Complex

 

 

1:1:1, 1:2:2 and 1:1:2 for Ni⁺² and Co⁺² for both the secondary ligands i.e. oxalic acid and malonic acid are stable. However very low value is obtained in case of 1:2:3 for Ni⁺² and Co⁺² when the secondary ligand is malonic acid indicating that primary complex of glycine is quiet stable and malonic acid forms complex with difficulty. The order of stability depends on pair of ligands used for complexation; it was reported that¹¹ Cu- Hydroxamate complexes do not significantly favor mixed ligand complex formation with amino acids having only the carboxylate and amino groups; however these complexes are strongly formed with study histidine, due to the presence of the imidazole group.

 

It was reported that¹² the stability of ternary complexes depends on several factors such as a double bond present in a ligand increases stability of the complex due to exocyclic conjugation, the OH group present in the ligand increases stability due to electron withdrawing nature, if the ring form is bigger, the stability will be low. The methylene group is present in the ligand decreases complex tendency of the ligand, if the ligand is trans isomer, stability is less and for six cis-isomer stability is higher. The stabilizing effect of chelation for histamine shows that stability is more compared to imidazole.¹³

 

The values of log K_MXY for all systems reveals that nickel form most stable complexes over entire range of concentrations of primary ligand and secondary ligand. The magnesium complexes shows low values for  ^{log K}_MXY ^{may be due to the fact that it does not possess empty d-orbital, hence a weak Lewis acid. For all ratios and for both ligands the trend remains almost constant, indicating that concentration of ligand has little effect on the complexation compared to the nature of ligand. The values for oxalate complexes are more than malanato complexes which confirm this hypothesis.}

 

Table-1: The log KMXY values for Ternary Complexes of   M:L1:L2  (where L1= glycine)

M:L1:L2 RATIO	METALS (M)	LIGANDS (L2)
		OXALIC ACID	MALONIC ACID
1:1:1	Ni	7.60313	8.07326
	Co	5.96976	6.22693
	Mg	4.58681	3.83411
1:2:2	Ni	8.27082	8.13707
	Co	6.57636	6.13104
	Mg	3.95566	3.15507
1:1:2	Ni	8.62160	8.02822
	Co	6.39175	6.31042
	Mg	4.70718	4.00290
1:2:3	Ni	8.32603	7.48651
	Co	6.28248	5.68769
	Mg	4.40791	4.09016

 

ACKNOWLEDGMENT:

I am thankful to Dr. Maqdoom Farooqui, Principal, Maulana Azad College, Rauza Baugh, Aurangabad, (M.S.) India for providing laboratory facilities, also thankful to Ansari Mohammed Nasir, Assistant Professor, Hinduja College, Charni Road, Mumbai and also thankful to Abbas Damberwala, Assistant professor, Rizvi College of Arts, Sci. and Commerce, Bandra (W), Mumbai 400050, for his valuable suggestions and discussion.

 

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