Thermodynamics and Stability of Co2+, Ni2+, Cu2+ And Zn2+ Complexes With DL-2-Amino Butanedioic Acid

Introduction

From the survey of chemical literature^1-9 it has been found that , the scanty work has been reported on the behaviour and metal complexes of DL-2-amino butanedioic acid as such the present work has been under taken reporting the formation, composition and stability constant of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ complexes at different temperature 20°C, 30°C and 40°C and varying ionic strengths 0.05 M, 0.1 M, 0.15 M and 0.2 M by pH metric titration technique. The thermodynamic function viz. free energy (∆G), enthalpy (∆H), and entropy (∆S) of the complexation reaction have also been determined and explained.

The stability constants and thermodynamic parameters of these metal complexes are discussed on the basis of several physical properties of the title metals such as ionic radii, electronegativity and second ionization potential of the title metal ion.

Experimental

DL-2-amino butanedioic acid was obtained from E. Merck Nickel sulphate, Copper sulphate, Cobalt chloride, Zinc chloride and other reagents of Anala–R B.D.H. grade.

Procedure

pH measurements were made on a EC digital pH-meter (accuracy ± 0.01 pH) with combined glass-calomel electrode assembly in a nitrogen atmosphere. The instrument was standardized with standard buffers. A thermostat having an accuracy of ± 0.1°C maintained the temperature of the cell. Requisite amounts of NaClO₄ were added to the solution mixture for maintaining the ionic strength.

The experimental procedure as described earlier (1) involves a series of pH titration of ligand with 0.1M NaOH in the absence and presence of metal ions at different metal ligand ratio, keeping the total volume 25 ml in each case. For qualitative detection of complex formation, stoichiometric pH titration were performed for the solutions having metal ligand ratio 1:1, 1:2 ,1:3 and 1:4. After each addition of titrant NaOH, pH meter readings were noted on reaching equilibrium. Curves were plotted between pH and moles of alkali required per mole of ligand ‘m’ the inflections in the curves reveal the formation of complexes in solutions from which the stoichiometry has been evaluated.

The stability constants of metal ligand complexes were evaluated by employing Irving Rossotti method^10-11 pH titration technique keeping the ratio of metal to ligand constant at 1:5 using 8 x 10^-3 M HClO₄ for initial lowering of buffer region.

Result and Discussion

The values of formation function _n were plotted against the corresponding free ligand concentration (pL) values to get the formation curves of the metal complexation equilibrium. From these formation curves, the values of stability constants, log K₁ and log K₂ were calculated which correspond to pL values at = 0.5 and 1.5 respectively. The log K stab values were determined at different temperature 20°,30°C and 40°C and at varying ionic strength 0.05M, 0.1M, 0.15M and 0.2M in the pH range 4.25-8.85 are summarized in table 1 and 2 which follow the sequence.

Co²⁺< Ni²⁺< Cu²⁺ > Zn²⁺

Which is in agreement with Irving William order of stability constants of bivalent metal complexes.

Effect of Temperature and Ionic strength

It is seen from Table 1 that the stability constants gradually increase with rise in temperature showing thereby that higher temperature favours the formation of stabler complexes. The effect of increase in ionic strength from 0.05M to 0.2M on the stability has been examined and it is observed that log K increases with rise in ionic strength. These changes have shown in Table 2.

Figure 1: Plot of log B vs 1/r at 30°C Click here to View figure

 

Figure 2: Plot of log B vs Xm × at 30°C Click here to View figure

 

Figure 3: Plot of log B vs ionization potential at 30°C Click here to View figure

 

Table 1: Stability constant of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ Complexes of DL-2- Amino-butanedioic acid at varying temperatures (Ionic strength 0.10 M)

Metal complexes	Log K		20°C	30°C	40°C
Co2+	Log K	1	5.8631	6.0532	6.2930
	Log K2		4.3676	4.4188	4.5184
	Log β		10.2391	10.4720	10.8114
Ni2+	Log K	1	6.8914	7.0914	7.2200
	Log K2		5.0663	5.1677	5.2305
	Log β		11.9577	12.2591	12.4505
Cu2+	Log K	1	8.4759	8.6833	8.8501
	Log K2		6.6564	6.7891	6.9665
	Log β		15.1323	15.4724	15.8166
Zn2+	Log K	1	5.6955	5.9002	6.0938
	Log K2		4.3686	4.4905	4.5681
	Log β		10.0641	10.3907	10.6619

 

Table 2: Stability constant of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ Complexes of DL-2-Amino-butanedioic acid at varying ionic strengths(Temp.30°C)

Metal complexes	Log K		20°C	30°C	40°C
Co2+	Log K	1	5.9785	6.0532	6.1959
	Log K2		4.3173	4.4188	4.4682
	Log β		10.2958	10.4720	10.6641
Ni2+	Log K	1	6.9977	7.0914	7.1958
	Log K2		5.1316	5.1677	5.2339
	Log β		12.2930	12.2591	12.4297
Cu2+	Log K	1	8.5829	8.6833	8.7822
	Log K2		6.7177	6.7891	6.8659
	Log β		15.3006	15.4724	15.6481
Zn2+	Log K	1	5.7925	5.9002	6.0536
	Log K2		4.4165	4.4905	4.5699
	Log β		10.2090	10.3907	10.6235

 

Table 3: Thermodynamic function of Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ Complexes of DL-2-Aminobutanedioic acid at 30° C

Metal complexes	Thermodynamic Function at 30°C (µ= 0.1M)
	− ∆G	∆H	∆S
	(kcal/mole)	(kcal/mole)	(cal/degree/mole
Co2+	14.52	12.08	87.79
Ni2+	16.99	12.23	96.43
Cu2+	21.45	14.36	118.18
Zn2+	14.41	12.50	88.81

 

The stability constants of these transitional metals have also been interpreted in relation to atomic properties of these metals such as ionic radii (A°) electronegativity (Pauling) and 2^nd ionization potential (eV).

Plot of log β against reciprocal of ionic radii of metal ions (Figure1) shows that the ligand forms less stable complexes with Co²⁺ and Zn²⁺ and more stable complexes with Cu²⁺ in comparison to Ni²⁺ complex.

The plot of Xm (Electronegativity of metal) against log β (Figure2) shows that stability of these metal complexes increases with increasing electronegativity which suggest that the metal ligand bond would be covalent. Similar observation has been reported by Van Uitert¹⁴ and Selbin¹⁵.

The second ionization potential rises along the transition series and it becomes maximum Cu²⁺. A correlation of potential and stability was pointed out by Irving Williams¹³ and Schwarzenbach Ackernmann and Prue¹⁶. A similar correlation was observed in the present case of the plots of log β against second ionization potential (Figure 3).

Thermodynamic Parameters

Thermodynamic functions such as free energy(∆ G), enthalpy(∆ H) and entropy (∆ S) accompanying complexation are determined at 30°C with the help of standard equations¹⁷ and are summarized in Table 3.

The negative values of ∆G show that the reaction tends to proceed spontaneously. The positive values of enthalpy indicate the endothermic nature of the reaction process in fair agreement with increasing stability suggesting higher temperature favours the chelation process. A positive change in the entropy strongly indicates the complex formation. The very large entropy changes are also justified by considering the greater availability of coordination sites of these metal ions.

To study the bonding nature of complexes, it is necessary to observe the relation between ionic radii of metal ions and thermodynamic parameters ∆G, ∆H and ∆S, by plotting them against reciprocal of ionic radii of divalent transition metals ions, which show somewhat linear relationship. Martell¹⁸ has suggested a dependence of linear relationship between ∆S and 1/r, for divalent metals. This indicate the pre-dominant role of electrostatic force involved in ion association process, a fact which is in agreement with Bjerrum’s ion pair concept.

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