*<sup>n</sup>* <sup>¼</sup> Total number of ligands coordinated to metal

**Table 1.**

*Stability constant values obtained for Ni, Mg and Co complexes having different metal to ligand ratio.*

*Stability of Metal Complexes DOI: http://dx.doi.org/10.5772/intechopen.90894*

constants, the concentrations of Mn+ and L� are varied, and such variations in the concentration will lead to changes in the ionic strength of the solutions. In order to maintain the constant ionic strength, a large excess of an ionic salt is added to the reaction mixture. The presence of large excess of ionic salt will compensate any changes in the ionic strength of the solution. The ionic salts that are added for such purpose should not react with M2+ or L�, and commonly used salts include KNO3

For example, KNO3 was added in excess during the binding study of the ligand p-aminobenzoic acid with Ni, Mg, and Co metal ions. The p-aminobenzoic acid has two coordination sites such as amino and carboxylate groups and has a pKa value of 5.9153. The stability constant values obtained for Ni, Mg, and Co complexes are

The stability constant values for Ni shows the trend 2:3 < 1:5 < 1:1 < 1:2, while the trend for Co is 1:2 < 2:3 < 1:1 < 1:5 and for Mg it is 1:5 < 1:2 < 1:1 < 2:3. The values obtained from the above study indicates that 1:2 complex of Ni complex is more stable, whereas Co complex is stable in 1:5 ratio and that of Mg is more stable

The Irving and Rossotti method for the determination of stability constant is also

Step 2: Calculation of formation functions n, nA, and PL using the values used/

The term formation function "n," also called as ligand number, is defined as the average number of ligands attached per metal center and is calculated using the

> *<sup>n</sup>* <sup>¼</sup> Total number of ligands coordinated to metal Total number of metal

Ni 8.492 14.8593 8.3598 3.4649 Mg 8.4664 8.3392 7.0794 11.1943 Co 8.590 5.3186 8.6337 6.2330

*Stability constant values obtained for Ni, Mg and Co complexes having different metal to ligand ratio.*

**1:1 1:2 1:5 2:3**

based on the principle of potentiometric method [21]. Using this method, the formation curve of metal complex can directly be calculated with the help of pH meter. Another major advantage of this method over the Bjerrum's method is that the calculation is simple and does not require hydrogen ion concentration. Moreover, this method can be used for types of ligands that are conjugate to weak acids. The calculation of stability constant using this method involves the following steps. Step 1: The following solutions were titrated separately against base solution

� and ClO4

� ions for most of the M2+ ions.

and NaClO4, due the low affinity of NO3

*Stability and Applications of Coordination Compounds*

depicted in **Table 1**.

in the ratio of 2:3.

**4.4 Irving and Rossotti method**

a. Titration with free acid (A)

obtained from above three titrations

following equation

**Table 1.**

**36**

b. Titration with free acid + ligand (A + L)

c. Titration with free acid + ligand + metal (A + L + M)

**Metal M:L ratio**

The term nA is similar to n and is defined as the average number of protons bound to the ligand which are not coordinated to the metal center. PL gives the free ligand exponent. All the three terms n, nA, and PL can be calculated with the help of following equations

$$\text{mA} = \chi - \frac{(\text{V}\_2 - \text{V}\_1)\left(\text{N} + \text{E}^0\right)}{(\text{V}\_0 + \text{V}\_1)\text{T}^0 \text{L}} \tag{25}$$

$$\mathbf{n} = \frac{(\mathbf{V\_3} - \mathbf{V\_2})(\mathbf{N} + \mathcal{E}^0)}{(\mathbf{V\_0} + \mathbf{V\_2})\mathbf{n}\mathbf{A}\mathbf{T}^0\mathbf{M}}\tag{26}$$

$$\text{PL} = \log\left\{ \mathbf{1} + \frac{\frac{\left[\mathbf{H}^{+}\right]}{\mathbf{K}\_{2}} + \frac{\left[\mathbf{H}^{+}\right]^{2}}{\mathbf{K}\_{1}\mathbf{K}\_{2}}}{\left(\mathbf{T}^{0}\mathbf{L} - \mathbf{T}^{0}\mathbf{M}\right)\overline{\mathbf{n}}} \mathbf{X} \frac{\left(\mathbf{V}\_{0} + \mathbf{V}\_{3}\right)}{\mathbf{V}\_{0}} \right\} \tag{27}$$

where N is the normality of base used; V0 is the initial volume of the solution; V1, V2, and V3 are the volume of base consumed during the (A), (A + L), and (A + L + M) titrations, respectively, at same pH value; T<sup>0</sup> L is the initial concentration of ligand; ℇ<sup>0</sup> is the initial concentration of acid; γ is the number of titrable or replaceable protons.

Step 3: Determination of formation curves: by plotting formation function (n) against PL and nA against pH for a HL (protonated ligand) system.

The value formation constants corresponding to formation of protonated ligand are obtained by plotting nA against pH. Similarly, the stepwise stability constants for the formation of metal complexes are obtained from the formation curve resulted by plotting n against PL.

#### **5. Conclusions**

The thermodynamic and kinetic stability of coordination compounds along with the various factors affecting the stability of metal complexes have been discussed in this chapter. Stability constant and its determination have also been listed.

### **Acknowledgements**

Authors acknowledge National Institute of Technology Kurukshetra, Haryana, India, for its support.

#### **Conflict of interest**

There is no conflict of interest.

*Stability and Applications of Coordination Compounds*

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*Stability of Metal Complexes*

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*DOI: http://dx.doi.org/10.5772/intechopen.90894*

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