Stability Constants of Mixed Ligand Complexes of Transition Metal(II) Ions with N -(2-Hydroxy-1-naphthylidene)-2,6-diisopropylaniline as Primary Ligand and N -(2-Hydroxybenzylidene)-2,3-dimethylaniline as Secondary Ligand

: Binary and ternary complexes of the type M-Y and M-X-Y [M = Co(II), Ni(II), Cu(II) and Zn(II); X = N -(2-hydroxy-1-naphthylidene)-2,6-diisopropylaniline and Y = N -(2-hydroxybenzylidene)-2,3-dimethylaniline] have been examined pH-metrically at 27±0.5 o C and µ = 0.1 M in 75: 25% (v/v) 1,4-dioxne-water medium. The logarithms of the values of stability constants for binary (M-Y) and for ternary (M-X-Y) systems were calculated.


Experimental
The pH-meter model no. EQ-614 supplied by Equiptronics, a precision research pH-meter with wide range of glass electrode and calomel reference electrode, was used for pH measurements. The pH-meter was standardized with potassium hydrogen phthalate and phosphate buffers before performing the titrations.
The solutions of ligands were prepared in 1,4-dioxane. All the metal ion solutions were prepared in double distilled water and standardized by using conventional procedures 8 . A solution of KOH (0.2 M) was prepared in double distilled water and standardized with standard solution of succinic acid. The titrations were carried out in an inert atmosphere of nitrogen. All the measurements were carried out at temperature 27±0.5 o C. The method of Bjerrum and Calvin as modified by Irving and Rossotti 5,6 was used to determine ñ A (average number of protons associated with secondary ligand); ñ (average number of secondary ligand molecules attached per metal ion; ñ mix (average number of secondary ligand molecules attached per (M.X)ion); pL and pL mix (free ligand exponent for binary (M-Y) and ternary (M-X-Y) system respectively) values. All the solvents and chemicals used were of A R grade.
For the determination of proton-ligand stability constants of secondary ligand (Y) and metal-ligand stability constants of binary (M-Y) and ternary (M-X-Y) complexes, the following sets of solutions were prepared keeping the total volume V o = 40 mL. All titrations were carried out at the ionic strength of 0.1 M using KCl as an electrolyte in 75:25 %(v/v) 1,4dioxane-water medium against standard carbonate free KOH (0.

Proton-ligand stability constants of secondary ligand (Y)
From the titration curves of solutions (i) and (ii), ñ A values at various pH were calculated. The proton ligand formation curve was obtained by plotting the values of ñ A vs. pH-meter readings. From the graph the values of log H K 1 and log H K 2 were evaluated by half integral method (A) and in the similar way the values of log H K 1 and log H K 2 were evaluated using graphical method (B) by plotting the graph of log [ñ A / (1-ñ A )] against pH and log [(2-ñ A ) / (ñ A -1)] against pH, respectively. The values obtained by method A and B are in agreement with each other, the average values of log H K 1 and log H K 2 are given in Table 1.

Metal-ligand stability constants of the Binary (M-Y) complexes
The metal ligand stability constants of binary complexes were evaluated assuming the polynuclear complexes and hydrolyzed product formation does not take place. The examination of titration curves indicate that complex formation takes place in the solution on the following grounds:  Table 2. The variations of ñ was found to be between 0.0-2.0 for the binary (M-Y) complexes of Co(II), Ni(II), Cu(II) and Zn(II) metal ions, which indicate that the composition of the complexes were 1:2 in solution. Table 2

Metal-ligand stability constants of the ternary (M-X-Y) complexes
The metal-ligand stability constants of ternary complexes were evaluated assuming that the formation of polynuclear complexes and hydrolyzed product does not take place. An examination of the titration curves indicated that the ternary complexes formation has taken place in the solution on the following grounds: (a) the horizontal distance was measured between ternary titration curves of solution (iv) and secondary ligand titration curve of solution (ii), the positive difference shows the earlier release of protons in the formation of ternary complexes; (b) the hydrolysis of the metal ion was suppressed and precipitation does not result.  Table 2. ). The ∆log K T values for all the metal(II) ions in the present study is negative. This indicates that ternary 1:1:1 (M-X-Y) complexes are less stable than binary 1:1 (M-Y) complexes 9 . The ∆ log K T values are given in Table 2.

Conclusion
The value of log MX MXY K are slightly lower than log M MY K 1 and higher than log MY MY K 2 , which is due to the fact that the tendency of the secondary ligand (Y) to get bound with aquated metal ion [M(aq)] 2+ is more than to combine with the metal ion already bound with primary ligand (X). The relative stability (∆ log K T ) values of the ternary complexes with corresponding binary complexes for all the metal(II) ions in the present study is negative indicating that ternary 1:1:1 (M-X-Y) complexes are less stable than binary 1:1 (M-Y) complexes. In the ternary system studied, the order of stability constants of mixed ligand complexes with respect to the metal ions was found to be Cu(II) > NI(II) > Co(II) > Zn(II); which is same as in the corresponding binary (M-Y) systems. This is in accordance with the Irving-Williams series of stability constant.