Stability Constants of Mixed Ligand Complexes of Transition Metal ( II ) ions with N-( 2-hydroxybenzylidene )-2 , 3-dimethylaniline as Primary Ligand and N-( 2-hydroxy-1-naphthylidene )-4-nitroaniline 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-hydroxybenzylidene)-2,3dimethylaniline and Y = N-(2-hydroxy-1-naphthylidene)-4-nitroaniline] have been examined pH-metrically at 27±0.5 C and at constant ionic strength, μ=0.1 M (KCl) in 75:25 (v/v) 1,4-dioxne-water medium. The stability constants for binary (M-Y) and 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 11 .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 (free ligand exponent for binary (M-Y) system) and pL mix (free ligand exponent for ternary (M-X-Y) system) values.All the solvents and chemicals used were of A R grade.
For the determination of proton-ligand stability constant 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) 1,4-dioxane-water medium against standard carbonate free KOH (0.2 M) solution.The ratio of metal (M): secondary ligand (Y) was maintained at 1:4 in the binary system.In the ternary system the ratio of metal (M): primary ligand (X): secondary ligand (Y) was maintained at 1:1:1.The pH-meter readings were plotted against the volume of alkali (KOH) used for each titrations.

Proton-ligand stability constant 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 value of log H K 1 was evaluated by half integral method (A) and in the similar way the value of log H K 1 was evaluated using graphical method (B) by plotting the graph of log [ñ A / (1-ñ A )] against pH.The values obtained by method A and B are in agreement with each other, the average value of log H K 1 has been found to be 8.625.

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 products were not formed.An examination of titration curves indicate that complex formation takes place in the solution on the following grounds: (I) The metal titration curve of solution (iii) shows displacement with respect ligand (Y) titration curve of solution (ii) along the volume axis.This indicates the affinity of the ligand to metal ions which release proton and produce volume difference.(II) The color change of ligand appeared in the presence of metal ion shows the formation of new species due to coordination.(III) The hydrolysis of the metal ion was suppressed due to the complex formation and precipitation did not appear during the titrations.
From the titration curves of solutions (ii) and (iii), ñ and pL values were calculated.The formation curves were obtained by plotting the values of ñ vs.  1.
The variations of ñ was found to be between 0.0-2.

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 products were not formed.An examination of the titration curves indicate that the ternary complexes formation has taken place in the solution on the following grounds: (i) the horizontal distance was measured between ternary titration curves of solution (v) and secondary ligand titration curve of solution (iv), the positive difference shows the earlier release of protons in the formation of ternary complexes.(II) The hydrolysis of the metal ion was suppressed and precipitation did not appear during the titrations.
From the titration curves of solutions (iv) and (v), ñ mix and pL mix values were calculated.The values of log MX MXY K have been evaluated from the formation curves (ñ mix vs.. pL mix ).At ñ mix = 0.5, in the formation curve, pL mix = log MX MXY K (method A).The values of log MX MXY K were also evaluated by the graphical method (B), by plotting the graph of log [ñ mix / (1-ñ mix )] against pL mix .The values of ñ mix vary from 0.0-1.0,thus confirm the formation of 1:1:1 mixed ligand complexes.The values obtained by method A and B are in agreement with each other, the average values of log 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) 12 .
The relative stability of the ternary complexes compared with corresponding binary complexes can be qualitatively expressed in many different ways.We have expressed the relative stabilities in terms of ).The ∆ log K T values for all the metal(II) ions in the present study (Table 1) is negative.This indicates that ternary 1:1:1 (M-X-Y) complexes are less stable than binary 1:1 (M-Y) complexes 13,14 .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 15,16 .

Conclusion
The value of log , 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.

2 ,
pL.From the graph the values of log by half integral method (A) and in the similar way the values of log M graphical method (B) by plotting the graph of log [ñ / (1-ñ)] against pL and log [(2-ñ) / (ñ-1)] against pL respectively.The values obtained by method A and B are in agreement with each other, the average values of log M along with metal-ligand stability constants the log β values of the binary complexes are given in Table

1 values
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.The log M MY K for the binary complexes of the metal ions are in the following order: Cu(II) > Ni(II) > Co(II) > Zn(II).