Stability Constants of Mixed Ligand Complexes of Transition Metal ( II ) Ions with Salicylidene-4-methoxyaniline as Primary Ligand and 5-Bromosalicylidene-4-nitroaniline as Secondary Ligand

Binary and ternary complexes of the type M-Y and M-X-Y [M = Mn(II), Ni(II), Cu(II) and Zn(II); X = salicylidene-4-methoxyaniline and Y=5-bromosalicylidene-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. The relative stability (∆ log KT) values of the ternary complexes with corresponding binary complexes for all the metal(II) ions in the present study found to be 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) > Mn(II) > Zn(II); which is same as in the corresponding binary (M-Y) systems.


Introduction
Metal complexes of the Schiff bases have occupied a central role in the development of coordination chemistry 1 .Many attempts have been made to evaluate different factors affecting the stability of the metal chelates along with their stability constants [2][3][4] .In the present study the stability constants of the mixed ligand complexes of Mn(II), Ni(II), Cu(II) and Zn(II) with salicylidene-4-methoxyaniline (X) as primary ligand and 5-bromosalicylidene-4-nitroaniline (Y) as secondary ligand in 75: 25(v/v) 1,4-dioxane-water medium at 27±0.5 o C have been reported by employing pH-metric titration technique [5][6][7][8][9][10] .Under identical conditions the stability constants of binary metal complexes of 5-bromosalicylidene-4nitroaniline (Y) have also been investigated.

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 n A (average number of protons associated with secondary ligand); n (average number of secondary ligand molecules attached per metal ion; n 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.

Proton-ligand stability constant of secondary ligand (Y)
From the titration curves of solutions (i) and (ii), n A values at various pH were calculated.The proton ligand formation curve was obtained by plotting the values of n 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).The values of log H K 1 and log H K 2 were also evaluated using graphical method

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.1.
The variations of ñ was found to be between 0.0-2.0 for the binary (M-Y) complexes of Mn(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 1 values for the binary complexes of the metal ions are in the following order: Cu(II) > Ni(II) > Mn(II) > Zn(II).

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: 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), n mix and pL mix values were calculated.The values 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) 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) > Mn(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) > Mn(II) > Zn(II); which is same as in the corresponding binary (M-Y) systems.

From the titration curves 2 ,
of solutions (ii) and (iii), n and pL values were calculated.The formation curves were obtained by plotting the values of n vs. pL.From the graph the values of log using graphical method (B) by plotting the graph of log [ n / (1n )] against pL and log [(2n ) / ( n -1)] against pL respectively.The values obtained by method A and B are in agreement with each other, the average values of log along with metal-ligand stability constants the log β values of the binary complexes are given in Table (B) by plotting the graph of log [ n A / (1n A )] against pH and log [(2n A ) / ( n 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 log H K 2 has been found to be 8.40 and 2.80 respectively.