Study of Ternary Complexes of Bivalent Metal Ions with Dipyridyl and -Diketones

The presence of -carbonyl group with proton on the carbon atom allows a Keto ⇌ Enol tautomerism. By the replacement of the proton by a metal ion -ketoenolate complexes are obtained. p-Bromobenzoylactone (p-BBA) and p-Chlorobenzoylacetone have been chosen to study the interaction with metal ions along with dipyridyl.

A digital pH meter, model pH 5651 (EC, India) was used for pH measurements. The solutions of p-BBA and p-BCA were prepared in freshly distilled dioxane. Dipyridyl was (E Merck) was dissolved in doubly distilled water. The solutions of bivalent metal ions were prepared from reagent-grade bottles. Chemically pure sodium perchlorate was used to maintain the ionic strength of the medium Tetramethylammonium hydroxide (TMAH) in 75% (v/v) aqueous dioxane was used as titrant

The following solutions were titrated potentiometrically against 0.04 M TMAH. (i) 3.0 ml HClO4 (0.02M) +1.0 ml NaClO4 (2.0 M) + 0.5 ml K2SO4 (0.02 M) + 15 .0 ml dioxan +0.5 ml H2O (ii) 3.0 ml HClO4 (0.02M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml K2SO4 (0.02 M) + 0.5 ml dipyridyl (0.02 M) + 15.0 ml dioxan. (iii) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml K2SO4 (0.02 M) + 0.5 ml sec. ligand (0.02 M) +0.5 ml H2O +14.5 ml dioxan. (iv) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml dipyridyl (0.02 M) +0.5 ml metal ion (0.02 M) +15 ml dioxan.

(v) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml dipyridyl (0.02 M) + 0.5 ml metal ion (0.02 M) + 0.5 ml sec ligand (0.02 M) + 14.5 ml dioxan.

The values of stability constants and reproportionation constants are given in table 1. The n values obtained lie below 1.0 in all the cases. Hence only 1:1:1 mixed ligand complex is formed in each system. The complex formation between metal ions and dipyridyl was found to take place at low pH whereas with secondary ligands it takes place at a high pH range. The titration curves for the solutions (iv) and (v) overlap in the lower pH range, indicating that secondary ligands do not combine with metal ions in this pH range and only complexation with dipyridyl takes place. At high pH values, the titration curve of solution (v) diverges from the solutions (iii) and (iv) shows the coordination of the secondary ligands with [M-dipy]2+. The reaction may be represented as KM.dipy.L can be written as The reactions can be written as The values of stability constants of the mixed ligand complexes have been calculated and are given in Table 3.

The correlation between the stability constants of the parent complexes and the mixed ligand complex formed with the same ligands is given as

(i + j = m) The value of Kd = Reproportionation constant. The value of Kd gives an idea about the compatibility of the two ligands in the coordination sphere of the metal ion. If the Kd > 1 then the ligands are compatible and the mixed ligand complex formed should be more stable than the parent complexes. In the present case, Kd values are less than one (Table 3) so the mixed ligand complex formed is less stable than the parent complexes. Hence dipyridyl and -diketones (p-BBA and p-CBA) are non-compatible ligands.

The strength of the individual MA and ML bonds in the complex MAjLi have been calculated from the stability constants of the mixed complex (MAjLi) and parent complexes (MAm and MLm) by using the following relations – The strengths of the metal-ligand bonds in the parent complexes MAm and MLm have been calculated from The results obtained are given in table 5. From the values of bond energies calculated it is found that the mixed ligand complexes have weaker bonds as compared to the parent complexes hence their stabilities will also be lower.

1. K. Yamasaki and M. Yasuda (1956), J. Am. Chem. Soc., 78, pp. 1324. 2. M. Yasuda, K. Sane and K. Yamasaki (1956), J. Phys. Chem., 60, pp. 1667. 3. R. Griesser and H. Sigal (1970), Inorg. Chem., 9, pp. 1238. 4. G. Anderegg (1963), Helv. Chim. Acta, 46, pp. 2397.