Investigations of ternary complexes of Co(II) and Ni(II) with oxydiacetate anion and 1,10-phenanthroline or 2,2′-bipyridine in solutions

Abstract

Potentiometric (PT) and conductometric (CT) titration methods have been used to determine the stoichiometry and formation constants in water for a series of ternary complexes of Co(II) and Ni(II) involving the oxydiacetate anion (ODA) and 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) ligands, namely [Co(ODA)(phen)(H2O)], [Co(ODA)(bpy)(H2O)], [Ni(ODA)(phen)(H2O)] and [Ni(ODA)(bpy)(H2O)]. The ternary complex formation process was found to take place in a stepwise manner in which the oxydiacetate ligand acts as a primary ligand and the phen or bipy ligands act as auxiliary ones. The stability of the ternary complexes formed is discussed in the relation to the corresponding binary ones. Furthermore, the kinetics of the substitution reactions of the aqua ligands in the coordination sphere of the Ni-ODA and Co-ODA complexes to phen or bipy were studied by the stopped-flow method. The kinetic measurements were performed in the 288–303 K temperature range, at a constant concentration of phen or bipy and at seven different concentrations of the binary complexes (4–7 mM). The influence of experimental conditions and the kind of the auxiliary ligands (phen/bipy) on the substitution rate was discussed.

Investigations of ternary complexes of Co(II) and Ni(II) with thiodiacetate anion and 1,10-phenanthroline or 2,2’-bipyridine in aqueous solutions

“…Due to the greater basicity of the phen ligand, the complexes with phen are more stable than the corresponding complexes with bipy. The same situation was also observed for the ternary oxydiacetate complexes of Co(II) and Ni(II), M(ODA)(B) (where M denotes Co(II) or Ni(II), B denotes phen or bipy) [3]. The ternary complexes undergo hydrolysis and the resulting hydroxo complex species reach the highest concentration in the 10-11 pH range.…”

“…The present study constitutes a continuation of our earlier efforts in characterization of the oxydiacetate complexes in solutions [1,3]. It seems to be worth investigating how the substitution of the ethereal donor atom in the oxydiacetate ligand (ODA) by the sulphur donor atom (the thiodiacetate ligand, TDA) affects the stabilities of the complexes in solutions.…”

“…It is also interesting to note that the binary thiodiacetate Co(II) and Ni(II) complexes in the system under study (M:L = 1:1) are more stable than the corresponding oxydiacetate complexes (M-ODA) [3]. This phenomenon can be explained by the differences in the electronegativity between the central donor atoms of the ODA and TDA ligands (oxygen and sulfur atoms, respectively) and the metal ion in the coordination centres of the complexes (Co 2+ or Ni Table 1.…”

“…For this reason the formula that reflects the structure of the ternary complexes in solutions should be written as M(TDA)(B)(H 2 O). However, it is worth emphasizing that the formation of the ternary complexes is not favoured over the corresponding binary ones, as occurs in the case of the oxydiacetate cobalt(II) and nickel(II) complexes [3]. This conclusion can be drawn from the assessment of the relative stability of the ternary and the corresponding binary complexes, according to the following equation [16]: (3) (4) where is the logarithm of stability constant represented by the equilibrium ML + B = MLB and .…”

“…The compositions of the compounds were established on the basis of the elemental analysis of carbon and hydrogen (Vario EL analyzer Cube CHNS). 3 ] were carried out according to the procedures described in the literature [4,5]. Thus, to the solution of thiodiacetic acid (H 2 TDA), obtained by dissolving 9 mmol of H 2 TDA in 60 mL of water, a stoichiometric quantity of Na 2 CO 3 was added.…”