N-(Pyridin-2-ylmethylene)isonicotinohydrazide, pmINH, has been synthesized and characterized on the basis of elemental and spectral data. The protonation constants of pmINH and the stability constants of the base with trivalent La, Pr, Nd, Sm, Eu, and Gd at constant ionic strength (I, mol • dm -3 ) 0.05 M NaClO 4 ) and at different temperatures, T ) (293, 303, and 313) K, were determined potentiometrically in water-dioxane (30 %) medium. The ligand forms only 1:1 complexes with the lanthanides. Both protonation and complexation reactions are found to be exothermic in nature. The trend in the formation constants follows the order: La 3+ < Pr 3+ < Nd 3+ < Gd 3+ < Sm 3+ < Eu 3+ and shows a break at gadolinium. The thermodynamic parameters associated with the complexation reactions were also calculated. The values of ∆G, ∆H, and ∆S are negative for all the systems and suggest that all reactions are enthalpy driven. The thermodynamic parameters of the complexation are correlated with the reciprocal ionic radii of the metal ions. Raw and normalized isothermal calorimetric data for the titrations of Gd 3+ -pmINH solutions at 303 K show that the reaction was exothermic and metal-ligand stoichiometry to be 1:1.

N- (2,ethylidene] hydrazino-2-oxoethyl)benzamide, aehb, was prepared by condensation of N-benzoyl glycine hydrazide with 3-aminoacetophenone and characterized on the basis of elemental and spectral data. Potentiometric studies of the Ni(II), Cu(II), and Cd(II) complexes of aehb were carried out at a constant ionic strength (I, mol • dm -3 ) 0.10 M KNO 3 ) at different temperatures, T ) (290, 300, and 310) K, in aqueous dioxane (40 %) and in nonionic and anionic micellar media. The ligand accepted one proton in the pH range of 5.0 to 11.0 and reacted with the Ni 2+ , Cu 2+ , and Cd 2+ ions in all media to form only 1:1 metal-ligand complexes. The protonation constant of aehb and the stability constants of the complexes had been evaluated from the pH metric titration curves employing Irving and Rossotti's pH metric titration technique. The stability constant of the complexes was in the following order: Cu(II) > Ni(II) > Cd(II). Thermodynamic parameters associated with the protonation and complexation reactions of aehb were also calculated. The values of ∆G and ∆H were negative for all of the systems in all of the media, which suggested that all of the reactions were exothermic and enthalpy-driven. The presence of surfactants in the media resulted in a decrease of the protonation constant of the ligand and stability constants of the complexes due to the compartmentalizing action of the micelles on the reacting species, and the values in different media followed this trend: aqueous > Triton X-100 (Tx-100) > Labs.

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The complexation of N′-[1-(2,4-dihydroxyphenyl)ethylidene]isonicotinohydrazide (DEH) with trivalent Pr, Nd, Gd, Tb, and Ho ions were studied employing potentiometric and spectroscopic techniques. The potentiometric studies carried out at different temperatures, (293.15, 303.15, and 313.15) K, in aqueous dioxane (30 %) medium at a constant ionic strength of I = 0.1 mol·dm–3 KNO3 indicate the presence of two ionizable protons in the ligand. The formation of Ln(III) complexes having 1:1 and 1:2 metal–ligand stoichiometry has been inferred from the metal–ligand formation curves and the elemental analysis data. The trend in the stability of the Ln(III) complexes follows the order: Pr < Nd < Gd > Tb < Ho. Thermodynamic parameters (ΔG, ΔH, and ΔS) of both protonation and complexation reactions are negative for all of the systems suggesting that the reactions are spontaneous, exothermic, and enthalpy-driven. A photoluminescence study reveals that the complexes are not luminescent due to the quenching effect of the ligand or aqueous or OH ions. The Tb(III) complex of the ligand isolated from ethanolic medium is paramagnetic, behaves as a 2:1 electrolyte, and is of 1:2 stoichiometry.