M. Tabata, T. Murase, Kousuke Kurisaki
Mar 30, 2005
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Influential Citations
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Journal
Inorganica Chimica Acta
Abstract
Abstract The kinetics of the complexation of Ni(II) with 1,10-phenanthroline(phen), 4,7-dimethyl-1,10-phenanthroline(dmphen), and 5-nitro-1,10-phenanthroline(NO 2 phen) in acetonitrile–water mixed solvents of acetonitrile mole fraction x AN = 0, 0.05, 0.1, 0.2 and 0.3 at 288, 293, 298 and 303 K have been studied by stopped-flow method at ionic strength of 1.0 (NaClO 4 ) and pH 7.4. The corresponding activation enthalpy, entropy, and free energy were determined from the observed rate constants. The complexation of Ni(II) with the three ligands has comparable observed rate constants; in pure water the observed rate constants are (×10 3 dm 3 mol −1 s −1 ) 2.31, 2.57, and 1.38 for phen, dmphen and NO 2 phen, respectively. The corresponding activation parameters for the three ligands are, however, considerably different; in pure water the Δ H ‡ /Δ S ‡ (kJ mol −1 /J K −1 mol −1 ) are 44.7/−30.2, 19.5/−114.1, and 32.2/−76.9 for phen, dmphen, and NO 2 phen, respectively. The effects of solvent composition on the kinetics are also markedly different for the three ligands. The Δ H ‡ and Δ S ‡ showed a minimum at x AN = 0.1 for phen; for dmphen and NO 2 phen, however, maxima at x AN = 0.2 were observed. Nevertheless, there is an effective enthalpy–entropy compensation for the Δ H ‡ /Δ S ‡ of all the three ligands, demonstrating the significant effects of the changes in solvation and solvent structure on the complexation kinetics. As the rate-determining step of Ni(II) complexation is the dissociation of a water molecule from Ni(II), the solvent and ligand dependencies in the Ni(II) complexation kinetics are ascribed to the change in solvation status of the ligands and the altered solvent structures upon changing solvent composition.