Kinetics of the complexation of nickel(II) with 1,10-phenanthroline and its derivatives in acetonitrile–water mixed solvents

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(NO2phen) in acetonitrile–water mixed solvents of acetonitrile mole fraction xAN = 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 (NaClO4) 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 (×103 dm3 mol−1 s−1) 2.31, 2.57, and 1.38 for phen, dmphen and NO2phen, respectively. The corresponding activation parameters for the three ligands are, however, considerably different; in pure water the ΔH‡/ΔS‡ (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 NO2phen, respectively. The effects of solvent composition on the kinetics are also markedly different for the three ligands. The ΔH‡ and ΔS‡ showed a minimum at xAN = 0.1 for phen; for dmphen and NO2phen, however, maxima at xAN = 0.2 were observed. Nevertheless, there is an effective enthalpy–entropy compensation for the ΔH‡/ΔS‡ 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.