Effect of lattice anions on the ligand substitution reaction of tris(1,10-phenanthroline)metal(II) complexes: [M(phen)3]SO4 and [M(phen)3]X2(M: Fe(II), Co(II), Ni(II); X: Cl-, Br-, I-).

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The kinetics of the ligand substitution reaction of the title complexes in the solid state have been studied using thermogravimetry (TG) and differential scanning calorimeter (DSC) techniques. For these complex salts, an exothermic peak appears in the DSC curve which correlates with a ligand substitution in which anionatobis(1,10-phenanthroline)metal(II) complexes are formed. The heats of the exothermic reaction for ligand substitution of the sulfates decrease in the order Co(II)>Ni(II)>Fe(II). Isothermal and nonisothermal kinetic analyses were carried out using the exothermic peak. Activation energies of the ligand substitution for the same counterion generally increase with the metal in the order Co(II)<Ni(II)<Fe(II) and for the same metal, Ni(II) or Fe(II), increase with the counterion in the order Cl−<Br−<I−<SO42−. However, for the cobalt complex, the activation energies are almost identical for this series of counterions. From the trends in activation parameters for the ligand substitution, we propose a dissociative mechanism for the sulfates of all complexes studied and for the cobalt complex halides studied. For the halides of the iron(II) and nickel(II) complexes studied, a mechanism involving nucleophilic attack of the halide ions is important.
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