Relaxation kinetic study of monophthalatonickel(II) complex formation

Abstract

The temperature-jump relaxation method has been used to study the kinetics of the reaction between nickel(II) ion and phthalic acid in aqueous solution, at 25 °C and ionic strength 0·10M, over the acidity range from 2 × 10^–6 to 4 × 10^–4M. The observed pH dependence of the overall rate of complex formation suggests that the nickel(II) ion reacts with both the dianionic and monoanionic species of the dicarboxylic acid and that the phthalate anion acts as a bidentate ligand, forming a seven-membered chelate ring with the metal ion. The kinetic results show, moreover, that chelate-ring closure contributes to some extent to the limiting rate in the overall chelation process and that the rate of chelate-ring opening is faster for phthalate anion than for methyl- and n-butyl-malonate, i.e. the larger chelate ring is easier to open than the smaller one.