Mono-complex Formation Kinetics of 2-Thenoyltrifluoroacetone with Nickel(II) and Copper(II) Ions in Aqueous Solution

Abstract

Abstract The mono-complex formation of 2-thenoyltrifluoroacetone (HTTA) with nickel(II) and copper(II) ions was studied in aqueous solution at 25.0 °C and an ionic strength of 0.20 mol dm−3 by means of a stopped-flow method. The rate-[H+] profiles for the formation of these mono-complexes revealed that the rate-determining step of the complex formation interchanges depending upon hydrogen-ion concentration, as in the case of the Ni(II)–bpy complex. The rate constants of the mono-complex formation with TTA−, kf, and that with HTTA, kfh, were determined from the rate-[H+] profile in the region of the high hydrogen-ion concentration, where the pseudo-equilibrium of acid dissociation of the ligand was maintained during the complex formation; kf=(9.2±3.0)×102, kfh=(1.3±0.3)×10−1 mol−1 dm3 s−1 for the Ni(II)–TTA complex and kf=(2.0±1.2)×103, kfh=(5.0±0.9)×10−1 mol−1 dm3 s−1 for the Cu(II)–TTA complex. The deprotonation-rate constants of HTTA could be evaluated from the apparent rate constants of the complex formation in low hydrogen-ion concentration. The values thus obtained agreed well with that determined independently by a pH jump method.