Reactions of metal ions with triketones. Part 3. Kinetics and mechanisms of the reactions of nickel(II) and cobalt(II) with 1-phenylhexane-1,3,5-trione in methanol–water (70 : 30 v/v) solution
Abstract
The kinetics and mechanisms of the reactions of nickel(II) and cobalt(II) with 1-phenylhexane 1,3,5-trione (H2phto) to form the 1 : 1 and 2 : 2 complexes have been investigated in methanol–water (70 : 30 v/v) solutions at 25 °C and ionic strength 0.5 mol dm–3 NaClO4. The rate constants for 1 : 1 complex formation with H2phto (KHL) and with Hphto–(KL) have been determined. For Ni2+, KHL= 3.48 and KL= 5.14 × 104 dm3 mol–1s–1 while for Co2+, KHL= 0.6 and KL= 1.26 × 106 dm3 mol–1 s–1 The conversion of the 1 : 1 into the 2 : 2 complexes takes place via a two-step mechanism, the first of which is second order in complex concentration and first order in hydroxideion concentration, while the second step is first order in complex concentration. The rate constants for Ni2+ are 1.99 × 107 dm3 mol–1 s–1 and 0.34 s–1 while for Co2+ they are 3.12 × 107 dm3 mol–1 s–1 and 0.16 s–1. Mechanisms are proposed which account for the kinetic data.