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 (K_HL) and with Hphto^–(K_L) have been determined. For Ni²⁺, K_HL= 3.48 and K_L= 5.14 × 10⁴ dm³ mol^–1s^–1 while for Co²⁺, K_HL= 0.6 and K_L= 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.