Reactions of metal ions with triketones. Part 2. Kinetics and mechanisms of the reactions of nickel(II) and cobalt(II) with heptane-2,4,6-trione in methanol-water (70: 30 v/v)

Abstract

The kinetics and mechanisms of the reactions of nickel(II) and cobalt(II) with heptane-2,4,6-trione (H₂hpto) to form the 1 : 1 and 2: 2 complexes have been studied in methanol–water (70:30 v/v) at 25 °C and an ionic strength of 0.5 mol dm^–3(NaClO₄). The rate constants of the mono complex formation with H₂hpto (k_HL) and with Hhpto^–(k_L) have been determined. For Ni²⁺, k_HL= 180, k_L= 4.88 × 10⁴ dm³ mol^–1 s^–1, while for Co²⁺k_HL= 476, and k_L= 1.36 × 10⁶ dm³ mol^–1 s^–1. The conversion of the 1 : 1 to the 2:2 complex takes place via a two-stage mechanism. The rate constants for Ni²⁺ are 6.1 and 0.21 s^–1 while for Co²⁺ they are 24.6 and 0.026 s^–1.