Kinetics and mechanism of the oxidation of oxalate ion by tris(acetylacetonato)-manganese(III) and its hydrolytic derivatives in aqueous perchlorate media

Abstract

In the pH range 6.6–8.6, [MnL 2 (H 2 O) 2 ] + and [MnL 2 (H 2 O)(OH)] (HL = acetylacetone) oxidize oxalate ion (ox 2− ) to CO 2 through the inner-sphere intermediates [MnL 2 (ox)] − and [MnL 2 (OH)(ox′)] 2− , where ox′ is a half-bonded (unidentate) oxalate ion. Their rate constants of decomposition are 1.0 × 10 −4 s −1 and 11.2 × 10 −2 M −1 s −1 at 30 °C and at I = 1.0 M (NaClO 4 ). Decomposition of these mixed complexes produces free radicals, presumably CO 2 − , which is further oxidized to CO 2 by another Mn(III) in a fast step. At pH 4.2, [Mn(ox) 3 ] 3− is produced quantitatively when [ox] 0 ≥ 0.12 M, which has been characterized spectrally, and its unimolecular decomposition rate constant k (= 2.7 × 10 −4 s −1 at 30 °C and I = 1.0 M) compares well with that reported earlier (2.44 × 10 −4 s −1 at 25 °C and I = 1.0 M).