Kinetics and mechanism of the aquation of the trinuclear cation, [μ 3-oxo-triaqua-hexakis(acetato)tris(iron(III))] +in perchloric acid media

Abstract

The aquation of the title complex cation in aqueous perchloric acid proceeded via two steps, both postulated to be the proton attack on the oxygen atom which binds the acetate ligand to the metal centre, followed by Fe–O bond cleavage. This was followed by rapid decomposition to produce aqueous iron(III) and acetate ions. The first-order rate constants for the first and second steps at 25 °C are: k 1 = (4.16 ± 0.58) × 10 −2 s −1 and k 2 = (2.09 ± 0.42) × 10 −3 s −1, respectively, and their corresponding activation parameters are Δ H 1 ‡ = 22 . 2 ± 1 . 6 kJ mol - 1 , Δ S 1 ‡ = - 197 ± 5 J K - 1 mol - 1 , Δ H 2 ‡ = 40 . 2 ± 3 . 8 kJ mol - 1 , Δ S 2 ‡ = - 161 ± 13 J K - 1 mol - 1 . The spontaneous hydrolysis rate constants for the first and second steps were also determined at 25 °C and ionic strength of 1 mol dm −3 and they are k 0 = (3.10 ± 0.82) × 10 −3 s −1 and k 0 ′ = ( 2 . 65 ± 0 . 68 ) × 10 - 4 s - 1 , respectively. The corresponding activation parameters are Δ H 0 ‡ = 32 . 5 ± 5 . 5 kJ mol - 1 , Δ S 0 ‡ = - 184 ± 18 J K - 1 mol - 1 ; Δ H 0 ′ ‡ = 39 . 0 ± 2 . 6 kJ mol - 1 , Δ S 0 ′ ‡ = - 182 ± 8 J K - 1 mol - 1 .