Kinetics and mechanism of the formation and reactivity of µ-hydroxo-µ-sulfito-bis[bis(1.2-diaminoethane)chromium(III)]

Abstract

The kinetics of formation of [(en)₂Cr(µ-OH)(µ-SO₃)Cr(en)₂]³⁺ cation from [(en)₂Cr(µ-OH)₂Cr(en)₂]⁴⁺(en = 1,2-diaminoethane) in aqueous buffered sulfite solution has been studied spectrophotometrically. This formation consisted of two steps; first a rapid uptake of SO₂ by the ringcleaved mono-µ-hydroxo species [(OH)(en)₂Cr(µ-OH)Cr(en)₂(OH)]³⁺ to form a terminal O-bonded sulfito intermediate. This reaction was studied at 600 nm in the ranges 3.78 ⩽ pH ⩽ 5.47, 20 ⩽θ⩽ 30 °C and 0.03 ⩽[S]_T⩽ 0.05 mol dm^–3([S]_T=[SO₂]+[HSO₃^–]+[SO₃^2–]). The rate constant for uptake of SO₂ at 25 °C was found to be (8.7 ± 0.1)× 10⁷ dm³ mol^–1 S^–1 with ΔH^‡= 6.5 ± 0.9 kJ mol^–1 and ΔS^‡=–71 ± 2 J K^–1. The second step in which the sulfito intermediate undergoes a ring closure to form the (µ-OH)(µ-SO₃) complex with the elimination of H₂O (or OH^–) was studied in the ranges 2.46 ⩽pH⩽ 4.46, 19.9 ⩽θ⩽ 29.9 °C and 0.03 ⩽[S]_T⩽ 0.05 mol dm^–3, and the rate constant for this process, k₃, was found to be 0.68 s^–1 at 25.2 °C with ΔH^‡= 28 ± 1 kJ mol^–1 and ΔS^‡=–155 ± 3 J K^–1 mol^–1. The rate constant for ring closure with the loss of OH^– was found to be 0.15 s^–1 at 25.2 °C with ΔH^‡= 66 ± 7 kJ mol^–1 and ΔS^‡=–40 ± 24 J K^–1 mol^–1. The (µ-OH)(µ-SO₃) complex decomposes in acid to give free sulfite with a rate constant of (7.8 ± 0.1)× 10^–5 s^–1 at 25 °C which is independent of [H⁺] over the range 0.01–1.0 mol dm^–3 HClO₄. It undergoes additional sulfite uptake (k= 3.01 × 10^–4 s^–1, 25 °C, ΔH^‡= 91 ± 8 kJ mol^–1, ΔS^‡=–8.7 ± 29.6 J K^–1 mol^–1) to form a possible disulfito product. The reaction was found to be independent of [S]_T but dependent on [H⁺] over the ranges 2.11 ⩽ pH ⩽ 7.28, 0.03 ⩽[S]_T⩽ 0.05 mol dm^–3 and 20 ⩽θ⩽ 30 °C.