Kinetics and mechanism of the reactions of hexaaqua rhodium (III) with sulphur (IV) in aqueous medium

Abstract

An O-bonded sulphito complex, Rh(OH₂)5(OSO₂H)²⁺, is reversibly formed in the stoppedflow time scale when Rh(OH₂)₆³⁺ and SO₂/HSO₃⁻ buffer (1 < pH < 3) are allowed to react. For Rh(OH₂)₅OH²⁺+ SO₂ □ Rh(OH₂)₅(OSO₂H)²⁺ (k₁/k₋₁), k₁ = (2·2 ±0·.2) × 103 dm⁻³ mol⁻¹ s⁻¹, k⁻¹ = 0·.58 ±0·16 s⁻¹ (25°C, I = 0·.5 mol dm⁻³). The protonated O-sulphito complex is a moderate acid (K _d = 3 × 10⁻⁴ mol dm⁻³, 25°C, I= 0·.5 mol dm⁻³). This complex undergoes (O, O) chelation by the bound bisulphite with k= 1·.4 ×10⁻³ s⁻¹ (31°C) to Rh(OH₂)4(O₂SO)⁺ and the chelated sulphito complex takes up another HSO₃⁻ in a fast equilibrium step to yield Rh(OH₂)₃(O₂SO)(OSO₂H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH₂)₃(O₂SO)(OSO₂)⁻→ Rh(OH₂)₃(O₂SO)(SO₃)- (k_iso = 3 × 10⁻⁴ s⁻¹, 31°C). A dinuclear (μ-O, O) sulphite-bridged complex, Na₄[Rh₂(μ-OH)₂(OH)₂(μ-OS(O)O)(O₂SO)(SO₃) (OH₂)]5H₂O with (O, O) chelated and S-bonded sulphites has been isolated and characterized. This complex is sparingly soluble in water and most organic solvents and very stable to acid-catalysed decomposition.