Redox Reactions of [RuIII(pic)3] (pic−=picolinate) with Sulfite in Aqueous Solution: Kinetic, and Mechanistic Studies

The reaction of [Ru^III(pic)₃] (pic⁻=picolinate) and sulfite (SO₃²⁻) resulting in the formation of [Ru^II(pic)₃] ⁻ and sulfate (SO₄²⁻) has been studied spectrophotometrically and kinetically in aqueous solution. The rate of the reaction studied by following the appearance of the MLCT (metal to ligand charge transfer) band of the [Ru^II(pic)₃)]⁻ complex at 466 nm was found to be first-order in both the complex and sulfite concentrations. The values of the observed rate constant depend on the pH, since it controls the speciation of oxoanions of sulfur(IV)-species viz. bisulfite (HSO₃⁻) and sulfite (SO₃²⁻) in the pH range 3.8 to 8.3. The product of the reaction, sulfate was ascertained by LDI-mass spectral analysis ofthe post-reaction solution. The activation parameters (ΔH^≠ =56±3 kJ mol⁻¹ and ΔS^≠= -179±9 J mol⁻¹ K⁻¹) corresponding to the reduction of [Ru^III(pic)₃] with SO₃²⁻ supports the operation of an outer-sphere electron transfer mechanism. The experimentally observed electron-transfer rate constant (k=5.60 M⁻¹s⁻¹ at 25 °C) is found to be in good agreement with that of calculated (k₁₂=4.75 M⁻¹s⁻¹) using Marcus cross-reaction relationship for outer-sphere electron-transfer reactions. A working mechanism consistent with the spectral and kinetic data is proposed for the aforesaid electron transfer reaction.