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

The reaction of [RuIII(pic)3] (pic−=picolinate) and sulfite (SO32−) resulting in the formation of [RuII(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 [RuII(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 [RuIII(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. [ABSTRACT FROM AUTHOR]