Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

Area of Science:

• Inorganic Chemistry
• Chemical Kinetics
• Electron Transfer Reactions

Background:

• Understanding the reaction mechanisms between metal ions and oxyanions is crucial in various chemical processes.
• Iron(II) and chlorite (ClO2) reactions are relevant in environmental chemistry and industrial applications.
• Previous studies have explored similar redox reactions, but detailed mechanistic insights into the Fe(II)/ClO2 system are needed.

Purpose of the Study:

• To elucidate the reaction pathways and kinetics for the reduction of chlorite (ClO2) by aqueous iron(II).
• To determine the contributions of outer-sphere and inner-sphere electron-transfer mechanisms.
• To characterize the intermediate species and their subsequent reactions.

Main Methods:

• Spectrophotometric monitoring of reaction progress under varying conditions.
• Kinetic analysis to determine rate constants and activation parameters.
• Computational modeling to support mechanistic interpretations.

Main Results:

• The reduction of ClO2 by Fe(aq)(2+) occurs via parallel outer-sphere (86%) and inner-sphere (14%) pathways.
• The outer-sphere pathway has a second-order rate constant of 1.3 x 10^6 M^-1 s^-1.
• An inner-sphere complex, FeClO2(2+), was identified as an intermediate, dissociating to Fe(aq)(3+) and ClO2(-) with a rate constant of 39.3 s^-1.

Conclusions:

• The Fe(II)/ClO2 reaction is predominantly governed by an outer-sphere electron transfer mechanism.
• The inner-sphere pathway involves a distinct intermediate, influencing the overall reaction kinetics and product distribution.
• Kinetic and thermodynamic data provide a comprehensive understanding of this important redox system.