Kinetics and mechanism of the reaction between the di-μ-cyanobis[tetracyanoferrate(III)] complex ion and sulfite in aqueous solution

The kinetics of the stepwise reduction of the title complex [Fe-2(CN)(10)](4-) by sulfite have been studied in the presence of air as a function of pH, sulfite concentration, temperature and ionic strength using stopped-flow and conventional spectrophotometric techniques. The k(obs) versus pH profile shows a marked increase in rate with increase of pH over the range 3.7 <= pH <= 6.1 due to the increase in concentration of the more reactive sulfite species SO32-. The reaction proceeds in several stages, the first of which involves a one electron transfer process with the formation of the radical anion SO3-. This then adds on in a rapid stage to form a species [Fe-2(III)(CN)(10)(SO3)](5-). The second and third stages also involve one electron transfer. In the third, or possibly a fourth stage cleavage occurs, the final product being [Fe-II(CN)(5)(SO3)](5-). The reaction rate is sensitive to the nature of the cation present with a reactivity sequence Li+ < Na+ < K+ < NH4+. (C) 2006 Elsevier B.V. All rights reserved.