Kinetics and mechanism of replacement of sulphite in the pentacyano(sulphito)ferrate(II) ion by cyanide ion

Abstract

The kinetics of replacement of sulphite by cyanide ion in Na₅[Fe(CN)₅SO₃] in aqueous solution follow the rate law –dln[Fe(CN)₅SO₃^5–]/dt=k₁k₂[CN^–]/(k_–1[SO₃^2–]+k₂[CN^–]), which is consistent with reactions (i) and (ii). [Fe(CN)₅SO₃]^5– [graphic omitted] [Fe(CN)₅]^3–+ SO₃^2–(i), [Fe(CN)₅]^3–+ CN^– [graphic omitted] [Fe(CN)₆]^4–(ii) At 43 °C, pH 10·80, and 1M ionic strength k₁=(9·50 ± 0·21)× 10^–4 s^–1. The [Fe(CN)₅]^3– intermediate is sufficiently long-lived to exhibit selective reactivity toward different reagents as shown by the competition factor k₂/k_–1= 8·76 ± 0·53. The kinetic data are considered as evidence for a limiting S_N1 mechanism. The competition factor has been interpreted in terms of free-energy changes resulting from desolvation processes of the competitors.