Kinetics and mechanism of replacements in pentacyano(ligand)ferrate(II) ions

Abstract

The kinetics of replacement of the ligand in the pentacyano(ligand)ferrate(II) ions have been examined for the leaving ligands NN-dimethyl(p-nitroso)aniline, nitrosobenzene, sulphite, and water, respectively, and for the entering ligands nitrosobenzene, 3-cyanopyridine. NN-dimethyl(p-nitroso)aniline, thiocyanate, nitrite, cyanide, and sulphite. Limiting reaction rates, at sufficiently large concentrations, of entering ligand have been observed with all the leaving ligands, except water, where the replacements obey the second-order rate law –d[Fe(CN)₅-OH₂^3–]/dt=k_Y[Fe(CN)₅OH₂^3–][Y]. When the entering ligand Y bears no electrical charge, the k_Y values are very similar and in the range 200–300 I mol^–1 s^–1 at 25 °C and 1M ionic strength. For singly negatively charged anions k_Y≃ 40–60, and for the doubly charged SO₃^2– ion k_Y= 3·3 I mol^–1 s^–1. The variations in k_Y are interpreted as being due to variations in diffusion rates since the reactions of the intermediate [Fe(CN)₅]^3–with the ligands are diffusion controlled.