Cation-Independent Electron Transfer between Ferricyanide and Ferrocyanide Ions in Aqueous Solution

Electron transfer between Fe(CN)6 3- and Fe(CN)6 4- in homogeneous aqueous solution with K+ as the counterion normally proceeds almost exclusively by a K+-catalyzed pathway, but this can be suppressed, and the direct Fe(CN)6 3-−Fe(CN)6 4- electron transfer path exposed, by complexing the K+ with crypt-2.2.2 or 18-crown-6. Fe(13CN)6 4- -NMR line broadening measurements using either crypt-2.2.2 or (with extrapolation to zero uncomplexed [K+]) 18-crown-6 gave consistent values for the rate constant and activation volume (k 0 = (2.4 ± 0.1) × 102 L mol-1 s-1 and ΔV 0 ⧧ = −11.3 ± 0.3 cm3 mol-1, respectively, at 25 °C and ionic strength I = 0.2 mol L-1) for the uncatalyzed electron transfer path. These values conform well to predictions based on Marcus theory. When [K+] was controlled with 18-crown-6, the observed rate constant k ex was a linear function of uncomplexed [K+], giving k K = (4.3 ± 0.1) × 104 L2 mol-2 s-1 at 25 °C and I = 0.26 mol L-1 for the K+-catalyzed pathway. When no complexing agent was present, k ex was roughly proportional to [K+]total, but the corresponding rate constant k K‘ (=k ex/[K+]total) was about 60% larger than k K, evidently because ion pairing by hydrated K+ lowered the anion−anion repulsions. Ionic strength as such had only a small effect on k 0, k K, and k K‘. The rate constants commonly cited in the literature for the Fe(CN)6 3-/4- self-exchange reaction are in fact k K‘[K+]total values for typical experimental [K+]total levels.