ELECTRON-TRANSFER AND LIGAND SUBSTITUTION-REACTIONS OF THE ION PENTACYANO(4-AMINOPYRIDINE)FERRATE(II)

The ion pentacyano(4-aminopyridine)ferrate(II) has been characterized in aqueous solution. The complex exhibits a strong metal-to-ligand charge-transfer absorption (εmax 4.4×103 M-1 cm-1) at 320 nm. In the presence of excess heterocycle, the rate law for formation of the complex from pentacyanoaquaferrate(II) is d[Fe(CN)5L3-]/dt=kf[Fe(CN)5OH23-] [L] with kf=432±10 M-1 s-1, ΔHf‡=15.0±0.6 kcal/mol, and ΔSf‡=4±2 cal deg-1 mol-1 (25°C, μ=0.1 M, pH ~7). Ligand-exchange studies yield a value for the specific rate of dissociation of 4-NH2py: kd=(2.5±0.1)×10-3 s-1 with μHd‡=21.4±0.2 kcal/mol and ΔSd‡=1.2±1.6 cal mol-1 deg-1. One-electron oxidation of the complex by cyclic voltammetry is reversible, with E°=-0.35±0.03 V (0.1 M LiClO4, 23°C). The oxidation product is the violet pentacyano(4-aminopyridine)ferrate(III) ion (λmax 560 nm, ε 2.7×103 M-1 cm-1). A study of the kinetics of oxidation of the iron(II) complex by ferricyanide yielded the rate law d[Fe(CN)5L2-]/dt=ket[Fe(CN)5L3-][Fe(CN)63-] where ket=(4.3±0.2)×105 M-1 s-1, ΔHet‡=2.0±0.5 kcal/mol, and ΔSet‡=-26±2 cal mol-1 deg-1 (25°C, μ=0.05 M (LiClO4), pH 9.0). © 1979, American Chemical Society. All rights reserved.