INTRA-MOLECULAR ELECTRON-TRANSFER FROM PENTACYANOFERRATE(II) TO PENTAAMMINECOBALT(III) MEDIATED BY VARIOUS 4,4'-BIPYRIDINES

The reactions of Fe(CN)5OH23- with Co(NH3)5L3+ (L = bis(4-pyridyl)methane, bis(4-pyridyl)ethane, bis(4-pyridyl)propane, bis(4-pyridyl)ethylene, and bis(4-pyridyl) ketone) produce binuclear complexes (NC)5FeIILCoIII(NH3)5 that exhibit iron t2g to ligand ύ* charge transfer bands at (same order) 370, 365, 360, 500, and 555 nm. Comparison of these values with those for the corresponding FeII(CN)5L3- complexes (370, 365, 360, 460, and 520 nm) suggests that the electron-withdrawing effect of the Co(III) moiety attached to the remote N is transmitted to the ring adjacent to the Fe(II) via ύ interactions, and that saturated hydrocarbon groups connecting the two pyridine rings insulate electronically the iron and cobalt centers. Rate constants for the formation and dissociation of the binuclear complexes have been measured and a dissociative mechanism is clearly indicated by the LFER of slope -1.0 between rate constants and equilibrium constants for the formation reactions. The rate constants for intramolecular electron transfer from Fe(II) to Co(III) in the binuclear complexes have been measured. At 25°C, μ = 0.10 M, the values are (same order of ligands) <0.6 X 10-3, 2.0 X 10-3, 4.8 X 10-3, 1.4 X 10-3, and <0.1 X 10-3 s-1. It is suggested that conjugation between the two pyridine rings is essential for electron transfer mediated by the ligand. When the two rings are separated by insulating methylene groups, electron transfer through the ligand is precluded, but ligands that permit close approach of the metal centers lead to intramolecular, outer-sphere electron transfer. © 1979, American Chemical Society. All rights reserved.