Electron-transfer kinetics of transition-metal complexes in lower oxidation states IV1. Electrochemical electron-transfer rates of tris-(2,2′-bipyridine) complexes of iron, ruthenium, osmium, chromium, titanium, vanadium and molybdenum

Abstract

The standard rate constants for tris(2,2'-bipyridine) complexes of iron, ruthenium, osmium, chromium, titanium, vanadium and molybdenum in low and high oxidation states are measured in N,N-dimethylformamide with a dropping mercury electrode and a platinum electrode by the galvanostatic double-pulse method. The standard rate constants for the redox systems whose reductant species has an excess electron in its ligand π*-orbital fall within 0.1 to 0.3 cm s−1 and those for the redox systems where an excess electron occupies a metal t2g-orbital, within 0.8 to 1.3 cm s−1. It is noted that this classification holds irrespective of half-wave potentials, oxidation states, ionic charges and kinds of electrodes. On the basis of this fact, it is concluded that the rate constants uncorrected for the potential in the diffuse double-layer are better than the corrected ones. It is suggested that the difference in rate constant between the two groups is attributed to the difference in the inner-sphere reorganization energy.