Electron-Transfer Reactions of Multidentate Ligand Cobalt(III) Complexes. I. The Reductions of Some Chloro(triethylenetetramine)cobalt(III) Complexes by Iron(II)

Abstract

Abstract The kinetics of Fe2+ reductions of cis-α-Co(trien)Cl2+, cis-α-Co(trien)OH2Cl2+ and cis-β1-Co(trien)OH2Cl2+(trien=triethylenetetramine) have been studied at 25°C, [H+]=0.10 m and Σ[ClO4−]=1.0 m. The second-order rate constants obtained are 4.0×10−4 m−1 sec−1, 1.6×10−4 m−1sec−1, and 9.8×10−5 m−1 sec−1 for cis-α-Co(trien)Cl+2, cis-α-Co(trien)OH2Cl2+, and cis-β1-Co(trien)OH2Cl2+ respectively. It is demonstrated, by comparing with other rate data for the Fe2+ reductions of cis-CoN4XCln+ (N=NH3, 0.5 en and 0.25 trien, and X=H2O and Cl−) that the rate decreases with an increase in the number of chelate rings in the Co(III) complexes. The chelate effect of the nonbridging ligand on the electron-transfer reactions of Co(III) complexes is discussed on the basis of a model which involves the ligand-field strength of the trans-ligand to the bridging one, the energy needed for stretching the trans-group away from the Co(III) center, and the effect of the solvation.