trans-[(en)~2(NO~2)Co(O~2CC~5H~5N)]^2^+(ClO~4)~2, which is first reported, has been synthesized and characterized by elemental analysis, UV-Vis and IR spectra, and X-ray structural analyses. Kinetic behaviours of the Co(Ⅲ) complex reduced by [Fe(CN)~6]^4^- and [Fe(CN)~5(H~2O)]^3^- have been investigated. The results indicated the electron transfer (ET) reactions followed the outer-sphere mechanism or the linner-sphere mechanism respectively. At 25℃, Ⅰ=0.50mol·L^-^1, the ion-pair formation constant Q~i~p and ET rate constant k~e~t of the precursor complex for trans-[(en)~2(NO~2)Co(O~2CC~5H~5N)]^2^+/Fe(CN)~6]^4^- are 29mol^-^1·L and 2.4×10^-^1s^-^1, the activation enthalpy △H^≠~e~t and the activation entropy △S^≠~e~t of this ET process are 1.2×10^2kJ·mol^-^1 and 5.0×10^2kJ·mol^-^1·K^-^1 respectively. The intramolecular ET rate constant of the precursor binuclear complex for trans-[(en)~2(NO~2)Co(O~2CC~5H~4N)]^+/[Fe(CN)~5(H~2O)]^3^- is 7.0×10^-^5s^-^1 at 40℃, pH=8.0 and Ⅰ=-0.10mol·L^-^1. Finally the influence of the molecular orbital symmetry and the redox potentials on ET rate have been discussed.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, PYRIDINE P, INFRARED SPECTROPHOTOMETRY, REACTION KINETICS, ELEMENTAL ANALYSIS, CRYSTAL STRUCTURE, COBALT COMPLEX, ELECTRON TRANSFER REACTION, DIFFRACTION ANALYSIS