Crystal structure of [ReO2(4-MeOpy)(4)][PF6] (4-MeOpy = 4-methoxypyridine) complex has been examined by the single crystal X-ray analytical method. This complex shows a trans-dioxo geometry (average Re-O bond length = 1.766(2) angstrom) and its equatorial plane is occupied by four 4-MeOpy molecules (average Re-N bond length = 2.156(4) angstrom). Electrochemical reaction of [ReO2(4-MeOpy)(4)](+) in CH3CN solution containing tetra-n-butylammonium perchlorate as a supporting electrolyte has been studied using cyclic voltammetry at 24 degrees C. Cyclic voltammograms show one redox couple around 0.65 V (E-pa) and 0.58 V (E-pc) [versus ferrocene/ferrocenium ion redox couple, (Fc/Fc(+))]. Potential differences between two peaks (Delta E-p) at scan rates in the range from 0.01 to 0.10 V s (1) are 65 mV, which is almost consistent with the theoretical Delta E-p value (59 mV) for the reversible one electron transfer reaction at 24 degrees C. The ratio of anodic peak currents to cathodic ones is 1.04 +/- 0.03 and the (E-pa + E-pc)/2 value is constant, 0.613 +/- 0.001 V versus Fc/Fc(+), regardless of the scan rate. Spectroelectrochemical experiments have also been carried out by applying potentials from 0.40 to 0.77 V versus Fc/Fc+ with an optically transparent thin layer electrode. It was found that the UV-visible absorption spectra show clear isosbestic points at 228, 276, and 384 nm, and that the electron stoichiometry is evaluated as 1.03 from the Nernstian plot. These results indicate that the [ReO2 (4-MeOpy) 4]+ complex is oxidized reversibly to the [ReO2(4-MeOpy)(4)](2+) complex. Furthermore, it was clarified that the [ReO2(4-MeOpy)(4)](2+) in CH3CN has the characteristic absorption bands at 236, 278, 330, 478, and 543 nm and their molar absorption coefficients are 4.3 x 10(4), 4.5 x 10(3), 1.0 x 10(4), and 6.1 x 10(3) M (1) cm (1) (M = mol dm (3)), respectively. (C) 2009 Elsevier B. V. All rights reserved.