Using in situ ESR/UV-Vis-NIR cyclovoltammetry at different temperatures, model systems for blue copper proteins like [Cu-1(bite)]BF4 and [Cu-11(bite)](BF4)(2) (bite = biphenyldiimino dithioether) were studied with respect to their redox behavior. Chemically reversible processes in the electrode reaction were observed by ESR and UV-Vis-NIR spectroscopy during the repetitive redox cycling of [Cu-1(bite)]BF4. The complex cyclovoltammetric feature of the compounds in acetonitrile can be explained by adsorption and a slow heterogeneous electron transfer with rate constant k(s) = 2 x 10(-4) cm s(-1). Spectroelectrochemical studies of the redox products at 77 K confirmed the formation of a metastable pseudo-tetrahedral [Cu-11 (bite)](2+) intermediate. This is direct experimental proof of processes in redox reactions of the [Cu-1(bite)](+) within a dual-pathway square reaction scheme indicating preferable electron transfer from the pseudo-tetrahedral [Cu-1(bite)](+) as the initial step. The DFT optimized geometries of quasi-tetrahedral and quasi-planar [Cu-1(bite)](+/2+) complexes are in reasonable agreement with experimental data. (C) 2003 Elsevier B.V. All rights reserved.