The structure of reversibly oxidizable [Cu(mmb)(2)](BF4) with 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) as bidentate N,S-donor ligand has been determined and compared with that of the copper(II) species [Cu(mmb)(2)(eta (1)-ClO4)](ClO4). In the complex ions of the equilibrium [Cu-I(mmb)(2)](+) + ClO4- reversible arrow e(-) + [Cu-II(mmb)(2)(eta (1)-ClO4)](+) the almost linear N-Cu-N backbone is invariant whereas the bonds to the thioether sulfur centers and especially the changing S-Cu-S angle (145.18(5)degrees for the Cu-II Species, 109.33(3)degrees for the Cu-I form) reflect the metal oxidation state. In contrast to the perchlorate coordinating copper(II) species, [Cu-I(mmb)(2)](BF4) contains a cation with a very large vacant site at the metal center, resulting in elliptical channels within the crystal. DFT calculations on [Cu-I(mb)(2)](+), [Cu-II(mb)(2)](2+), and [Cu-II(mb)(2)(OClO3)](+) with mb = 2-methylthiomethyl-1H-benzimidazole confirm the essential role of the metal-sulfur bonds in responding to the reversible Cu-I/II electron transfer process, even in the absence of electronically stronger interacting thiolate sulfur centers or sophisticated oligodentate ligands.