A new chemical and electrochemical synthesis of the linear tetranuclear manganese complex, [(Mn4O6)-O-IV(bpy)(6)](4+) (bpy = 2,2'-bipyridine) (4) is described. This complex can be synthesized by a two-electron oxidation of an aqueous solution of the mononuclear [Mn-II(bpy)(3)](2+) (1) one. The aqueous solutions were buffered at pH 4.5 with acetate for the chemical synthesis and bpy for the electrochemical one to stabilize the intermediate binuclear [(Mn2O2)-O-III,IV(bpy)(4)](3+) (2) formed. In the case of chemical synthesis, KMnO4 is used as oxidizing agent. Electrochemical behavior of these complexes, including the trinuclear [(Mn3O4)-O-IV (bPY)(4)(H2O)(2)](4+) derivative (3), have been investigated in organic (CH3CN) and bpy/bpyH(+) buffer (pH 4.5). Carefully controlled potential oxidations of a solution of the mononuclear Mn(II) 1 in this latter medium lead to the successive formation of the high-valent oxo binuclear Mn-2(III,IV) (2) and tetranuclear Mn-4(IV) (4) complexes. These two dimerization reactions are quantitative. They result from the decoordination of bpy ligands and interaction with water of the unstable [Mn-III(bpy)(3)](3+) and [(Mn2O2)-O-IV,IV(bpy)(4)](4+) electrochemically generated species. In CH3CN, the higher stability of [Mn-2(IV,IV) O-2(bpy)(4)](4+) avoids the second dimerization process. All transformations are chemically reversible by reduction processes, although in aqueous buffered medium, the two steps are not clearly separated. Indeed, 2 cannot be generated quantitatively and selectively by reduction of 4 without any formation of 1, since the reduction potentials of 2 and 4 are too close. The electrochemical behavior of the trinuclear 3 complex in aqueous buffered solution follows the same process as that of 4 although its reduction is slightly easier than 2 and 4. Pertinent aspects of the electrochemistry of these oxo manganese complexes toward the modeling of the Mn-4 active site of PSII are also discussed.