The tetranuclear species [(Mn4O6)-O-IV(bpy)(6)] (ClO4)(4).H2O was isolated from an aqueous nitric acid solution (pH = 2) of Mn-III(bpy)Cl-3(H2O) upon addition of NaClO4. It crystallizes in the triclinic space group with a = 20.213(7) Angstrom, b = 13.533(7) Angstrom, c = 13.411(8) Angstrom, alpha = 112.01(8)degrees beta = 96.72(11)degrees, gamma = 100.34(12)degrees, V = 3276.9 Angstrom(3), and Z = 2. The cation has a nonrectilinear chain structure [(bpy)(2)Mn-IV aO2MnIV (b)(bpy)O2MnIV (c)(bpy)O2MnIV (d)(bpy)(2)](4+). The metal-metal distances are Mn-a-Mn-b = 2.746(5) Angstrom, Mn-b-M(c) = 2.760(5) Angstrom, Mn-c-Mn-d = 2.735(4) Angstrom, Mn-a-Mn-c = 4.899(10) Angstrom, Mn-b-Mn-d = 4.897(6) Angstrom, and Mn-a-Mn-d = 6.419(11) Angstrom. The variable temperature magnetic susceptibility data for [Mn-IV O-4(6)(bpy)(6)](ClO4)(4).H2O in the range 12-294 K were fit using the spin Hamiltonian H-S = -J(ab)S(a)S(b)J(bc)S(b)S(c) - J(cd)S(c)S(d), with J(ab) = J(cd) = -176 cm(-1) and J(bc) = -268 cm(-1). The lowest spin states are organized similarly to those for a dinuclear Mn-IV-Mn-IV unit. This is due to the stronger antiferromagnetic coupling between the two central Mn-IV ions relative to the coupling between a and b on one hand and c anc d on the other hand. This compound is EPR inactive at 4 K but shows a spectrum around 100 K which was attributed to the triplet and quintet excited states. Electrochemically, the cation [Mn-IV O-4(6)(bpy)(6)](4+) is irreversibly reduced to Mn-II at +0.5 V/SCE. Its lower oxidizing power than those of [Mn-IV (2)O(2)L(4)](4+) (L = bpy or phen) was attributed to the dispersion of the positive charge and the surrounding of Mn-IV central ions by oxo groups. The [Mn-IV O-4(6)(bpy)(6)](4+) differs by the nature of the central bridge from the proposal by Klein et al. (Science 1993, 260, 675-679) for the structure of the oxygen evolving center (OEC) in plants.