The tetranuclear Mn(III) complex [Mn(4)(III)L(2)(mu-O)(2)(mu-OAC)(4)[(ClO4)(4) . 3CH(3)NO(2) (L = alpha,alpha’-bis(bis(2-pyridylmethyl)amino)-m-xylene), 2, has been synthesized, and its structural, spectroscopic, and magnetic properties have been investigated. The kinetics of the reaction of its precursor Mn(II) complex (formulated as [Mn(2)L(OAc)(2)(CH3OH)](ClO4)(2), 1, with O-2 that leads ito the formation of 2 have been studied. Complex 2 crystallizes on the orthorhombic crystal system, space group Pnma, with the cell dimensions a = 24.755(5) Angstrom, b = 28.854(4) Angstrom, c = 12.495(2) Angstrom, and Z = 4. The structure shows four Mn(III) ions in a dimer of two dinuclear centers. Each dinuclear center has two Mn(III) ions triply bridged by two acetate groups and one oxo group with three nitrogen donors coordinated, forming a distorted octahedral coordination. The two dinuclear units are related to each other by a mirror plane. Within a dinuclear unit, the two Mn(III) ions show coordination differences in bond angles and bond distances arising from differences in the Jahn-Teller and lattice distortions of the octahedral coordination. Temperature-dependent magnetic susceptibility studies show very weak magnetic coupling (J = -0.4 cm(-1)) between Mn(III) ions. The stoichiometry of the O-2/complex 1 reaction to form complex 2, the EPR, molar conductance, and magnetic susceptibility studies, and the structural data for complex 2 all support the formulation of complex 2 as having four Mn(III) ions. IR spectral studies of samples of complex 2 prepared by the reaction of complex 1 with isotopically pure O-16(2) and O-18(2) show that the bridging oxo group is derived from dioxygen. Kinetic studies show that the reaction of the precursor Mn(II) complex 1 with O-2 is first order in the concentrations of both the dinuclear Mn(II) complex and O-2. The reactivity of the Mn(II) complex 1 with O-2 is in contrast to the lack of any reaction with O(?)2 under ordinary conditions of the Mn(II) complexes of dinucleating ligands with two bis(2-pyridylmethyl)amine donor groups connected by straight-chain hydrocarbons (C(3) to C(5)). The rigidity of the m-xylene group connecting the two donor sets in the ligand L of complex 1 may be responsible for the contribution of the critical entropic factor in favor of the reaction, which may account for this difference in reactivities. The overall reaction of complex 1 with O-2 to form complex 2 is the reverse of the reaction at the water-oxidizing complex of photosystem II (PS II). The four Mn/dioxygen, four-electron redox reaction feature of the reaction complex 1 + O-2 --> complex 2 is relevant to the chemistry of the water-oxidizing complex. Also the dinuclear unit structure and the UV-visible absorption spectrum of 2 are features with relevance to the active site of manganese catalase.