A thiolate-bridged binuclear complex [PPN](2)[(Mn-II(T(MS)PS3))(2)] (1, PPN = bis(triphenylphosphine)-iminium and (TMS)PS3H(3) = (2,2',2"-trimercapto-3,3',3"-tris(trimethylsilyl)triphenylphosphine)), prepared from the reaction of MnCl2/[PPN]Cl and Li-3[TMSPS3], converts into a mononuclear complex [PPN][MnII(TMSPS3)(DABCO)] (2) in the presence of excess amounts of DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane). Variable temperature studies of solution containing 1 and DABCO by UVvis spectroscopy indicate that 1 and 2 exist in significant amounts in equilibrium and mononuclear 2 is favored at low temperature. Treatment of 1 or 2 with the monomeric O2-side-on-bound [PPN][MnIV(O-2)(TMSPS3)] (3) produces the mono-oxo-bridged dimer [PPN](2)[(Mn-III((TMS)PS3))(2)(mu-O)] (4). The electrochemistry of 1 and 2 reveals anodic peak(s) for a MnIII/MnII redox couple at shifted potentials against Fc/Fc(+), indicating that both complexes can be oxidized by dioxygen. The O-2 activation mediated by 1 and 2 is investigated in both solution and the solid state. Microcrystals of 2 rapidly react with air or dry O-2 to generate the Mn(IV)peroxo 3 in high yield, revealing a solid-to-solid transformation and two-electron reduction of O-2. Oxygenation of 1 or 2 in solution, however, is affected by diffusion and transient concentration of dioxygen in the two different substrates, leading to generation of 3 and 4 in variable ratios.