The stability of the peroxide ligand bridging two manganese ions in the trinuclear oxomanganese complex [Mn-3(III)(mu(3)-O)(mu-O-2)(AcO)(2)(dien)(3)](2+), one of only two structurally characterized Mn clusters possessing a mu(1,2)-peroxo bridge, has been investigated using density functional theory. Although the peroxide O-O bond in the related bis(mu-oxo)-bridged complex [Mn-2(IV)(mu-O)(2)(mu-O-2)(NH3)(6)](2+) undergoes spontaneous cleavage upon two-electron reduction to the Mn-2(III) dimer, calculations on the model complexes [Mn-2(III)(mu-O)(mu-O-2)(NH3)(8)](2+) and [Mn-2(III)(mu-O)(mu-O-2)(NH3)(6)(H2O)(2)](2+) , which contain the same mu-oxo-, mu-peroxo-bridged core present ir, the trimer, indicate that the peroxide bridge remains intact, in agreement with experiment. Its stability can be attributed to a Jahn-Teller distortion resulting in elongation of the axial Mn-N bonds perpendicular to the Mn-2(mu-O)(mu-O-2) plane which in turn stabilizes the high-spin Mn-III oxidation state. However, the difference in the energies of the bridged and cleaved peroxide structures is small (ca, 0,5 eV), the lowest energy structure depending on the nature of the terminal ligands. Calculations on the model trimer complex [Mn-3(III)(mu(3)-O)(mu-O-2)(HCO2)(2)(NH3)(9)](2+) indicate that the energetic differences between the cleaved and uncleaved structures is even smaller (ca. 0.2 eV), and although the peroxo-bridge remains more or less intact, it is likely to be quite facile.