The role of the axial ligand L ("push" effect) in the O-O bond cleavage in Mn-SaIen based trans-acylperoxo complexes, [(Salen)Mn-III(RCO3)L] (2) and [(Salen)Mn-IV(RCO3)L](+) (3), was investigated by the density functional (B3LYP) method for L = NH3 and HCO2-. It was shown that coordination of an axial ligand L to the trans isomer of 2, 2-trans, substantially reduces the triplet-quintet energy gap from +12.4 to +1.8 and -0.3 kcal/mol, and consequently decreases the overall barrier for the O-O bond cleavage producing 2-trans-L-oxo species from 19.5 to 10.2 and 10.9 kcal/mol for L = none, HCO2-, and NH3, respectively. Because the O-O bond cleavage in 2-trans-L becomes slightly more favorable than that in the isomeric 2-cis acylperoxo complexes producing 2-cis-Mn-V-oxo species [Khavrutskii, I. V.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 2003, 125, 13879-13889], the axial ligand may be considered as a switch between cis- and trans-oxo species formation. The product 2-trans-L-oxo unlike 2-cis-Mn-V-oxo species is chameleon in nature and can be viewed either as Mn-V-oxo species associated with RCO2- (heterolytic cleavage) or as Mn-IV-oxo species associated with RCO2 radical (homolytic cleavage). The mode of dissociation of this species into corresponding heterolytic or homolytic fragments is controlled by the nature of the axial ligand and the solvent employed.