Redox-inactive metal ions play pivotal roles in regulating reactivities of high-valent metal-oxo species in a variety of enzymatic and chemical reactions. A mononuclear nonheme manganese(IV)-oxo complex bearing a pentadentate N5 ligand has been synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes are characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding Sc3+ ions in oxidation reactions, such as ~2500-fold increase in the oxidation of thioanisole (i.e., oxygen atom transfer) but ~180-fold decrease in the C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a nonheme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.