Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn IV (O)] + -M n+ (1-M n+ , M n+ = Ca 2+ , Mg 2+ , Zn 2+ , Lu 3+ , Y 3+ , Al 3+ , and Sc 3+ ) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn III (OH)] + , with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M n+ ). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M n+ on the reactivity of 1-M n+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M n+ , such as 1-Sc 3+ ≈ 1-Al 3+ > 1-Y 3+ > 1-Lu 3+ > 1-Zn 2+ > 1-Mg 2+ > 1-Ca 2+ , follows the Lewis acidity of M n+ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M n+ , the higher the reactivity of 1-M n+ becomes. In sharp contrast, the reactivity of 1-M n+ in the HAT reaction was reversed, giving the reactivity order 1-Ca 2+ > 1-Mg 2+ > 1-Zn 2+ > 1-Lu 3+ > 1-Y 3+ > 1-Al 3+ ≈ 1-Sc 3+ that is, the higher is Lewis acidity of M n+ , the lower the reactivity of 1-M n+ in the HAT reaction. The latter result implies that the Lewis acidity of M n+ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M n+ cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.