Redox-inactive metal ions play vital roles in biological O-2 activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt(III) complex bearing a tetradentate macrocyclic ligand, [Co-III(TBDAP)(O-2)](+) (1) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), toward nitriles that afforded a series of hydroximatocobalt(III) complexes, [Co-III(TBDAP)(R-C(=NO)O)](+) (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn2+, La3+, Lu3+, and Y3+, the reaction does not form the hydroximatocobalt(III) complex but instead gives peroxyimidatocobalt(III) complexes, [Co-III(TBDAP)(R-C(=NH)O-2)](2+) (R = Me (2) and Ph (2(Ph))). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al3+, Sc3+, or H+, 1 is converted to [Co-III(TBDAP)(O2H)(MeCN)](2+) (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co-eta(1)-O-2 intermediate through the charge reorganization from a Co-II-superoxo to a Co-III-peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.