Mononuclear nonheme iron(IV)-oxo complexes with two different topologies, cis-alpha-[Fe(IV)(O)(BQCN)](2+) and cis-beta-[Fe(IV)(O)(BQCN)](2+), were synthesized and characterized with various spectroscopic methods. The effect of ligand topology on the reactivities of nonheme iron(IV)oxo complexes was investigated in C-H bond activation and oxygen atom-transfer reactions; cis-alpha-[Fe(IV)(O)(BQCN)](2+) was more reactive than cis-beta-[Fe(IV)(O)(BQCN)](2+) in the oxidation reactions. The reactivity difference between the cis-alpha and cis-beta isomers of [Fe(IV)(O)(BQCN)](2+) was rationalized with the Fe(IV/III) redox potentials of the iron(IV)-oxo complexes: the Fe(IV/III) redox potential of the cis-alpha isomer was 0.11 V higher than that of the cis-beta isomer.