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Two oxoiron(IV) isomers ( R 2a and R 2b) of general formula [Fe IV (O)( R PyNMe 3 )(CH 3 CN)] 2+ are obtained by reaction of their iron(II) precursor with NBu 4 IO 4 . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between R 2a and R 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that R 2a reacts one order of magnitude faster than R 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in R 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)