Prior work based on the crystal structures and spectroscopic studies of nonheme iron halogenases along with the related non-heme iron dioxygenases and hydroxylases has led to the proposal of the following mechanism in the catalytic cycle for non-heme iron halogenases. In its resting state (1), the hexa-coordinated iron active site consists of an Fe(II) coordinated to two His ligands, a bidentate aKG ligand, a halide ligand, and a water molecule, which is loosely bound to the iron center (Figure ). When the substrate enters the binding pocket in the active site, the water molecule is displaced, followed by the binding of molecular oxygen (2) to the iron center (Figure ). The bound molecular oxygen immediately attacks the carbonyl carbon of the bidentate aKG ligand, leading to O-O bond cleavage and oxidative decarboxylation of aKG (Figure ). During this catalytic step, the decarboxylation of an initially bidentate coordinating aKG leads to release of a carbon dioxide molecule, leaving behind the succinate substrate that has a carboxylate group that can bind iron in a monodentate or bidentate fashion (Figure ). This conversion leads to the appearance of an active site consisting of a highly reactive terminal Fe(IV)-oxo intermediate (3), which abstracts a hydrogen atom from the substrate, forming a radical substrate and an Fe(III)-OH intermediate (4) (Figure ). This rate-determining HAT step is followed by rebound halogenation of the substrate radical by the halide ligand, which returns the active site to its resting state (1) (Figure ). We assume that the Fe(IV)=O intermediate can form after oxidative decarboxylation based on our prior study of an engineered SadX enzyme with fluoride and chloride bound, where both could form hydroxylated products, and therefore the Fe-oxo intermediate must have formed, but only the chloride bound active site led to chlorinated products. Thus, we focus on the HAT and rebound steps (i.e., 3 and 4), and we study this mechanism for three halides: fluoride, chloride, and bromide. Hydrogen, carbon, nitrogen, oxygen, fluorine, chlorine, iron, and bromine atoms are shown in white, gray, blue, red, cyan, green, brown, and maroon, respectively.