Nonheme iron enzymes have garnered strong research interests in the past decades due to their ability to catalyze a broad array of oxidative reactions to lead to regio- and stereo-selective C-H functionalization. Significant progresses have been made on the mechanistic understanding of nonheme iron enzymes containing a 2-His-1-carboxylic iron binding motif, such as iron- and 2-oxoglutarate (Fe/2OG) dependent enzymes. An high-spin (S = 2) oxoferryl (Fe(IV)=O) intermediate has been identified in Fe/2OG enzymes as the conserved intermediate for C-H activation. However, the mechanistic understanding of nonheme iron enzymes containing different iron binding motif, such as the 3/4-His motif, is much less developed. No consensus on the reactive intermediate used in the native enzymatic reactions of these 3/4-His-Fe containing enzymes has been reached. In this talk, I will present our recent discovery of the first enzymatic S = 1 Fe(IV)=O intermediate ligated by 4-His ligand environment found in a nonheme iron enzyme that catalyzes a novel oxidative carbon-sulfur bond formation between histidine and cysteine. The implication of this discovery on how the spin state of this novel enzymatic S = 1 Fe(IV)=O intermediate affects its reactivity will be further discussed. As a comparison, I will also discuss our recent study on the electronic structure and reactivity correlation on an S = 2 Fe(IV)=O intermediate found in an Fe/2OG dependent halogenase that catalyze a stereo-selective chlorination on nucleosides.