All resting state and propanol-bound d 6 intermediates of the Fe and Ru catalysts were studied in three spin states: low spin (LS) i.e., S = 0 (singlet), intermediate spin (IS) i.e., S = 1 (triplet), and high spin (HS) i.e., S = 2 (quintet) (Supporting Information Table ). The metal-oxo intermediates for d 4 Fe(IV)=O and Ru(IV)=O adopt an identical spin state as their corresponding d 6 resting states. During HAT, for Fe catalysts, we assumed a ferromagnetically coupled H• (aradical transfer) with an antiferromagnetically coupled •CH(CH3)2 that recombines with the metalhydroxo species during the rebound step, as done in prior work. For Ru catalysts, however, we assumed an antiferromagnetically coupled H• (b-radical transfer) with a ferromagnetically coupled •CH(CH3)2 recombining with the metal-hydroxo species because higher spin multiplicity Ruhydroxo species are highly unfavorable, as observed from differences in spin state energies (see Sec. 4b). Thus, the overall spin state is conserved in the metal-hydroxo intermediate when coupled with the isopropyl radical as well as in the isopropanol-bound intermediate and ultimately, throughout the catalytic cycle.