Previously we have characterized two high-valent complexes [LFe IV(μ-O)2FeIIIL], 1, and [LFe IV(O)(μ-O)(OH) FeIVL], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-OH and 1-F. Electron paramagnetic resonance (EPR) and Mössbauer studies show that both complexes have an S = 1/2 ground state which results from antiferromagnetic coupling of the spins of a high-spin (Sa = 5/2) FeIII and a high-spin (Sb = 2) FeIV site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an FeIV=O site. Radiolytic reduction of 4 at 77 K yields a Mössbauer spectrum identical to that observed for 1-OH, showing that the latter contains an FeIV=O. Interestingly, the FeIV=O moiety has Sb = 1 in 4 and Sb = 2 in 1-OH and 1-F. From the temperature dependence of the S = 1/2 signal we have determined the exchange coupling constant J (ℋ = Ĵ a·̂b convention) to be 90 ± 20 cm -1 for both 1-OH and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J = 135 cm-1 for 1-OH and J = 104 cm -1 for 1-F, in good agreement with the experiments. DFT analysis shows that the Sb = 1 → Sb = 2 transition of the FeIV=O site upon reduction of the FeIV-OH site to high-spin FeIII is driven primarily by the strong antiferromagnetic exchange in the (Sa = 5/2, Sb = 2) couple.