Two mononuclear nonheme FeIII complexes, [PPh4][Fe-III(PS3 '')(OCH3)] (1) and [PPh4][Fe-III(PS3 '')(Cl)] (2), supported by a tris(benzenethiolato)phosphine derivative PS3 '' (PS3 '' = P(C6H3-3-Me3Si-2-S)(3)(3)) have been synthesized and characterized. The structures resolved from X-ray crystallography show that Fe-III centers in both complexes adopt distorted trigonal-bipyramidal geometry with a methoxide or a chloride binding in the axial position. The magnetic data for both are consistent with intermediate-spin FeIII centers with a C-3 symmetry (S = 3/2 ground state). The bound methoxide in 1 is labile and can be replaced by a CH3CN molecule. The forming (FeCH3CN)-C-III species can be further reduced by cobaltcene quantitatively to a stable (FeCH3CN)-C-II complex, [Fe(PS3 '')(CH3CN)](-). One-electron oxidation of 2 by ferrocenium gave a FeIV analogue, [Fe-IV(PS3 '')(Cl)]. Importantly, the (FeOCH3)-O-III moiety in complex 1 acts as a strong nucleophile that activates the CCl bond in CH2Cl2, leading to the formation of complex 2 quantitatively. Complex 1 also reacts with other electrophiles, benzyl chloride and benzyl bromide, to generate (FeX)-X-III species (X = Cl or Br). The reactions were investigated and monitored by UVvisNIR, NMR, and ESI-MS spectroscopies.