Density functional theory studies on the nature of the cis effect and cis influence of ligands on oxoiron nonheme complexes have been performed. A detailed analysis of the electronic and oxidizing properties of [Fe IV=0(TPA)L]+ with L = F-, C1-, and Br- and TPA = tris-(2-pyridylmethyl)amine are presented and compared with [FeIV=0(TPA)NCCH3]2+. The calculations show that the electronic cis effect is determined by favorable orbital overlap between first-row elements with the metal, which are missing between the metal and second- and third-row elements. As a consequence, the metal 3d block is split into a one-below-two set of orbitals with L = C1- and Br -, and the HOMO/LUMO energy gap is widened with respect to the system with L = F-. However, this larger HOMO/LUMO gap does not lead to large differences in electron affinities of the complexes. Moreover, a quantum mechanical analysis of the binding of the ligand shows that it is built up from a large electric field effect of the ligand on the oxoiron species and a much smaller quantum mechanical effect due to orbital overlap. These contributions are of similar strength for the three tested halogen cis ligands and result in similar reactivity patterns with substrates. The calculations show that [Fe IV=0(TPA)L]+ with L = F-, Cl-, and Br- have closely lying triplet and quintet spin states, but only the quintet spin state is reactive with substrates. Therefore, the efficiency of the oxidant will be determined by the triplet-quintet spin state crossing of the reaction. The reaction of styrene with a doubly charged reactant, that is, [Fev=0(TPA)L]2+ with L = F-, C1-, and Br- or [Fev=0(TPA)NCCH3]3+, leads to an initial electron transfer from the substrate to the metal followed by a highly exothermic epoxidation mechanism. These reactivity differences are mainly determined by the overall charge of the system rather than the nature of the cis ligand.