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Abstract

The electronic structures of a series of five‐coordinate complexes of iron containing zero, one, or two bidentate, organic π‐radical ligands and a monodentate ligand (pyridine, iodide) have been studied by broken‐symmetry (BS) density functional theoretical (DFT) methods. By analyzing the set of corresponding orbitals5 (CO) a convenient division of the spin‐up and spin‐down orbitals into 1) essentially doubly‐occupied molecular orbitals (MO), 2) exactly singly‐occupied MOs, 3) spin‐coupled pairs, and 4) virtual orbitals can be achieved and a clear picture of the spin coupling between the ligands (non‐innocence vs. innocence) and the central metal ion (d^N configuration) can be generated. We have identified three classes of complexes which all contain a ferric ion (d⁵) with an intrinsic intermediate spin (S_Fe= 3/2) that yield 1) an S_t=3/2 ground spin state if the two bidentate ligands are closed‐shell species (innocent ligands); 2) if one π‐radical ligand is present, an S_t=1 ground state is obtained through intramolecular antiferromagnetic coupling; 3) if two such radicals are present, an S_t=1/2 ground state is obtained. We show unambiguously for the first time that the pentane‐2,4‐dione‐bis(S‐alkylisothiosemicarbazonato) ligand can bind as π‐radical dianion (L._TSC)²⁻ in [Fe^III(L._TSC)I] (S_t=1) (6); the description as [Fe^IV(L_TSC³⁻)I] is incorrect. Similarly, the diamagnetic monoanion in 14 must be described as [Fe^III(CN)₂(L._TSC)]⁻ (S_t=0) with a low‐spin ferric ion (d⁵, S_Fe=1/2) coupled antiferromagnetically to a π‐radical ligand; [Fe^II(CN)₂(L_TSC⁻)]⁻ is an incorrect description.