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density functional calculations; intermediate spin; iron; ligand effects; radicals
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 (dN configuration) can be generated. We have identified three classes of complexes which all contain a ferric ion (d5) with an intrinsic intermediate spin (SFe= 3/2) that yield 1) an St=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 St=1 ground state is obtained through intramolecular antiferromagnetic coupling; 3) if two such radicals are present, an St=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 [FeIII(L.TSC)I] (St=1) (6); the description as [FeIV(LTSC3−)I] is incorrect. Similarly, the diamagnetic monoanion in 14 must be described as [FeIII(CN)2(L.TSC)]− (St=0) with a low‐spin ferric ion (d5, SFe=1/2) coupled antiferromagnetically to a π‐radical ligand; [FeII(CN)2(LTSC−)]− is an incorrect description.
