title: Electronic structures of five-coordinate complexes of iron containing zero, one, or two pi-radical ligands: a broken-symmetry density functional theoretical study.
abstract: The electronic structures of a series of five-coordinate complexes of iron containing zero, one, or two bidentate, organic pi-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 orbitals[5] (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 pi-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 pi-radical dianion (L.TSC)2- in [FeIII(L.TSC)I] (St=1) (6); the description as [Fe IV(L TSC(3-))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 pi-radical ligand; [FeII(CN)2(L TSC-]- is an incorrect description.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Chłopek
authors.forename: Nicoleta
authors.initials: N
authors.lastname: Muresan
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
mesh_terms: D004583:Electrons; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure
chemical_list: D008024:Ligands; D007501:Iron