Electronic structures of five-coordinate complexes of iron containing zero, one, or two π-radical ligands: A broken-symmetry density functional theoretical study

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 orbitais[5] (CO) a convenient division of the spin-up and spin-down orbitals into 1) essentially doubly-occupied molecular orbitais (MO), 2) exactly singly-occupied MOs, 3) spin-coupled pairs, and 4) virtual orbitais 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 (LTSC)I] (S t = 1) (6); the description as [FeIV-(LTSC 3-)I] is incorrect. Similarly, the diamagnetic monoanion in 14 must be described as [FeIII(CN)2(LTSC)]- (St = O) with a low-spin ferric ion (d5, SFe = 1/2) coupled antiferromagnetically to a π-radical ligand; (Fe II(CN)2(LTSC-)]- is an incorrect description. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.