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 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 (d 5 ) 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 TSC 3− )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 (d 5 , S Fe =1/2) coupled antiferromagnetically to a π‐radical ligand; [Fe II (CN) 2 (L TSC − )] − is an incorrect description.