| Sumario: | The four-coordinate heteroleptic complex [Fe (pda)(Fdad)] (1) and its homoleptic analogue [Fe(dad)] (2), where pda is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenedia- mide(2-) and dad is the singly reduced N,N'-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diazabutadiene π-radical anion, have been synthesized. X-ray crystallographic studies reveal a twisted tetrahedral geometry of the FeN coordination polyhedron in both 1 and 2. The electronic structures of 1 and 2 were probed by magnetic susceptibility measurements, Fe MOssbauer and electronic spectroscopy, and density functional theory (DFT) calculations. In spite of their similar geometries and a common triplet ground state (S = 1), the electronic structures of 1, 2, and the previously reported homoleptic analogue [Fe(pda)(pda)] (3), where pda is a one-electron-oxidized form of pda, differ. The electronic structure of 2 consists of two dad radicals coupled antiferromagnetically to a high-spin Fe center, whereas in 3, only one pda radical is coupled antiferromagnetically to an intermediate-spin Fe ion. This ligand mixed-valent species exhibits class-Ill behavior. Heteroleptic 1 contains a single dad radical coupled antiferromagnetically to an intermediate-spin F center but behaves as a class-Il ligand mixed-valent species. The observed diversity in the electronic structures of 1-3 is ascribed to the difference in the redox potentials of the ligands. Analysis of reduced orbital charges and spin densities obtained from DFT calculations also suggests that the electronic structures of 1-3 are best described as either a high-spin Fe ion coordinated to two radical monoanions (2) or as an intermediate-spin Fe ion coordinated to one radical monoanion and one closed- shell dianion (1, 3).
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