Electronic Structures and Spectroscopy of the Electron Transfer Series [Fe(NO)L₂]^<i>z</i> (<i>z</i> = 1+, 0, 1–, 2–,3–; L = Dithiolene)

The electronic structures and spectroscopic parameters for the electron transfer series of [Fe(NO)(L)2]z (z = 1+, 0, 1–, 2–, 3–; L = S2C2R2; R = p-tolyl (1) and CN (2)) were calculated and compared to experiment. Some compounds in the series were isolated and characterized by spectroscopy. The calculations support the notion that all the monocation (St = 0), neutral (St = 1/2), and monoanion (St = 0) complexes contain NO+ (SNO = 0), in which the redox active fragment is either the bis-dithiolene (2 L) or the central iron. The calculated electronic structures give insight into how p-tolyl and CN substituents and the redox states of the 2 L ligand impact the spin density on the iron in the monocation and neutral species. The electronic structure of [1]0 has some [FeI(NO+)(L22–)]0 character in resonance with [FeII(NO+)(L22–)]0 whereas [2]0 has a smaller amount of a [FeI(NO+)(L22–)]0 description in its ground state wavefunction. Similarly, the electronic structure of [1]1+ also has some [FeI(NO+)(L21–)]1+ character in resonance with [FeII(NO+)(L22–)]1+ whereas [2]1+ is best described as [FeII(NO+)(L•)2]1+. For the monoanion, the bis-dithiolene fragment is fully reduced and both [1]− and [2]− are best formulated as [FeII(NO+)(L24–)]−. The reduction of the monoanion to give dianions [1]2– and [2]2– results in {FeNO}7 species. The calculated 57Fe isomer shift and hyperfine couplings are in line with the experiment and support a description of the form [FeIII(NO–)(L24–)]2–, in which Fe(III) SFe = 3/2 is antiferromagnetically coupled to NO– (SNO = 1). Finally, the calculated redox potential and ν(NO) frequency for the {FeNO}8 trianionic species [2]3– is in agreement with experiment and consistent with a triplet ground state [FeII(NO–)(L24–)]3–, in which Fe(II) (SFe = 2) is involved in antiferromagnetic coupling with NO– (SNO = 1).