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Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO}(n) notation. In this work, we succeeded in characterizing a series of {Fe-NO}(6-9) complexes, including a reactive {Fe-NO}(10) intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyllamine (WTIMENmes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENmes)Fe](2+) resulted in formation of the {Fe-NO}(6) compound [(TIMENmes)Fe(NO)(CH3CN)](BF4)(3) (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO}(7,8,9) (2-4). Reduction of {Fe-NO}(9) with Cs electride finally yields the highly reduced {Fe-NO}(10) intermediate, key to formation of [Cs(crypt-222)][(TIMENmes)Fe(NO)], (5) featuring a metalacyclic [Fe-(NO-NHC)(3-)] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as Fe-57 Mossbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO}(8) complex was found via oxygenation of the nitride complex [(TIMENMes)Fe(N)](BF4). Surprisingly, the resulting {Fe-NO}(8) species is electronically and structural similar to the [(TIMENmes)Fe(N)(+) precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)Fe(N)](+) as a {Fe-N}(8) species.
