A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}8-10

Given the importance of Fe-NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine) borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark-Feltham numbers: {FeNO}(8), {FeNO}(9), and {FeNO}(10). These {FeNO}(8-10) compounds are each low-spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mossbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}(8-10) are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark-Feltham numbers.