Bridging\nCarbonyl and Carbyne Complexes of Weak-Field\nIron: Electronic Structure and Iron–Carbon Bonding

Carbon\nmonoxide inhibited forms of nitrogenases have carbonyl (CO)\nand carbide (C<sup>4–</sup>) bridges, which are common in synthetic\niron complexes with strong-field ligand environments but rare in iron\nsites with weak-field ligand environments analogous to the enzyme.\nHere, we explore the fundamental bonding description of bridging CO\nin high-spin iron systems. We describe the isolation of several diiron\ncarbonyls and related species, and elucidate their electronic structures,\nmagnetic coupling, and characteristic structural and vibrational parameters.\nThese high-spin iron complexes exhibit equivalent π-backbonding\nabilities to low-spin iron complexes. Sequential reduction and silylation\nof a formally diiron(I) bridging CO complex ultimately gives a formally\ndiiron(IV) bridging carbyne complex. Despite the large range of formal\noxidation states across this series, X-ray absorption spectroscopy\nand density functional theory calculations indicate that the electron\ndensity at the iron sites does not change. Thus, the [Fe(μ-CO)]<sub>2</sub> core undergoes redox changes at the bridging carbonyls rather\nthan the metal centers, rendering the metal’s formal oxidation\nstate misleading. The ability of the Fe<sub>2</sub>C<sub>2</sub> core\nto easily shift charge between the metals and the ligands has implications\nfor nitrogenases, and for other multinuclear systems for redox catalysis.