Bridging Carbonyl and Carbyne Complexes of Weak-Field Iron: Electronic Structure and Iron-Carbon Bonding

Carbon monoxide inhibited forms of nitrogenases have carbonyl (CO) and carbide (C^4-) bridges, which are common in synthetic iron complexes with strong-field ligand environments but rare in iron sites with weak-field ligand environments analogous to the enzyme. Here, we explore the fundamental bonding description of bridging CO in high-spin iron systems. We describe the isolation of several diiron carbonyls and related species, and elucidate their electronic structures, magnetic coupling, and characteristic structural and vibrational parameters. These high-spin iron complexes exhibit equivalent π-backbonding abilities to low-spin iron complexes. Sequential reduction and silylation of a formally diiron(I) bridging CO complex ultimately gives a formally diiron(IV) bridging carbyne complex. Despite the large range of formal oxidation states across this series, X-ray absorption spectroscopy and density functional theory calculations indicate that the electron density at the iron sites does not change. Thus, the [Fe(μ-CO)]₂ core undergoes redox changes at the bridging carbonyls rather than the metal centers, rendering the metal's formal oxidation state misleading. The ability of the Fe₂C₂ core to easily shift charge between the metals and the ligands has implications for nitrogenases, and for other multinuclear systems for redox catalysis.