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 (C4-) 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 pi-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(mu-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.