Valence Delocalization and Metal–Metal Bonding in Carbon‐Bridged Mixed‐Valence Iron Complexes**

Abstract

The carbide ligand in the iron‐molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed‐valent diiron complexes with C‐based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2(m‐CCO)2]+ complex with two carbon bridges (4) is S = 1/2 and is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe–C force constant. In contrast, one‐electron reduction of the [Fe2(m‐CCO)] complex with one carbon bridge (2) affords a mixed‐valence species with a high‐spin ground state (S = 7/2), and the Fe–Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe–Fe bonding interaction that leads to complete valence delocalization. Analysis of near‐IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780–965 cm–1. These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed‐valent iron dimers.