Imido complexes of high-spin 3d metals can feature N subvalency, wherein configurations with hole character at N contribute significantly to the ground state electronic structure. Reported herein are ...

The Mo nitrogenase is more efficient for N2 reduction than the alternative nitrogenases for reasons that are not fully understood. A set of related hypotheses center on the Mo ion facilitating electron delocalization, which could aid in substrate activation and/or provide enhanced stability during turnover, among other possibilities. To understand how the incorporation of Mo or other ‘heterometals’ influences the electronic structures of Fe-S clusters, we prepared and characterized an isostructural and isoelectronic series of [MFe3S4] clusters (M = V, Cr, Mo, W) that includes the first example of a [CrFe3S4] cluster. Comparison of the clusters’ structural, spectroscopic, and computed properties reveals that the Mo- and W-containing clusters have greater M–Fe delocalization than those containing only 3d metals. Although the [CrFe3S4] cluster adopts the same ground spin state as the other clusters (S = 3/2), the smaller Cr atom engages in substantially attenuated─though not fully obliterated─M–Fe delocalization. Moreover, electrochemical and Mössbauer spectroscopic studies reveal that the clusters with more substantial M–Fe delocalization have more electron-rich Fe sites. Such trends, if they apply to the nitrogenase cofactors, could account for the greater efficiency of the Mo nitrogenase in biological nitrogen fixation.