Finally , t he biomimetic tetranuclear [ MoFe 3 S 4 ] 2 + and [ Fe 4 S 4 ] 2 + and the MoFep rotein of nitrogenase were studied ( Figure 6C ) . All samples contain Fe II , a nd the resonant excitation into the pre - edge yields two distinct features split by~0.9 - 1.3 eV at their maxima due to the additional 5 Dterm discussed above . Once again , inspection of the D RXES and D XES highlights the ability of 1s3p RXES at the pre - edge region to identify the presence of ferrous iron , which is not possible using the Kb XES spectra alone . Future studies including 1s3p RXES full planes collected with higher resolution instruments may allow for amore rigorous quantification of the changes in electronic structure , e nabling better distinction between the different intermediate states . N evertheless , t hese results demonstrate that even in highly covalent systems , meaningful differences in 3d metal valencypersist in the 1s3p RXES data .