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A Reduced (β-Diketiminato)iron Complex with End-On and Side-On Nitriles: Strong Backbonding or Ligand Non-Innocence?



Treatment of the iron(I) synthon LMeFeNNFeLMe (LMe = β-diketiminato ligand) with pivalonitrile gives a formally iron(I) complex LMeFe(NCtBu)2, which has one strongly bound side-on nitrile and one weakly bound end-on nitrile ligand. The side-on nitrile ligand has N–C–C bending and N–C weakening that suggests partial reduction of the nitrile group. The S = 3/2 ground state of this molecule can be described as either an iron(II) atom with a nitrile radical anion or an iron(I) atom that is strongly backbonding. DFT computations show that strong backbonding comes from a doubly occupied orbital, and that there is little ligand radical character. Therefore, the iron(I) formulation is the most appropriate. This study exemplifies how one-electron transfer to ligands and two-electron backbonding to ligands may be distinguished by using spectroscopically calibrated and validated computations.