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

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

Abstract

Treatment of the iron(I) synthon L^MeFeNNFeL^Me (L^Me = β-diketiminato ligand) with pivalonitrile gives a formally iron(I) complex L^MeFe(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.

In the S = 3/2 molecule L^MeFe(NCtBu)₂, the side-on nitrile ligand has N–C–C bending and N–C weakening that suggests partial reduction of the nitrile group. Even though the molecule has three unpaired spins, the interaction between the iron atom and the nitrile ligand is two-electron backbonding rather than redox non-innocence.