Bonding in Low-Coordinate Environments: Electronic Structure of Distorted Square-Planar Iron-Imido Complexes With Pincer-Type Ligands.

Low-coordinate architectures sustain unusual chemistry for middle and late transition metals, of which imido complexes are an excellent example. Recent DFT studies have uncovered a number of unusual features in the bonding in trigonal-planar and pseudotetrahedral imido complexes. Herein, we have extended these studies to a unique, distorted square-planar iron-imido complex with a pincer-type pyridine-2,6-diimine (PDI) supporting ligand. DFT calculations indicate that the iron center in the formally Fe(II) complex Fe(PDI)(NPh) is better described as intermediate-spin Fe(III), antiferromagnetically coupled to a b2-symmetry PDI π-anion radical. A comparative analysis of the major classes of low-coordinate imido complexes has uncovered a certain similarity between Fe(PDI)(NPh) and a trigonal-planar Fe(III)-nacnac-imido complex. Both ligand architectures afford a total of four energetically accessible d orbitals, resulting in intermediate-spin Fe(III) centers.