Bonding in Low-Coordinate Environments:  Electronic Structure of Pseudotetrahedral Iron-Imido Complexes

A detailed density functional theory study of pseudotetrahedral Fe(III/IV)-imido-phosphine complexes has yielded a host of new insights. The calculations confirm dxy(2)dx(2)-y(2)(2)dz(2)(1) (or dδ(2)dδ'(2)dσ(1)) electronic configurations for Fe(III)-imido complexes of this type, as previously proposed, where the z direction may be identified with the Fe-Nimido vector. However, geometry optimization of a sterically unencumbered model complex indicated a bent (162°) imido linkage, in sharp contrast to the linear imido groups present in the sterically hindered complexes that have been studied experimentally. Under C3v symmetry, the Fe(III)-imido molecular orbital (MO) energy-level diagram indicates the existence of near-degenerate (2)A1 and (2)E states, and accordingly, the bending of the imido group appears to be ascribable to a pseudo-Jahn-Teller distortion. For Fe(IV)-imido complexes, our calculations indicate a dxy(2)dx(2)-y(2)(1)dz(2)(1) (or dδ(2)dδ'(1)dσ(1)) electronic configuration, which is somewhat different from the dxy(1)dx(2)-y(2)(1)dz(2)(2) (or dδ(1)dδ'(1)dσ(2)) configuration proposed in the literature. Not surprisingly, for a sterically unencumbered Fe(IV)-imido complex, the degenerate (3)E state (under C3v symmetry) results in a mild Jahn-Teller distortion and a slightly bent (173°) imido linkage (on relaxing the symmetry constraint). The calculations also shed light on the surprising stability of the dz(2)-based MO, which points directly at the imido nitrogen, relative to the dπ-based MOs. The low-coordinate nature of the complexes [Formula: see text] the absence of equatorial ligands and of a ligand trans with respect to the imido ligand [Formula: see text] plays a key role in stabilizing the dz(2) orbital as well as the complexes as a whole. The electronic configurations of Fe(IV)-imido porphyrins are radically different from that of the pseudotetrahedral complexes studied here, and we have speculated that these differences may well account for the nonobservation so far of Fe(IV)-imido porphyrins.