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

Abstract

A detailed density functional theory study of pseudotetrahedral FeIII/IV−imido−phosphine complexes has yielded a host of new insights. The calculations confirm dxy2dx2-y22dz21 (or dδ2dδ‘2dσ1) electronic configurations for FeIII−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 FeIII−imido molecular orbital (MO) energy-level diagram indicates the existence of near-degenerate 2A1 and 2E states, and accordingly, the bending of the imido group appears to be ascribable to a pseudo-Jahn−Teller distortion. For FeIV−imido complexes, our calculations indicate a dxy2dx2-y21dz21 (or dδ2dδ‘1dσ1) electronic configuration, which is somewhat different from the dxy1dx2-y21dz22 (or dδ1dδ‘1dσ2) configuration proposed in the literature. Not surprisingly, for a sterically unencumbered FeIV−imido complex, the degenerate 3E 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 dz2-based MO, which points directly at the imido nitrogen, relative to the dπ-based MOs. The low-coordinate nature of the complexesthe absence of equatorial ligands and of a ligand trans with respect to the imido ligandplays a key role in stabilizing the dz2 orbital as well as the complexes as a whole. The electronic configurations of FeIV−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 FeIV−imido porphyrins.