Electronic structure of an iron-porphyrin-nitrene complex

Middle and late transition metal imido complexes (which may also be viewed as metal-nitrene adducts) are rather rare, especially for square-pyramidal and octahedral coordination geometries. However, an iron(II) porphyrin aminonitrene adduct, denoted here as Fe(Por)(NN), has been known for almost a quarter of a century. Unlike the corresponding S = 1 oxene and S = 0 carbene adducts, Fe(Por)(NN) exhibits an S = 2 ground state. DFT calculations reported herein provide a molecular orbital description of this unusual species as well as a rationale for its S = 2 ground state. The electronic configuration of Fe(Por)(NN) may be described as d(pi)(2)d(xy)(1)d(z(2))(1)d(x(2)-y(2))(1)d(pi')(1), where the z direction corresponds to the Fe-NN axis. The stability and double occupancy of one of the d(pi) orbitals may be attributed to a pi-backbonding interaction with the N-N pi* orbital. The weak sigma-donor ability of the aminonitrene ligand results in a relatively low-energy d(z(2)) orbital and an overall d orbital splitting pattern that engenders a high-spin ground state.