Electronic structure and bonding in unligated and ligated FeII porphyrins

The electronic structure and bonding in a series of unligated and ligated Fe-II porphyrins (FeP) are investigated by density functional theory (DFT). All the unligated four-coordinate iron porphyrins have a (3)A(2g) ground state that arises from the (d(xy))(2)(d(z)(2))(2)(d(pi))(2) configuration. The calculations confirm experimental results on Fe tetraphenylporphine but do not support the resonance Raman assignment of Fe octaethylporphine as E-3(g), nor the early assignment of Fe octamethyltetrabenzporphine as B-5(2g). For the six-coordinate Fe-P(L)(2) (L=HCN, pyridine, CO), the strong-field axial ligands raise the energy of the Fe d(z)(2) orbital, thereby making the iron porphyrin diamagnetic. The calculated redox properties of Fe-P(L)(2) are in agreement with experiment. As models for deoxyheme, the energetics of all possible low-lying states of FeP(pyridine) and FeP(2-methylimidazole) have been studied in detail. The groundstate configuration of FeP(2-methylimidazole) was confirmed to be high-spin (d(xy))(2)(d(z)(2))(1)(d(pi))(2)(d(x)(2)-y(2))(1); FeP (pyridine) is shown to be a poor model for high-spin deoxyheme. (C) 2002 American Institute of Physics.