Perferryl Fe^V-oxo nonheme complexes: Do they have high-spin or low-spin ground states?

Nonheme perferryl Fe^V = O species are studied herein by means of coupled cluster (CCSD(T)) calculations with a complete basis set limit estimate and density functional B3LYP computations. It is shown that the high-spin/low-spin (HS/LS) energy order in these Fe^V = O species is highly dependent on the electronic nature of the ligand sphere and the geometric position of ligands relative to the Fe^V = O moiety. When only σ-donor amines ligate Fe^V = O, the LS state is slightly lower than the HS states. However, when a strong π-donor ligand such as hydroxyl is cis to Fe^V = O, the HS state becomes highly favored. And on the contrary, if the π-donor ligand is trans to Fe^V = O, the LS state is predicted here to be highly favored. This last type of perferryl complex has not yet been made by experimental means. Generally, our findings are consistent with the available experimental data.(4a, 6, 7)Some implications of these findings on the behavior of experimental systems are discussed.