Multireference Electronic Structures of Fe-Pyridine(diimine) Complexes over Multiple Oxidation States.

Noninnocent (redox-active) ligands are increasingly exploited in the design of coordination compounds of earth-abundant transition metals having interesting reactive and catalytic activities. Particular examples of such ligands include those in the pyridine(diimine) (also referred to as bis(imino)pyridine) family. The electronic structures of these compounds are characterized by significant complexity, such that routine single-reference methodologies, for example, Kohn-Sham density functional theory, may be challenged to describe them physically, that is, with quantitatively accurate descriptions of geometries, charge distributions, and spin-state energy separations. We report here RASSCF/RASPT2 calculations on iron pyridine(diimine) complexes over three formal oxidation states that illustrate the highly multiconfigurational characters of these compounds in general and that also offer insights into their electronic ground states and charge distributions.