Corrole complexes, or metallocorroles, have been synthesized with a wide variety of transition metals and are of interest for their ability to catalyze hydrogen evolution reactions, oxygen evolution reactions, and oxygen reduction reactions. Metallocorroles also have diverse applications in other areas of molecular catalysis, small molecule sens-ing, and in medicinal applications. In addition to the highly functionalized hang-man corroles designed to play a more specialized role in catalysis, substituting metallocorroles in the meso and β positions can significantly alter their reactivities. The formally tri-anionic corrole can bind transition metals in the +3 oxidation state; however, both experimental and computational work supports ligand non-innocence in copper corroles, where researchers have assigned the electronic structure as formally Cu(II)(corrole • ). This behavior has been rationalized as an electron transfer from the corrole ligand π orbitals to copper 3d orbitals, resulting in a corrole radical dianion and a reduced metal center. The ligand to metal charge transfer is thought to play a significant role in governing the molecular geometries of copper corroles where the radical character in copper corroles is suggested to be the leading factor driving the formation of the characteristic saddled molecular geometry. This saddled structure is not commonly observed in other metallocorrole systems where ligand non-innocence is not reported. At a given molecular geometry, different descriptions of the ground state electronic struc-ture in the copper corrole complex have been reported. These differences are often understood by taking language from the way in which porphyrin electronic structure is understood, specifically by expressing the valence orbitals in terms of the four so-called Gouterman π orbitals and the d x 2 -y 2 orbital (Figure ). An extended discussion of the corrole π-system in porphyrins and corroles is included in a later section. Note that the complex is taken to have C 2V symmetry and the orbitals are labeled accordingly. In a strictly d 8 Cu(III) corrole complex, the d x 2 -y 2 orbital would be unoccupied, while the lowest energy Gouterman π a 2 and π b 1 orbitals would be doubly occupied. However, if the ligand was perfectly non-innocent, the metal charge transfer in the S 0 ground state would result in an electron transfer from the ligand to metal resulting in a pure d 9 Cu(II) complex. The d x 2 -y 2 orbital would now be singly occupied leaving a ligand-centered radical anion localized in one of the π orbitals. The current consensus is that copper corrole complexes are neither pure Cu(II) d 9 nor pure Cu(III) d 8 complexes, but present with a multiconfigurational electronic structure that is intermediate between the two extremes. This type of electronic structure has also been reported to a lesser extent in other square-planar copper complexes and in other metalled macrocycles including porphyrins. Nevertheless, questions remain with respect to the details of the unique electronic structure in copper corroles. Early DFT (PW91) reports predicted a closed shell singlet ground state for the planar C 2V geometry with the broken-symmetry solution, corresponding to a Cu(II)(corrole • ) state, lying only 3.7 kcal/mol higher in energy. Later, Bröring et al. performed DFT (B3LYP) calculations on the saddled C 2 structure in addition to the planar C 2V geometry. They determined that the broken symmetry singlet was the ground state on the saddled structure, but, in contrast to previous reports, predicted a triplet state as most stable on the planar structure, lying only 1.9 kcal/mol higher in energy. Furthermore, they assign a Cu(II) oxidation state for both geometries. After this time, the oxidation state of copper in the corrole complexes arose as a matter of debate. It was not only early DFT computations that led to contrasting pictures, but experimental results also contributed to the discussion. Early results in the 1970s were incorrectly interpreted, suggesting that copper(II) binds to a dianionic corrole generated by removing two, not three, protons from the pyrrole nitrogen atoms. However, Will et al.