characterized the copper complex of the formally trianionic β-tetramethyltetraethyl corrole by X-ray crystallography in 1997 suggesting that it was nearly planar and assigned an oxidation state of Cu(III) in the ground state. This conclusion was subsequently supported by the aforementioned DFT computations imposing C 2V symmetry. In the early 2000s, several experimental and DFT studies on meso and β substituted copper corroles supported the assignment of a closed-shell Cu(III) ground state. However, towards the end of the decade, reports assigning the ground state as the Cu(II) corrole diradical cation on the basis of experimental diffraction and theoretical studies become more prominent. In 2016, Lemon et al. examine the question of copper oxidation state by means of X-ray photoelectron spectroscopy, ultimately assigning a Cu(II) oxidation state to the ground state singlet. Furthermore, they identifying a triplet state that is only 3.3 kcal/mol higher in energy by variable temperature 1 H NMR spectra. Specifically, they note that at 23 • C the triplet state is 1.1% populated, but this increases to 3.3% at 100 • C. A Cu(II) oxidation state was also assigned via X-ray absorption spectroscopy by Lim et al. in 2019. Finally, EPR spectroscopy confirmed that the ground state of the β-octaethyl-5,15-diphenyl and 5,15-dipentafluorophenyl-10-methylbenzoate copper corroles were diamagnetic. While Cu(II) and Cu(III) are the most commonly discussed oxidation states in the copper corrole literature, a Cu(IV) oxidation state was reported for a copper corrole complex based on experimental and computational studies, although this was generated via an oxidation event and was not assigned to the ground state. Finally, a triplet ground state can be stabilized by extending π conjugation at the β position of the corrole or by inducing a change in spin state via solvent coordination. In addition to experiments, a significant number of theoretical studies have been carried out by employing both DFT and high-level wave-function theory (WFT) approaches. As is often the case, the nature of the DFT ground state and, in turn, the assigned oxidation state of copper are sensitive to functional choice. For example, pure DFT functionals tend to predict a closed shell singlet ground state with an innocent (no radical character) corrole ligand; however, hybrid DFT functionals lead to ground states with diradical character that increases with the amount of Hartree-Fock (HF) exchange. Most notably, an extensive theoretical study was performed by Pierloot et al. on the bare copper corrole and the mesosubstituted 5,10,15-triphenyl copper corrole. They optimized the molecular geometries of the complexes with four different DFT functionals (BP86, OLYP, PBE0, and B3LYP), imposing C 2V symmetry for the planar structure and C 2 symmetry for the saddled structure. In addition to describing the DFT electronic structure, they performed a subsequent electronic structure study on PBE0 geometries with complete or restricted active space second-order multireference methods (e.g., CASPT2 or RASPT2). Specifically, both BP86 and OLYP result in closed-shell singlet ground states for the planar and saddled geometries, consistent with a Cu(III) oxidation state and a d 8 π 2 a 2 π 2 b 1 electron configuration. However, the hybrid PBE0 and B3LYP functionals predict a triplet ground state for the planar geometry, consistent with a Cu(II) oxidation state in which the unpaired electrons are ferromagnetically coupled in the copper 3d x 2 -y 2 and corrole π b 1 orbitals. On the other hand, both hybrid functionals predict a broken symmetry singlet as the ground state for the saddled structure (i.e., an antiferromagnetically coupled singlet state) that is assigned as Cu(II). Using CASPT2 and RASPT2, the two lowest lying singlet spin states for the planar and saddled geometries were computed. For both the bare copper corrole and 5,10,15-triphenyl copper corrole, Pierloot et al. assign the two singlet states based on their dominant electron configurations, and localized-orbital CAS configuration interaction (CASCI) calculations, as a "Cu(III)" singlet