Collman and Arnold conducted experimental (UV/Vis/NIR/EPR spectroelectrochemical) and theoretical (DFT) approaches to establish the electronic nature of the electrogenerated species. Interestingly, transition metal corrole dimer {M[TpXPC]}2 (M = Re or Os and TpXPC refers to a mesotris(para-X-phenyl) corrole and X= CF3, H, Me, and OMe) have been synthesized recently by Ghosh and coworkers. The cyclic voltammetry measurements of the {Re[TpMePC]}2 and the {Os[TpMePC]}2 exhibit three reversible oxidations and one reversible reduction. The X-ray data is available just for the neutral structure. Thus, there is little information regarding the nature of the metal-metal bonding in the oxidized and reduced species from electrochemistry and DFT calculations. Hight level computational calculations offer fundamental insight into the nature of metal-metal bonding in the neutral, oxidized, and reduced species of the [{Re[TpMePC]}2] 0/1+/1-and the [{Os[TpMePC]}2] 0/1+/1-complexes. However, rationalizing the nature of metal-metal bonding is challengeable from theoretical perspectives. The most popular computational method is DFT. It has been used for dealing with large systems, in particular, those that contain transition metals. This method, however, in some cases fails to give a clear description of the nature of the metal-metal bonding because it is inherently a single determinant in nature, and the accuracy of the available exchange-correlation functionals is still open to debate. Metalmetal bonding forms from the overlap between d orbitals on each metal of the dimer. This leads to forming different types of bonds that are 𝜎, 𝜋, and 𝛿. These bond components have different strengths; therefore, they cannot be described by DFT because it does not take into account the very different contributions of the 𝜎, 𝜋, and 𝛿 components to the overall bond strength. An alternative approach is multi-configurational self-consistent field (MCSCF) method, specifically, the complete active space self-consistent field (CASSCF) method. This approach takes into account the different contributions of the 𝜎, 𝜋, and 𝛿 components of overall bond strength, thus, gives a better description of the nature of the metal-metal bonding. The main challenge in using the CASSCF methodology is the choice of the active space due to its dependence on the chemical intuition of the system under study. The occupation numbers in single determinant methods (DFT) are restricted to integer values whereas in the CASSCF wavefunction can be fractions between zero and two which can capture all points between the strongly bonded limit and the dissociation limit, where occupation numbers of the bonding and antibonding orbitals are 0.5. In 2003, Gagliardi and Roos studied the electronic structure of [Re2Cl8] 2-using CASSCF/PT2 methodology. 10a