The electronic structures and redox properties of the Co (bpy) ₃ ^{2+/3+} and Co (phen) ₃ ^{2+/3+} redox couple were investigated using the two different exchange–correlation functional, namely CAM-B3LYP and TPSSh, with the def2-SVP and def2-TZVP basis sets. Our results indicate that Co (bpy) ₃ ^{2+} and Co (phen) ₃ ^{2+} complexes exhibit a high-spin ground state, whereas Co (bpy) ₃ ^{3+} Co (phen) ₃ ^{3+} complexes adopt a low-spin ground state. Both basis sets and exchange–correlation functionals consistently predict the same ground-state spin configurations for these complexes. For Co (bpy) ₃ ^{2+}, the energy gap between the doublet and quartet spin states is relatively small when using the TPSSh functional, amounting to 2. 21 kcal/mol with the Def2-SVP basis set and 0. 09 kcal/mol with the Def2-TZVP basis set. In contrast, when the CAM-B3LYP functional is employed, the energy splitting becomes significantly larger, with values of 7. 91 kcal/mol and 6. 12 kcal/mol for the Def2-SVP and Def2-TZVP basis sets, respectively. We also observed similar trends for Co (phen) ₃ ^{2+}. In contrast, Co (bpy) ₃ ^{3+} exhibits a significantly larger energy separation between spin states. Using the CAM-B3LYP functional, the energy difference between the singlet ground state and the quintet excited state is calculated to be 41. 42 kcal/mol and 42. 07 kcal/mol with the Def2-SVP and Def2-TZVP basis sets, respectively. When the TPSSh functional is employed, this singlet-quintet energy gap becomes slightly larger, further reinforcing the strong preference for the low-spin singlet configuration in the oxidized complex. Additionally, we have observed that adiabatic ionization potentials are less affected by the choice of basis when the CAM-B3LYP functional is used. In contrast, vertical ionization potential exhibits a more pronounced basis set dependence. However, ionization potentials calculated with the TPSSh functional exhibit significantly reduced sensitivity to basis set choice. The calculated oxidation potentials (E^) using CAM-B3LYP functional for the Co (bpy) ₃ ^{2+/3+} couple are 5. 32 V and 5. 26 V employing the def2-SVP and def2-TZVP basis sets, respectively, whereas (E^) calculated at the TPSSh functional is 4. 62 V and 4. 55 V. We also have observed that the standard oxidation potential of Co (phen) ₃ ^{2+/3+} is relatively higher than the values of Co (bpy) ₃ ^{2+/3+}. Computational analyses of Co (bpy) ₃ ^{3+/2+} and Co (phen) ₃ ^{3+/2+} establish a unified redox picture grounded in Gaussian MO compositions and NBO donor–acceptor metrics. In both families, Co (III) displays ligand– HOMOs with a Co (3d) block immediately below, while Co-centered d acceptors comprise the LUMO/LUMO+1. One-electron reduction populates a metal-centered acceptor, yielding Co (II) doublets with a two-long/four-short Co–N pattern and quartets with uniformly longer Co–N bonds. NBO E^{ (2) } values for LP (N) LV (Co) donation decrease systematically from Co (III) to Co (II), rationalizing bond elongation and modest increases in Co natural charges. Phenanthroline narrows the d/ ^* gap relative to bipyridine, making ligand-centered pathways slightly more competitive; nevertheless, the primary Co (III) Co (II) event remains metal-centered across spin states examined. These findings contribute to a deeper understanding of redox property calculations via DFT and may support the potential application of cobalt-containing redox couples in thermoelectric materials.
Ibragimov et al. (Sat,) studied this question.