The observation in this study should be applicable to photochemistry and spin chemistry of heavy element systems. Further expansion of this study to the vibrational properties, including the dynamic JT effect, is also interesting. In the context of theoretical chemistry, these kinds of heavy metal complexes with high symmetry would be an attractive target for relativistic correlated theory aimed at strong correlation, such as density matrix renormalization group (DMRG) theory The excitation from the GAS1 to GAS2 spaces also contributes. b The excitation from the GAS2 to GAS3 spaces also contributes. , driven similarity renormalization group (DSRG) theory , and DMRG-tailored coupled cluster theory . Feature calculations using these theories may provide results closer to the experimental ones. This study also provides fundamental knowledge about the chemistry of heavy metal complexes. The results would serve as useful examples for a lecture on inorganic chemistry.