The calculated electronic structure of the Bi4 cluster (shown in Fig. ) are in general consistent with the previous results by Lohr and Pyykkö obtained by means of relativistically-parametrized extended Hückel method , although the ordering of virtual orbitals is slightly different. The stabilizing effect of SOC on ABi2 can be qualitatively understood by considering the electronic structure of a Td Bi4 cluster. Firstly, in a heavy element, such as Bi scalar-relativistic effects result in a complete energetic separation of 6s and 6p orbitals . Thus, only the p orbitals participate in the bonding. Secondly, the SOC splits the 6p atomic level into p1/2 and p3/2 separated by ca. 2 eV in a free atom. In the scalar-relativistic approximation the neutral Bi4 cluster is a closed-shell system with four electrons occupying the doubly-degenerate e HOMO level. With 2 additional electrons donated by electropositive species, the Bi4 2-is first-order Jahn-Teller (JT) active due to two electrons occupying a triply degenerate t1 level. SOC splits the 6p-derived levels (6s-derived lower t2 level is also weakly split due to small admixture of the p character), lifting the triple degeneracy of t1 and t2. The two additional electrons now occupy a e' (Γ6) level, quasi-degenerate (ΔE ≈ 0.02 eV) with the next e'' (Γ7) leading to structural stabilization. However, the splitting of the e' and the two u' (Γ8) levels is relatively small (ΔE ≈ 0.9 and 1.2 eV, respectively), resulting in a possibility of a second-order JT instability, as pointed out by Lohr and Pyykkö . While the electronic structure of the crystalline ABi2 compound is much more complex than of isolated cluster, the stabilizing effects of SOC similarly stem from the lifted p orbital degeneracy while the weakened but persistent mode softening can be considered an effect of a second-order JT (SOJT) effect. Examples of such SOC stabilized superconductors can be found among Sb-, Bi-and Pb-rich intermetallic phases featuring hypervalent polyanions, listed in Table .