AbstractThe equilibrium structural parameters, symmetry groups, and dipole moments of the free hollow electron‐deficient TlBi19, Tl2Bi18, p‐, m‐, o‐Tl2Pb10 shells and the endohedral (He, Ne)@Tl2Pb10, (Y, La, Lu, Tl, Bi, Ce)@TlPb11, Tl@(Y, La, Lu) Pb11, Sr@TlBi19, Tl@SrBi19, Yb@TlBi19, Tl@YbBi19, [Y@TlBi19]+, [Lu@TlBi19]+, Sr@Tl2Bi18, and Yb@Tl2Bi18 complexes are predicted by using DFT (U)PBE0 hybrid method. The isomerization of (Sr, Yb)@TlkBi20–k into Tl@(Sr, Yb) Tlk–1Bi20–k is endothermic. The isodesmic substitution of an endo‐atom in the Tl@TlPb11 cluster for a rare‐earth atom is exothermic. The hollow TlkBi20–k (Sr, Yb) complexes with Tl in the shell and with Sr or Yb exo‐atom are impossible. The endo‐atoms plunge in hollow TlPb11 and hollow TlBi19 shells without delay on its surface. The (C5v)‐TlPb11 and (C3v)‐TlBi19 shells are stabilized by an endo‐atom X with oxidation number ΞX = +3 and +2, respectively. No one cerium IV oxidation state has been predicted. The integer number ΞX is presented as a function of the populations of one‐electron states in the shell surrounding the endo‐atom X. Each Pb or Bi atom in the electron‐deficient clusters retains an entirely localized lone electron pair. The change in the IR spectrum when a metal atom is embedded into its cavity indicates a strong interaction between the components of the endohedral complex. Structural aspects of the β‐decay of a radioactive endo‐atom in the cavity of a metal cluster are discussed.