Ab initio Hartree-Fock SCF calculations on the compounds Si(4)A(4)O(12)H(8)M and Si4B4O12H8M, where M Ti, Zr, indicate that endohedral complexes, in which the M cation occupies the center of the aluminosilicate or borosilicate cage, will be stable compared to their Na complexes and the M cation in aqueous solution. The Si4Al4O12H84- cage is strongly disorted by the Ti cation, but the calculated cage distortions in the other compounds are much smaller, consistent with a better match between the interior dimensions of the cage and the size of the cation. Incorporation of Ti or Zr in the center of the Si4Al4O12H84- cage substantially reduces the NMR shieldings of the Si, Al, and O, providing spectral signatures for the formation of such compounds. For Si4Al4O12H8Ti the S-4 symmetry equilibrium structure has a local geometry about the Ti and calculated UV excitation energies very close to those in Ti(OH2)(4)(4+), but for the other endohedral complexes, i.e., Si4Al4O12H8Zr, Si4B4O12H8Ti, and Si4B4O12H8Zr, the calculated excitation energies are considerably smaller than those for the hydrated M cations, consistent with their longer nearest-neighbor M-O distances. Thus, such endohedral complexes provide new spectral environments for the Ti4+ and Zr4+ cations.