The symmetry (Oh) and internuclear distances in (O@Nb₆O₁₈)^8–/aq, (O@Ta₆O₁₈)^8–/aq, O@Nb₆O₁₈(Na/K)₈, and O@Ta₆O₁₈(Na/K)₈ clusters were determined by DFT PBE0 calculations. The models of a multicharged cluster in a polarizable aqueous medium and a cluster stabilized by the Na⁺ and K⁺ cations provide the agreement of calculations for the hexaniobate and hexatantalate octaanions wuth X-ray diffraction data.