The pentagonal K₁₀[(UO₂)₅(μ-O₂)₅(C₂O₄)₅] species have been identified as the building blocks of uranyl-peroxide nanocapsules. The computed complexation energies of different alkali cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) with [(UO₂)₅(μ-O₂)₅(O₂)₅]^10- and [(UO₂)₂₀(μ-O₂)₃₀]^20- species suggest a strong cation templating effect. In the studied species, the largest complexation energy occurs for the experimentally used alkali cations (Na⁺ and K⁺).