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Abstract The accurate distribution of countercations (Rb⁺and Sr²⁺) around a rigid, spherical, 2.9‐nm size polyoxometalate cluster, {Mo₁₃₂}⁴²⁻, is determined by anomalous small‐angle X‐ray scattering. Both Rb⁺and Sr²⁺ions lead to shorter diffuse lengths for {Mo₁₃₂} than prediction. Most Rb⁺ions are closely associated with {Mo₁₃₂} by staying near the skeleton of {Mo₁₃₂} or in the Stern layer, whereas more Sr²⁺ions loosely associate with {Mo₁₃₂} in the diffuse layer. The stronger affinity of Rb⁺ions towards {Mo₁₃₂} than that of Sr²⁺ions explains the anomalous lower critical coagulation concentration of {Mo₁₃₂} with Rb⁺compared to Sr²⁺. The anomalous behavior of {Mo₁₃₂} can be attributed to majority of negative charges being located at the inner surface of its cavity. The longer anion–cation distance weakens the Coulomb interaction, making the enthalpy change owing to the breakage of hydration layers of cations more important in regulating the counterion–{Mo₁₃₂} interaction. 

Abstract The accurate distribution of countercations (Rb⁺and Sr²⁺) around a rigid, spherical, 2.9‐nm size polyoxometalate cluster, {Mo₁₃₂}⁴²⁻, is determined by anomalous small‐angle X‐ray scattering. Both Rb⁺and Sr²⁺ions lead to shorter diffuse lengths for {Mo₁₃₂} than prediction. Most Rb⁺ions are closely associated with {Mo₁₃₂} by staying near the skeleton of {Mo₁₃₂} or in the Stern layer, whereas more Sr²⁺ions loosely associate with {Mo₁₃₂} in the diffuse layer. The stronger affinity of Rb⁺ions towards {Mo₁₃₂} than that of Sr²⁺ions explains the anomalous lower critical coagulation concentration of {Mo₁₃₂} with Rb⁺compared to Sr²⁺. The anomalous behavior of {Mo₁₃₂} can be attributed to majority of negative charges being located at the inner surface of its cavity. The longer anion–cation distance weakens the Coulomb interaction, making the enthalpy change owing to the breakage of hydration layers of cations more important in regulating the counterion–{Mo₁₃₂} interaction.