Alkali‐Metal‐Controlled Self‐Assembly of Crown‐Shaped Ring Complexes of Lanthanide/[α‐AsW 9 O 33 ] 9− : [K⊂{Eu(H 2 O) 2 (α‐AsW 9 O 33 )} 6 ] 35− and [Cs⊂{Eu(H 2 O) 2 (α‐AsW 9 O 33 )} 4 ] 23−

Crowning glory: The L‐shaped [Ln(H 2 O) 2 (α‐AsW 9 O 33 )] 6− (Ln=lanthanide) building unit provides an elegant approach to develop crown‐shaped structures of [{Ln(H 2 O) 2 (α‐AsW 9 O 33 )} n ] 6 n − ( n =even number of ≥4) which can encapsulate a cation. The watermill‐propeller structure, left, is a K + ion encapsulated by six [α‐AsW 9 O 33 ] 9− ligands, whereas the windmill structure, right, is a Cs + ion encapsulated by four [α‐AsW 9 O 33 ] 9− ligands.