Large cations for large anions 1

Alkali metal ions encapsulated within crown ether or cryptand ligands are often used as a source of ‘large cations’ to aid the crystallization of salts containing large anions. An example is the compound [K(crypt-222)]₂[C₆₀].4C₆H₅Me which contains the fulleride [C₆₀]^2-. The space-filling diagram shows part of the packing diagram of [K(crypt- 222)]₂[C₆₀]_.4C₆H₅Me; solvent molecules have been removed for clarity. The [K(crypt-222)]‏⁺ cations have similar overall dimensions to the fulleride dianions, allowing the ions to pack efficiently in the crystal lattice. Colour code: C, grey; K, orange; N, blue; O, red.