Figure 2. Crystal structures of 3M and 4M.

Molecular structures with displacement ellipsoids at 40%, and minor disorder, hydrogen atoms, counter ions and lattice solvent omitted, with selective labelling of: (a) 3Li, (b) 3Rb, (c) 3Cs and (d) 4Cs. The structures of 3Na, 4Na and 3K have been reported previously²⁴,²⁷, but the latter was previously prepared by KC₈ reduction of 6 rather than 7. We report here a new polymorph of 3K (Supplementary Methods). The anion components of 4Na–4Cs are very similar and differ mainly in only the nature of the alkali metal bis(crown) cation component.