and 4 represents the number of coordinating atoms, here oxygen),102 dicyclohexano-
18-crown-6 (15), and 15-crown-5 (16). These compounds have the property103
of forming complexes with positive ions, generally metallic ions (though not usually
ions of transition metals) or ammonium and substituted ammonium ions.104 The
crown ether is called the host and the ion is the guest. In most cases, the ions are
held tightly in the center of the cavity.105 Each crown ether binds different ions,
depending on the size of the cavity. For example, 14 binds Liþ 106 but not Kþ,107
while 15 binds Kþ but not Liþ.108 Similarly, 15 binds Hg2þ, but not Cd2þ or Zn2þ,
and Sr2þ but not Ca2þ.109 18-Crown-5 binds alkali and ammonium cations >1000
times weaker than 18-crown-6, presumably because the larger 18-crown-6 cavity
involves more hydrogen bonds.110 The complexes can frequently be prepared as
well-defined sharp-melting solids.