Complex ions of group 1 metals

    Unlike simple inorganic ligands, polyethers and, in particular, cyclic polyethers complex alkali metal ions quite strongly. The crown ethers are cyclic ethers which include 1,4,7,10,13,16-hexaoxacyclooctadecane (Figure 1.1a), the common name for which is 18-crown-6; this nomenclature  gives the total number (C +‏ O) and number of O atoms in the ring. Figure 1.1b shows the structure of the [K(18- crown-6)]⁺‏ cation; the K‏ ion is coordinated by the six Odonors.

The radius of the cavity† inside the 18-crown-6 ring is 140 pm, and this compares with values of rion for the alkali metal ions ranging from 76pm for Li‏ to 170pm for Cs‏ (Table 1.1).

The radius of the K‏ ion (138 pm) is well matched to that of the macrocycle, and stability constants for the formation of [M(18-crown-6)]‏   in acetone follow the sequence K⁺> Rb⁺> Cs⁺‏ ≈ Na⁺> Li⁺‏.

   Different crown ethers have different cavity sizes, although the latter is not a fixed property because of the ability of the ligand to change conformation. Thus, the radii of the holes in 18-crown-6, 15-crown-5 and 12-crown-4 can be taken to be roughly 140, 90 and 60pm respectively. It is, however, dangerous to assume that an [ML]‏⁺ complex will fail to form simply because the size of M⁺ is not matched correctly to the hole size of the macrocyclic ligand L. For example, if the radius of M‏⁺ is slightly larger than the radius of L, a complex may form in which M+‏ sits above the plane containing the donor atoms, e.g. [Li(12-crown-4)Cl] .

   Alternatively a 1 :2 complex [ML₂]‏ ⁺ may result in which the metal ion is sandwiched between two ligands, e.g.  [Li(12-crown-4)₂]‏ ⁺. Note that these latter examples refer to complexes crystallized from solution.