Trend 1.9 of the Lithium Group (Group IA) Trends is that Alkali metal ions do not readily form complexes with ligands such as NH3 or CN-, but polydentate ligands (chelates) such as crown ethers and cryptands with oxygen and nitrogen donor atoms (lewis bases) do form stable complex ions.
The bonding in the complex ions is primarily electrostatic (ionic, very hard) and the relative sizes of the cation and the cavity are important.
This trend is almost entirely dependent on the radius of the cationic species. The larger the cation, the larger the polydentate ligand must be to accommodate the cation. In the same manner, the smaller the cation, the smaller the ligand must be to bind with the cation. If the cavity within the ligand is too large, then the cation will be held too loosely, and won’t bind effectively, and if the cavity is too small for the cation, the cation simply won’t fit inside the ligand.
For example, for complex ions with the 18-crown-6 ether ligand, the order of stability is: Li+<Na+<K+>Rb+>Cs+
The potassium coordinates with 6 partially negatively charged oxygen atoms to make a stable complex ion. Potassium chloride dissociates, allowing the potassium to bind in the cavity, while the anionic chloride stabilizes the complex cationic species.
While the cation is bound in the ligand cavity, all of the charge is pulled into the cavity making the outside of the ring non-polar, making the species soluble in other non-polar (organic) solvents. This feature of polydentate ligands is very useful in phase transfer catalysis.
Similarly, cryptands can also be used to form stable complex ions. Cryptands are generally more selective, hold the cation thousands of times more tightly than crowns, and usually contain oxygen and nitrogen to stabilize the cation.
Other alkali metals with their preferred (parent) crown
Remember: The bigger the ion, the bigger the crown needs to be to accommodate the ion.
Li+ 12-Crown-4 (2 crowns sandwich the Li+, forming an 8 coordinate complex ion)