Crown ethers and cryptands are types of ligands that can form complexes with alkali metals. These ligands have a unique structure that allows them to coordinate with metal ions, particularly alkali metal ions such as lithium, sodium, and potassium.

1. Crown ethers:
- Crown ethers are cyclic compounds that contain several oxygen atoms arranged in a ring structure. The most common crown ethers have a ring size of 12 atoms, with 3-4 oxygen atoms.
- The oxygen atoms in the crown ether ring can act as electron pair donors and coordinate with metal ions. The size and shape of the crown ether ring are designed to accommodate specific metal ions, such as alkali metals.
- When a crown ether coordinates with an alkali metal ion, it forms a complex in which the metal ion is encapsulated within the crown ether ring. The crown ether acts as a "crown" or a "cage" that surrounds the metal ion.
- Crown ethers can selectively bind alkali metal ions due to their size and charge. The oxygen atoms in the crown ether ring interact with the metal ion, forming coordination bonds.
- These crown ether complexes with alkali metals have various applications, such as in ion transport and separation, as well as in chemical analysis and synthesis.

2. Cryptands:
- Cryptands are another type of ligand that can form complexes with alkali metals. They are macrocyclic compounds that contain multiple nitrogen atoms arranged in a ring structure.
- Similar to crown ethers, cryptands have a specific size and shape that allows them to selectively bind alkali metal ions. The nitrogen atoms in the cryptand ring coordinate with the metal ion.
- Cryptands typically have larger ring sizes than crown ethers and can accommodate larger metal ions, such as potassium and rubidium.
- The formation of cryptand complexes with alkali metals is driven by the coordination between the nitrogen atoms and the metal ion, resulting in the encapsulation of the metal ion within the cryptand structure.
- Cryptands are used in various applications, including ion recognition, molecular recognition, and catalysis.

In summary, crown ethers and cryptands are ligands that can form complexes with alkali metals. These complexes have important applications in various fields, including chemistry, materials science, and biology. The selective binding of alkali metal ions by crown ethers and cryptands is due to their specific size, shape, and coordination properties.