Donor groups cryptand metal complexation

The crown ethers and cryptands generally form more stable complexes with hard metal ions, such as the group 1, 2, and lanthanide ions. The introduction of softer heteroatoms into the macrocycle can change the selectivity dramatically. There is much interest in tuning macrocycles in this way to favor complexation of certain metal atoms, but O-donor macrocycles will dominate the discussion here. 

---