text: In chemistry, metallacrowns are a macrocyclic compounds that consist of metal ions and solely or predominantly heteroatoms in the ring. Classically, metallacrowns contain an [M–N–O] repeat unit in the macrocycle. First discovered by Vincent L. Pecoraro and Myoung Soo Lah in 1989, metallacrowns are best described as inorganic analogues of crown ethers.  To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 12-MC-4 to 60-MC-20 have been synthesized.