Metallacycles

Metallacycles are cyclic organometallic compounds in which a metal atom is an integral part of the ring. The ring typically contains four to eight members, with the metal serving as one vertex, and it combines metal–carbon bonds with organic framework atoms. They appear across a range of transition metals and are important in both fundamental coordination chemistry and catalysis.

Most metallacycles form by cyclometalation, a process in which a metal inserts into a C–H or other skeletal bond of a ligand to generate a metal–carbon bond that closes the ring. They can also arise as intermediates in cycloaddition reactions or in migratory insertions of unsaturated substrates into existing metal–carbon bonds. Common motifs include metallacyclopentadienes (five-membered rings) and metallacyclobutanes; larger rings are also known.

Metallacycles involve a variety of metals, including Fe, Co, Ni, Cu, Ru, Rh, Pd, Pt, Ir, and group 4–6 elements, and they can adopt different oxidation states. Ring size and geometry influence their electronic structure and reactivity, with smaller rings often more strained and reactive. Their chemistry is characterized by the ability of the metal center to activate substrates, mediate bond-forming steps, and participate in electron-transfer processes.

Significance and applications include serving as key intermediates in catalytic cycles for cross-coupling, hydrofunctionalization, and cycloadditions, as well as precursors to more complex cyclic architectures. Their study provides insight into bond formation processes, cyclometalation mechanisms, and the design of metal-assisted synthetic transformations.