Rearrangements of alkyne and 1,3-diyne in transition metal center forming small ring complexes

CC bond coupling of alkynes(s) and alkynyl(s) mediated by transition metal complexes have recently lead to isolations of the small organometallic ring constituted of unsaturated carbon atoms of the dimer ligand. The ring structures of four-membered 1-metallacyclo-2-butene and five-membered MCp compounds reveal a diverse array of bond multiplicity that undergoes complexation. Some of these cyclometallated alkynes(s) and 1,3-diyne(s) in solution undergo unusual isomerization, alkyne trimerization, tetramerization, and also condensation, which may be included in an important intermediate of metal-catalyzed organic syntheses of hetero- and carbocyclic molecules. This context is reviewed mainly along syntheses and anomalous crystal structures of these metallacycles formed by the rearrangement of the linear alkyne(s) and 1,3-diyne(s) in the metal center. Recently explored angular phenylene compounds of cobaltacene, and metallapentalene are also described.