Metal Complexes of Small Cycloalkynes and Arynes

Cyclooctyne is the smallest unsubstituted cycloalkyne that can be isolated in the free state and it is more reactive than acyclic alkynes towards transition metal complexes. Smaller cycloalkynes such as cycloheptyne, cyclohexyne, benzyne and cyclopentyne, which are transient molecules in the free state, can be stabilized by coordination either to mononuclear, electron‐rich, transition metal‐containing fragments, e.g. [ZrCp 2 (PMe 3 )] and M(PR 3 ) 2 (M = Ni, Pt), or by formation of dinuclear or polynuclear metal complexes, e.g. [Os 3 H 2 (CO) 9 (C 6 H 4 )] and [Pt 2 (μ‐PPh 2 )(μ‐C 5 H 6 )(PPh 3 ) 3 ]. The alkynes can donate between two and four π‐electrons to the metal centers, the higher number being favored for the early transition metals. The metal‐cycloalkyne and metal‐aryne bonds in the mononuclear complexes readily insert molecules containing CO, CC, CC and CN bonds, a feature that may be useful in organic synthesis. The highly unsaturated species 1, 4‐benzdiyne acts as a bridging ligand between two metal centers in [{Ni(Cy 2 PCH 2 CH 2 PCy 2 )} 2 (μ‐C 6 H 2 )].