Synthesis and Ligand Properties of a Persistent, All‐Carbon Four‐Membered‐Ring Allene

A single donor substituent at each terminus is sufficient to make the CCC skeleton of allenes very flexible and give carbon(0) character to the central carbon atom. This allows the synthesis of a four-membered carbocyclic allene, which can be doubly protonated and behaves as a very strong eta(1)-donor ligand for transition metals (see scheme).