Ligand Exchange at Carbon: Synthetic Entry to Elusive Species and Versatile Reagents

How different is carbon compared to other elements in the periodic table? Can carbon compounds be regarded as coordination complexes with carbon as the central element undergoing a facile exchange of its ligands? Although carbon clearly plays a special role among the elements of the periodic table, recent studies have drawn parallels between the bonding situation and the reactivity of carbon compounds to transition metal complexes. This Perspective summarizes recent reports about ylidic and zwitterionic compounds that were shown to exhibit ambiguous bonding situations that can be interpreted as donor-acceptor interactions similar to the bond between a metal and a neutral ligand. Based on this conception, ligand exchange reactions prototypical of transition metal complexes were realized at carbon atoms, enabling new synthetic strategies for the synthesis of reactive species and building blocks. In particular, the exchange of N₂, CO, and phosphine ligands led to the development of a mild method for accessing new compounds and reagents with unusual properties, such as vinylidene ketenes or stable ketenyl anions, that open up a diverse but still poorly explored follow-up chemistry.