Coordination chemistry at carbon.

Electron-rich allenes and heterocumulenes are commonly described by the regular notations of organic chemistry. Following on from published results and recent computational studies, we present here a host of crystallographic and reactivity data, as well as theoretical results, that indicate a highly non-canonical bonding situation in many members of this series. These must actually be interpreted as coordination compounds, in which carbon serves as a ‘central atom’ that interacts with its ‘ligand sphere’ via donor–acceptor bonds, even if these internal ligands themselves are carbon based. This captodative description is not limited to compounds that supposedly comprise a carbon(0) centre, a peculiar oxidation state that can be probed experimentally by geminal diauration. As the available data suggest that this unconventional interpretation of C–C and C–X bonds is more generally applicable than previously anticipated, it may well affect our understanding of organic chemistry in general. A systematic variation of ligand properties allows an in-depth experimental and theoretical study of a highly non-canonical bonding situation in certain organic compounds, and provides insight into the criteria that must be fulfilled for such compounds to be truly considered as carbon(0)-containing entities.