Carbophilic Lewis acids can polarize a coordinated π‐bond by a slippage mechanism. A series of stable ylid‐ or enolate gold complexes of ene‐1,1‐diamines not only emulate this property, but also reveal the exceptional donor capacity of such electron‐rich olefin ligands. Moreover, the first metal complex of a tetraaminoallene is reported, which features a prototype “carbodicarbene” ligand bound to a transition‐metal template.

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.

Thumbnail image of graphical abstract Accept it! Although the π-acceptor properties of N-heterocyclic carbenes (NHCs) are often considered negligible, they can be enhanced to the extent that they start to dominate the catalytic behavior of gold–NHC complexes (see scheme). As it seems to be easier to tune the π acidity of NHCs than to alter their σ-donor properties, the general perception of this important class of ancillary ligands needs to be revised

2 3 4 5 6 7 8 9 12 Unfortunately, however, different authors have used different metal carbonyl templates for this purpose, which requires careful consideration upon comparison of the different scales. The "Rhodium and the Iridium Scale" By far the largest data set is available for rhodium carbonyl complexes of type 1 (X = Cl, I), which have therefore also been chosen in the present study to allow for a proper comparison of the new carbenes with the established ligands. Thereby we rely on the stretching frequency (ν) of t trans-CO moiety because it is least influenced by any steric effects and hence enables the most rigorous assessment of the electronic properties of the ligand in question. Although this is widely practiced in the literature, some authors chose the average CO stretching frequency of the cisand the transdisposed CO in 1 as parameter for such comparisons. It is of note that such an averaged scale is narrower, but the ranking of the ligands remains unaltered unless severe steric effects come into play.

The imidazo[1,5-a]pyridine skeleton provides a versatile platform for the generation of new types of stable N-heterocyclic carbenes. Rh(I) mono- (6) and biscarbenes (7) from imidazo[1,5-a]pyridin-3-ylidenes (ImPy) and derivatives such as 13 from a mesoionic carbene were synthesized and characterized.