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PUBLISHED ONLINE: 14 JUNE 2009 | http://www.nature.com/doifinder/10.1038/nchem.248
Web End =DOI: 10.1038/NCHEM.248
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 donoracceptor 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 CC and CX bonds is more generally applicable than previously anticipated, it may well affect our understanding of organic chemistry in general.
Organic compounds are commonly understood as substances based on carbon atoms that involve all four valence electrons in bonding. However, if two electrons form a lone pair, as is
the case in carbon monoxide, isonitriles or singlet carbenes, we cross the traditional borders to coordination chemistry and/or enter the realm of reactive intermediates. This long-lasting paradigm was challenged only two decades ago when carbenes were prepared that are sufciently stable to be isolated in the pure form1,2. Since
then, the chemistry of dicoordinate carbon(II) as a discrete entity has ourished and become a vital part of contemporary organic chemistry and catalysis research35.
At rst sight, carbogenic compounds, wherein carbon contains all four valence electrons in the form of two lone pairs, may seem even more elusive. Recent theoretical studies, however, suggest that such an uncommon bonding situation may materialize in perfectly stable compounds (some of which have actually been known for decades) even though their true nature went largely unrecognized6,7. Following on from insightful early reports8,9, it was proposed that carbodiphosphoranes, such as http://www.nature.com/compfinder/10.1038/nchem.248_comp1
Web End =1 , should be considered as consisting of two phosphine ligands coordinated to a central, formally zerovalent, carbon atom with two orthogonal lone pairs in a disposition for coordination to external partners (Fig. 1); one resides in an orbital of s symmetry (highest occupied...