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A journalist recently asked me if I foresaw when my group prepared the first stable carbene that this topic would become a field in its own right. My answer was clear-cut: certainly not! Indeed, in 1988 and even for a few years after, I believed that these species were too fragile to be really useful, and that they would remain laboratory curiosities. I was not totally wrong as far as our group’s first stable carbene is concerned, and it is obvious that Arduengo carbenes, the famous NHCs, have been responsible for the fantastic development of this field of chemistry.

As can be seen in this book, during the last 15 years or so, following the pioneering work by Herrmann et al., carbenes have mostly been used as ancillary ligands for the preparation of transition metal-based catalysts. Compared to phosphorus ligands, carbenes tend to bind more strongly to metal centers, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and have proven remarkably resistant to oxidation. It is noteworthy that, although the first carbene–transition metal complexes were prepared as early as 1915 by Chugaev (Fischer and Maasböl were the first to fully characterize a carbene–metal species), the recent developments in their application in catalysis have been facilitated considerably by the availability of carbenes stable enough to be bottled. Moreover, the existence of metal-free carbenes has allowed their use as organocatalysts.

It is amazing to realize that the first method of taming a carbene was by attaching it to a transition metal, whereas nowadays carbenes can be used to stabilize transition metal centers that otherwise are not accessible. Similarly striking are some recent developments that show that carbenes, which were considered for years as the prototypes of reactive intermediates, can be used for stabilizing highly reactive main group species.

For the future, since the robustness of carbene complexes is largely due to the presence of strong carbon–metal bonds, other types of stable low-valent carbon species are highly desirable, and I believe that a second generation of carbon-based L ligands will soon appear.

Lastly, I wish to say here that, for Bo Arduengo and myself, the enormous amount of results summarized in this excellent book is a wonderful gift.

Guy Bertrand

University of California, USA

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