I clearly remember ending the preface to the first edition of this book hoping the volume would be soon outdated. And yet, when I was approached by the Royal Society of Chemistry regarding a second edition my very first thought was: be careful what you wish for! Still, this has been an enjoyable journey, particularly thanks to the outstanding group of NHC experts who have agreed to join me in this project. My sincere gratitude goes to all of them for their invaluable contributions to this book.
In this second edition, all 14 chapters from the first edition have been thoroughly revised in order to include the most exciting recent findings, while keeping their main focus on the state of the art. This edition has been completed with the addition of a new chapter on NHC–main group adducts, one of the fastest growing areas of research in this area of chemistry.
Having been considered as transient species, at the most, for long years, nobody could have foreseen the current importance of NHCs at the heart of numerous advances in different chemical fields. In spite of the well-established status of NHCs, there is debate around their Lewis representation and bonding to different elements. A number of representations can be found in the literature (in Figure 1, five-membered ring diaminocarbenes are represented as examples). Representation A was soon abandoned, with early data showing the poor π-backdonating ability of NHCs, making the NHC–M bond single in nature (at least with cyclic diaminocarbenes).1 Although the current picture of NHCs is far more complex and it is now known that π-backdonation can represent over 30% of the bonding (hardly negligible…),2 the analytical data still point relentlessly towards NHC–M single bonds. Similarly, the representation of unsaturated NHCs as aromatic derivatives (B) has gradually decreased following reports on the predominance of the carbenic form over ylidic resonance structures.3 Even if subsequent studies pointed towards a cyclic electron stabilization, the resulting aromatic character of imidazol-2-ylidenes would be substantially smaller than in benzene or imidazolium salts.4 Most probably, structure E might be the one closer to the real bonding situation in these species; however, it is also the least straightforward to use. C and D are the most popular versions nowadays, despite the fact that they symbolize a trivalent carbon bond (D) in a confusing manner for any chemist unfamiliar with the field.
In this book, version D has been consistently used all chapters, with two notable exceptions. In Chapter 3, abnormal carbenes are represented in their mesoionic form, whereas in Chapter 5 dative arrow bonds are used in compounds with negligible backbonding and a formal C = E π bond is used for those molecules where substantial multiple bonding has been evidenced. These are arbitrary decisions and do not pretend to make any statement in this debate. For the reader’s convenience, the general representations used for azolium salts, free NHCs and NHC–metal complexes are depicted in Figure 2. Also, the structures of NHC acronyms used throughout the different chapters can be found in Figure 3.