CHAPTER 3: Non-Classical N-Heterocyclic Carbene Complexes

This chapter provides an overview of the application of abnormal/mesoionic N-heterocyclic carbenes in organometallic chemistry in the last five years. This update is compiled as an addition to the chapter in the previous edition of this book rather than an extended version. In particular, it highlights recent advances in the use of abnormal imidazolylidenes and the rapid emergence and diverse application of 1,2,3-triazol-4-ylidenes as a distinct subclass of non-classical carbenes. The chapter introduces synthetic methodologies, specific reactivity and stability properties, and discusses the catalytic activity of abnormal/mesoionic carbine–metal complexes. Significant progress has been made in oxidative transformations, whereas cross-coupling catalysis is compromised by low catalyst stability and heterogeneization when using complexes with abnormal carbene ligands.