Exploring the stability of the NHC-metal bond using thiones as probes

The metal-carbon bond in N-heterocyclic carbene (NHC) metal complexes, which are ubiquitous in modern homogeneous catalysis, is often conjectured to be robust. Here, carbene dissociation was evaluated from a series of complexes with metals of relevance in catalysis containing either an Arduengo-type 2-imidazolylidene or a mesoionic 1,2,3-triazolylidene ligand through thione formation, revealing remarkable kinetic lability of the NHC-metal bond for, e.g. Ir III , Rh III , and Ni II complexes. The stability of Arduengo-type N-heterocyclic carbenes and mesoionic triazolylidenes to a range of catalytically active metals was probed. Remarkably labile bonding was observed with some metals, which impacts mechanistic considerations in catalysis.