Mesoionic ligands and systems beyond singlet carbenes.

Mesoionic carbenes (MICs) based on 1,3-imidazole and 1,2,3-triazole frameworks - namely, iMICs and tMICs - are versatile carbon donor ligands widely employed in organometallic chemistry and related fields. Functionalization of these scaffolds with a donor atom or group has led to the discovery of new classes of mesoionic ligand systems with distinctive stereoelectronic properties. These developments provide a valuable toolkit for advancing molecular chemistry, catalysis, and materials science. This review presents a current overview of iMIC- and tMIC-derived mesoionic compounds featuring carbon, nitrogen, phosphorus, sulfur, or other heavier main-group donor atoms as ligands in organometallic and inorganic chemistry. Although structurally and electronically related to iMICs and tMICs, these ligands incorporate a remote carbon or heteroatom at the donor site, thereby expanding the landscape of mesoionic ligands and main-group systems with diverse electronic structures, properties, and reactivity profiles.