Recent advances and application of triazol-5-ylidene (MIC) adducts of the type MIC·E (E = BR3, CS2, CO2, and CSN) and related mesoionic MICX (X = CR2 and NR) species.

N-heterocyclic carbenes (NHCs) are distinguished as exceptional nucleophiles capable of forming stable adducts when treated with a variety of electrophiles. It has been reported that the nucleophilic attack of free NHCs on heteroallene electrophiles, such as carbon dioxide, carbon disulfide, isothiocyanates, and carbodiimides, among others, allows for the generation of adducts with a variety of structural architectures and applications. Compared to the well-developed area of adducts based on classical NHC ligands, the related species based on mesoionic triazol-5-ylidenes have been under development recently. Taking advantage of the enhanced sigma-donor properties of the triazol-5-ylidene (MIC) ligands compared to classical NHCs, a new series of stable adducts of the type MIC·electrophile and related mesoionic MICX (X = CR2 and NR) species have been recently prepared and applied in coordination chemistry and catalytic applications. In this frontier article, the recent progress on the design and synthesis of mesoionic triazol-5-ylidene adducts with boranes, heteroallenes (CS2, CO2, and CSNARr), and related mesoionic N-heterocyclic olefins (mNHOS) and imines (MIIs) is discussed. Special emphasis is placed on their structural features, electronic properties, and applications.