Pyridylidene transition metal complexes – An underexplored class of organometallic carbenes

Pyridylidene compounds, carbene isomers of related pyridines, remain a fascinating class of intermediates in organic synthesis. Although of high interest as ligands in organometallic and coordination chemistry, this aspect is by far underexplored because of obvious lack of general and efficient synthetic methodologies. This review summarizes the relatively few reported examples of organometallic compounds with pyridylidene ligands. Several important reported examples recently open the way to a promising development of this class of carbenes in organometallic catalysis, biology or materials sciences. • General and historical aspects of metal-free carbenes and the place of pyridylidenes are presented. • The particular case of metal-free pyridylidenes, their reactivity and electronic properties are discussed through the Hammick intermediate and theoretical investigations. • Advancements in the organometallic chemistry of pyridylidenes (until-2025) and their synthetic strategies are summarized, with detailed examination according to the electronic configuration of the metal centre (d 6 , d 8 and d 10 ).