LAUSR

Transition metals form a variety of alkylidyne complexes with either a d0 metal center (high-valent) or a non-d0 metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR+. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.