We report in this communication the first example of catalytic alkyne metathesis reactions mediated by well-defined non-d 0 alkylidyne complexes. The air-stable d 2 Re(V) alkylidyne complex Re4, bearing two PO-chelating ligands and a labile pyridine ligand, could catalyze homometathesis of internal alkynes with a broad substrate scope, including alcohols, amines, and even carboxylic acids. The catalyst can tolerate heating, air, and moisture in both solid and solution states, and the catalytic metathesis reactions could proceed normally in wet solvents.

Metallacyclobutadienes are analogues of cyclobutadienes in which one of the cyclobutadiene CR groups has been formally replaced by a transition-metal fragment. These metallacycles are interesting because they can play an important role in catalysis and can serve as starting materials for the syntheses of organometallic compounds such as metallabenzene, η -cyclopentadienyl, and η -cyclopropenyl complexes. Unlike cyclobutadienes, metallacyclobutadienes can be significantly more stable. A number of metallacyclobutadienes have now been isolated and thoroughly characterized, especially for those that contain transition metals of groups 5-9. Their properties have also been actively investigated. This article highlights the chemistry of metallacyclobutadienes with reference to their syntheses, reactivity, and structural properties.

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.