Square‐Planar Ruthenium Alkylidyne Complexes Undergo Stepwise Rather Than Concerted [2 + 2] Cycloadditions with Alkynes

Abstract A new entry into square‐planar, formally d 4 ‐configured ruthenium alkylidyne complexes is disclosed, using p ‐tolyl(trimethylsilyl)diazomethane as a convenient and safe alkylidyne synthon. The method furnished complex 12 supported by an electron‐rich PNP‐pincer ligand, which undergoes remarkably facile [2 + 2] cycloaddition with electron‐rich, electron‐deficient, and strained alkynes; these reactions represent the first examples of metallacyclobutadiene formation by a d 4 ‐configured transition metal alkylidyne complex. Strikingly, however, the cycloadditions proceed by a stepwise mechanism, which stands in marked contrast to the concerted pathway entertained by all prototypical d 0 and d 2 Schrock‐type alkylidynes, including the molybdenum, tungsten, and rhenium complexes that currently dominate the field of alkyne metathesis. In essence, it is the non‐bonding lone pair forming the largely metal‐centered HOMO of significant d z2 character that accounts for this unorthodox behavior. The newly gained insight into this key reactivity determinant also allows the few other known reactions of formally d 4 ‐configured alkylidene complexes previously described in the literature to be explained and will empower further explorations of their chemistry.