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The reactivity of a [V^V] alkylidyne with unsaturated substrates such as NCR (R = Ad or Ph) and PCAd leads to the formation of rare transition metal complexes featuring an α-aza-vanadacyclobutadiene and a β-phospha-vanadacyclobutadiene moieties. 

Abstract Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{κ²‐C,C‐(Me₃SiC₃SiMe₃)}] (2‐M) (BDI=[ArNC(CH₃)]₂CH⁻, Ar=2,6‐ⁱPr₂C₆H₃;M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl₂] (1‐M,M=Ti,V) with 1,3‐dilithioallene [Li₂(Me₃SiC₃SiMe₃)]. Complexes2‐Mhave been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size‐exclusion chromatography (SEC) and intrinsic viscosity studies. Two‐electron oxidation of2‐Vwith nitrous oxide (N₂O) cleanly yields a [V^V] alkylidene‐alkynyl oxo complex [(BDI)V(=O){κ¹‐C‐(=C(SiMe₃)CC(SiMe₃))}] (3), which lends support for how this scaffold in2‐Mmight be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3‐dianionic allene as a segue into M−C multiple bonds.