Mono- and Bimetallic Zwitterionic Chromium(0) and Tungsten(0) Allenyls

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M­(CO)<sub>5</sub>[C­(OEt)CCR­(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene, 1,3-bis­(2,4,6-trimethylphenyl)­imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis­(dimethyl)­imidazol-2-ylidene) were employed as nucleophiles in these C–C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV–vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,β-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.