Vinylidene Complexes

Abstract

Abstract The parent vinylidene CCH 2 and its substituted analogs CCRR′ are highly unstable isomers of the corresponding alkyne, HCCH and RCCR′, respectively. However, vinylidene complexes, which can be denoted as MCCRR′ in the mononuclear case, are often more stable than η 2 ‐alkyne complexes, and in fact, the most common preparation method for vinylidene complexes is the direct reaction of alkynes (terminal alkynes in most cases) with coordinatively unsaturated metal complexes, although the relative stability of vinylidene and η 2 ‐alkyne complexes depends on the nature of the metal center. The vinylidene ligand may be viewed as an α,β‐unsaturated type of carbene, and the MCCRR′ backbone may also be regarded as a metallaallene. The vinylidene ligand often exhibits a π‐acidic nature, and its character somewhat resembles Fischer‐type carbenes. As a result, vinylidene complexes are widely seen with middle to late transition metals, most typically groups 6, 7, 8, and 9 metals in relatively low oxidation state (typically 0 to III). Owing to its versatile reactivities, vinylidene complexes work as intermediates in many catalytic and stoichiometric conversions of alkynes. Surface vinylidene species are also considered to take part in heterogeneous catalysis such as Fischer–Tropsch reaction, and multinuclear vinylidene complexes are expected to serve as molecular models for such surface species. This article summarizes bonding and structure, physicochemical properties, synthesis, and reactivities including catalytic applications of vinylidene complexes.