Evidence for a facile switch in the bonding mode of µ-vinylidene ligands from σ, η2 to σ,σ; synthesis and structure of a dinuclear µ-σ,η2-allenylidene complex

Abstract

Methylation (MeOSO₂CF₃) of the adducts formed between MeC₂Li and [Me₂(CO)₄(η-C₅H₅)₂](M = Mo, W) forms [M₂(µ-MeC₂Me)(η-C₅H₅)₂] and [M₂(µ-σ,η²-CCMe₂)(CO)₄(η-C₅H₅)₂], variable temperature ¹H n.m.r. studies with the latter showing that these species can undergo a facile µ-σ,η²(4e) to µ-σ,σ(2e) switch; protonation of the corresponding adduct from CH₂C(Me)CCLi and [M₂(CO)₄(η-C₅H₅)₂] provides access to the first µ-σ,η²(4e) allenylidene complexes which show dynamic behaviour in solution and which have been structurally identified by X-ray crystallography.