Do transition metal sigma complexes of main group metal E–C bonds exist?

Abstract

Starting with the first σ-H2 complex reported by Kubas in 1984, the 3-center 2-electron bonding between σ-E–H bonds of the main group elements and transition metals has become a common topic of fundamental importance in organometallic chemistry. In many cases, the E atom is a metal of the main group series, as Mg, Al, or Zn. However, despite the ubiquity of organometallic reagents with metal–carbon bonds (i.e. MgR2, AlR3 or ZnR2) that are commonly used in conjunction with transition metals, related transition metal σ-E–C complexes (E = main group metal) have not been described as such. Those would involve as the main bonding component the donation of a highly polar E–C bond, that retains some covalent character, towards an empty orbital of a transition metal. In this perspective work we question ourselves whether the definition of such σ-E–C complexes could indeed be coined to compounds that revolve around M−C−E motifs. In most of the cases we have based our analysis strictly on reported structural X-ray diffraction data, though key information extracted from spectroscopic and computational techniques is discussed when available.