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

Starting with the first & sigma;-H2 complex reported by Kubas in 1984, the 3-center 2-electron bonding between & sigma;-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 & sigma;-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 & sigma;-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.

Automated summary

Starting with the first σ-H2 complex reported by Kubas in 1984, the bonding between σ-E-H bonds of main group elements and transition metals has become a fundamental topic in organometallic chemistry. Although organometallic reagents with metal-carbon bonds are commonly used, the description of transition metal σ-E-C complexes (E = main group metal) is lacking. This perspective work aims to question the possibility of defining such complexes that revolve around M-C-E motifs.