Main Group Carbenes

Abstract

This article describes the chemistry of main group carbene complexes. Although not limited to, the article particularly summarizes the coordination of N‐heterocyclic carbenes to main group species. Usually, carbene ligands cannot be stabilized by electron‐poor coordination fragments because traditional carbenes show a pronounced tendency toward metal‐to‐ligand π back‐donation. However, N‐heterocyclic carbenes do not rely on back‐donation for stabilization because of the delocalization of the nitrogen lone pair in these carbenes. Thus, they are able to form stable complexes with metals that normally show no tendency for π back‐donation. In general, the carbene acts as a two‐electron donor ligand in a similar fashion to phosphines. Carbene complexes of the alkali and alkaline earth metals, which show a preference for σ‐bonding, are described. The high nucleophilicity of the N‐heterocyclic carbenes is shown by alane, gallane, and indane complexes that have much greater stability than normally observed for Lewis base adducts of group 13. Nucleophilic carbenes also react with Lewis acidic group 14 complexes with the formation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. Furthermore, N‐heterocyclic carbenes are sufficiently nucleophilic to enable depolymerization of cyclopolyphosphines and cyclopolyarsines to occur with the isolation of carbene–pnictinidene adducts. These carbenes can also form stable adducts with strong electron acceptors, such as pnictogen pentafluorides, if the reactivity is moderated. In addition, stable chalcogen–carbene and halide–carbene complexes are also discussed.