A bonding model for gold(I) carbene complexes

The last decade has witnessed dramatic growth in the number of reactions catalysed by electrophilic gold complexes. Although proposed mechanisms often invoke the intermediacy of gold-stabilized cationic species, the nature of bonding in these intermediates remains unclear. Herein, we propose that the carbon-gold bond in these intermediates comprises varying degrees of both sigma- and pi-bonding; however, the overall bond order is generally less than or equal to one. The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation is dictated by the carbene substituents and the ancillary ligand. Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalysed cyclopropanation reactions.