Charge-transfer bonding in metal-arene coordination

X-ray crystallographic structures of donor-acceptor complexes of aromatic hydrocarbons with transition metals are re-examined with the focus on the arene ligands. Thus, significant structural and electronic changes are revealed in the arene moiety due to coordination to the metal center including: (i) expansion of the aromatic six-carbon ring; (ii) endocyclic pi-bond localization; (iii) distortion of the planarity (folding) of the arene ring; and (iv) shortening of the metal-arene bond distances. All structural features are characteristic of metal-arene (pi- or sigma-) complexes that exhibit various degrees of (metal-to-ligand) charge transfer. The concept of charge-transfer bonding not only explains the structural details but also the various facets of chemical reactivity of metal-coordinated arenes including efficient carbon-hydrogen bond activation and nucleophilic-electrophilic umpolung, both of which are critical factors in homogeneous metal catalysis. (C) 2000 Elsevier Science S.A. All rights reserved.