X-ray crystallographic structures of donor-acceptor complexes of aromatic h
ydrocarbons with transition metals are re-examined with the focus on the ar
ene ligands. Thus, significant structural and electronic changes are reveal
ed in the arene moiety due to coordination to the metal center including: (
i) expansion of the aromatic six-carbon ring; (ii) endocyclic pi-bond local
ization; (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 v
arious degrees of (metal-to-ligand) charge transfer. The concept of charge-
transfer bonding not only explains the structural details but also the vari
ous facets of chemical reactivity of metal-coordinated arenes including eff
icient carbon-hydrogen bond activation and nucleophilic-electrophilic umpol
ung, both of which are critical factors in homogeneous metal catalysis. (C)
2000 Elsevier Science S.A. All rights reserved.