pi-Arene/cation structure and bonding. Solvation versus ligand binding in iron(III) tetraphenylporphyrin complexes of benzene, toluene, p-xylene, and[60]fullerene

Benzene, toluene, p-xylene, and [60]fullerene are shown to be weak ligands to a hard metal such as iron(III) in Fe(TPP)(+) cation (TPP = tetraphenylpo rphyrinate). X-ray crystal structures of [Fe(TPP)(C6H6)][CB11H6Br6].3.5C(6) H(6) (1), [Fe(TPP)(C7H8)][CB11H6Cl6].2C(7)H(8) (2), [Fe(TPP)(C8H10)][Ag(CB1 1H6Br6)(2)].arene (3), and [Fe(TPP)(C-60)][F-20-BPh4].2.5dichlorobenzene (4 ) show distinctively short Fe ... C contacts, in; the range 2.65-2.95 Angst rom, which distinguish ligation from pi-pi cocrystallized solvation. Dihedr al angles between the arene and metalloporphyrin planes are also diagnostic of ligand versus solvate roles. The essential features of the arene coordi nation canbe faithfully reproduced using density functional theory. In cont rast to alkali metal cation/p-arene interactions, a component of the covale nt bonding can be recognized. In a broader context, this study suggests tha t solvents such asbenzene should always be viewed as potential ligands in t he presence of coordinatively unsaturated cations. The common use in supram olecular chemistry of the term "noncovalent" to describe entities with meta l-ligand bonds is viewed as misleading.