Electron redistribution of aromatic ligands in (arene)Cr(CO)(3) complexes.Structural (bond-length) changes as quantitative measures

Arene ligands experience significant ring expansion upon coordination with chromium tricarbonyl, as established by precise X-ray crystallographic anal yses of various (eta (6)-arene)Cr(CO)(3) complexes. Such changes in ligand structures result from the charge (electron) redistribution, Ar+-Cr-, upon arene coordination, since they are closely related to those found in the in termolecular 1:1 complexes of the corresponding series of arenes with nitro sonium cation (NO+). The latter are prototypical examples of charge-transfe r complexes as described by Mulliken. As such, they show enhanced degrees o f charge (electron) transfer that approach unity, which is confirmed by qua ntitative comparison with the structural changes measured in the one-electr on (oxidative) transformation of electron-rich arene donors (Ar) to the cat ion-radicals (Ar.+). Such a charge redistribution thus readily accounts for the enhanced reactivity to nucleophilic attack of the arene ligand in vari ous ArCr(CO)(3) complexes and related transition-metal/arene analogues.