Electron Redistribution of Aromatic Ligands in (Arene)Cr(CO)₃Complexes. 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 analyses of various (η 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 intermolecular 1:1 complexes of the corresponding series of arenes with nitrosonium cation (NO + ). The latter are prototypical examples of charge-transfer complexes as described by Mulliken. As such, they show enhanced degrees of charge (electron) transfer that approach unity, which is confirmed by quantitative comparison with the structural changes measured in the one-electron (oxidative) transformation of electron-rich arene donors (Ar) to the cation-radicals (Ar • + ). Such a charge redistribution thus readily accounts for the enhanced reactivity to nucleophilic attack of the arene ligand in various ArCr(CO) 3 complexes and related transition-metal/arene analogues.