Arene donor strength

The additions of other (polycyclic) aromatic donors to solutions of dichlorine, dibromine or diiodine afford similar new bands, which show significant red shifts with increasing strength of the arene donor. For example, the absorption maximum of the dibromine complexes varies from 280 nm (with chlorobenzene) to 369 nm (with hexamethylbenzene) and similar variations of the new absorption maxima are observed with diiodine complexes (Fig. 2). 

The increased importance of charge transfer in proceeding up the series of NO+ complexes with the enhanced donor strength of arenes that vary from benzene with IP = 9.23 eV to the electron-rich hexamethylbenzene (IP - 7.85 eV) has its chemical consequences with respect to thermal (adiabatic) electron transfer. Thus the benzene complex with Z = 0.52 is persistent in acetonitrile solution for long periods, provided the solution is protected from adventitious moisture and light. By contrast, the hexamethylbenzene complex with Z = 0.97 slowly liberates nitric oxide under... 

The energy difference AGET° between the vibronically equilibrated reactant and product states can be considered as the difference between the IP of the donor and the EA of the acceptor (in the gas phase) or between the corresponding electrochemical potentials (in solution). For a set of structurally related arene donors in the same solvent, a linear (Mulliken) correlation is usually observed experimentally between the donor strength and the CT energy (due to the relatively small changes in A), i.e. ... 

CT transition energy of the high-energy (hvH) and with sterically hindered donors) on the arene donor low-energy (hvi.) bands with the oxidation potential strength. Data from ref. [28]. 

Thus, in the methylanthracene-induced deligation, the trend in the quantum yields for (DUR)2Fe2+ > (HMB)2Fe2+ follows predictably from the lifetimes (/c,-1) of the labile 19-electron radicals (DUR)2Fe+< (HMB)2Fe+, as evaluated by transient electrochemical methods (136). Furthermore, the remarkable trends in the quantum yields to decrease with the increasing strength of the arene donor must take specific cognizance of the rate of back electron transfer (fc, ). Since the latter results in the annihilation of the radical ion pair Ar2Fe+/D+-, it is readily evaluated from the separate redox couples,... 

Localization of re-bonds (also termed double-bond fixation ) is an important structural feature frequently observed upon arene coordination to a metal center. To rationalize this effect, the formation of covalent (a) bonds between the metal and particular carbon atoms of the arene ring is commonly invoked. However, this cannot explain all the unusual bond distances observed and is not generally applicable to the analogous findings with organic acceptors. Analysis based on the CT concept allows a comparative treatment of all types of donor/acceptor complexes, and predicts a close relationship between the degree of bond localization and the donor/acceptor strengths of the complexed partners. 

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