General Spectral Features of Charge Transfer Complexes

General Spectral Features of Charge Transfer Complexes 

Any theory of charge transfer complexes should explain satisfactorily the following general features of the absorption spectra and energy changes associated with uch complex formation. 

The characteristic absorption of visible or ultra-violet radiation by the complex. This is ascribed to the excitation of an electron in the complex. Often, the Amax is several thousand Angstroms greater than the wavelengths of absorption by the individual components of the complex. This indicates a decrease in energy of the order of 20 kcal/mole for the most loosely bound electron in the complex. In some instances where the two peaks are fairly close, it has been suggested by Bayliss38 39 that it is mainly due to a perturbation of the optical transition and not due to complex formation. 

Although the electron excitation energy decreases considerably on complex formation, the heats of formation for several of these complexes are of a much smaller order2 40 1-5 kcal/mole) This indicates the weak nature of binding in the ground state of the complex. 

The high intensity of the absorption band. The extinction coefficient is often of the order of 104 litre/mole cm. Several of the earlier theories of such complex formation could not account for the high intensities of charge transfer absorption bands. 

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