Intensity of Intervalence Transition

5 Intensity of Intervalence Transition Using Equation 10.38 the transition dipole moment is 

We assume that the donor and acceptor are at some distance, and hence the integrals that contain both ( )d and ( )a are small compared with the integrals that contain only one of the terms, as assumed in Equation 10.46. We let the donor and acceptor be located on the x-axis with distance R in between and the origin at the donor D. 

This equation tells us that the intensity of the allowed intervalence transition decreases fast with the distance between the two metal ions. 

In the delocalized case, the excitations are predominantly in the avoided crossing region. Using Equation 10.47 with 4 = 45°, we obtain 

The intervalence transition is often in the visible region and since the transition is allowed, a mixed valence complex often has a very strong color. A good example is Prussian blue, which can be made by adding Fe + to K4Fe(CN)( , where the valence state of Fe is +2. Alternatively, Fe + may be added to KjFelCN), where the valence state of Fe is -1-3. If compounds with the same valence are mixed, there is no strong color. In the present case, the system is not strictly symmetric since one Fe is connected to C while the other is connected to N in CN . The lack of symmetry guarantees a unique product ( TumbuH s blue is the same as Prussian blue). 

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