Re: Why does intervalence charge transfer take place?

There is a nice description of the general theory of intervalence charge 
transfer at http://www.webexhibits.org/causesofcolor/8.html

In very broad, general terms, there needs to be two charge centers that 
are closely spaced relative to each other. The electron distribution 
between localized charge centers must be unbalanced so electrons can move 
from one charge center to the other. Intervalence charge transfer or 
cooperative charge transfer occurs in order to balance the charge 
distribution and lower the overall energy.

Transfer can only occur when there is enough overlap between the outer 
orbitals of charge centers to allow an electron to pass from one to the 
other. This process requires an activation energy which is either thermal 
or optical but is usually through optical absorption of light of a 
particular wavelength corresponding to the energy difference. 


See also relevant literature that describes the theoretical aspects of 
intervalence charge transfer.


1.  Crutchley R J 1994 Adv. Inorg. Chem. 41 273 and references there in; 
Ward M D 1995 Chem. Soc. Rev. 34 121; Glockle M, Kaim W and Fiedler J 1998 
Organometallics 17 4923; Kaim W, Klein A and Glockle M 2000 Acc. Chem. 
Res. 33 755 and references therein 
2.  McCleverty J A and Ward M D 1998 Acc. Chem. Res. 31 842; Chen P and 
Meyer T J 1998 Chem. Rev. 98 1439 
3.  Creutz C and Taube H 1969 J. Am. Chem. Soc. 91 3988